draft-ietf-rtgwg-bgp-routing-large-dc-09.txt   draft-ietf-rtgwg-bgp-routing-large-dc-10.txt 
Routing Area Working Group P. Lapukhov Routing Area Working Group P. Lapukhov
Internet-Draft Facebook Internet-Draft Facebook
Intended status: Informational A. Premji Intended status: Informational A. Premji
Expires: September 3, 2016 Arista Networks Expires: October 29, 2016 Arista Networks
J. Mitchell, Ed. J. Mitchell, Ed.
March 2, 2016 April 27, 2016
Use of BGP for routing in large-scale data centers Use of BGP for routing in large-scale data centers
draft-ietf-rtgwg-bgp-routing-large-dc-09 draft-ietf-rtgwg-bgp-routing-large-dc-10
Abstract Abstract
Some network operators build and operate data centers that support Some network operators build and operate data centers that support
over one hundred thousand servers. In this document, such data over one hundred thousand servers. In this document, such data
centers are referred to as "large-scale" to differentiate them from centers are referred to as "large-scale" to differentiate them from
smaller infrastructures. Environments of this scale have a unique smaller infrastructures. Environments of this scale have a unique
set of network requirements with an emphasis on operational set of network requirements with an emphasis on operational
simplicity and network stability. This document summarizes simplicity and network stability. This document summarizes
operational experience in designing and operating large-scale data operational experience in designing and operating large-scale data
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 3, 2016. This Internet-Draft will expire on October 29, 2016.
Copyright Notice Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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7.3. Impact of Clos Topology Fan-outs . . . . . . . . . . . . 23 7.3. Impact of Clos Topology Fan-outs . . . . . . . . . . . . 23
7.4. Failure Impact Scope . . . . . . . . . . . . . . . . . . 24 7.4. Failure Impact Scope . . . . . . . . . . . . . . . . . . 24
7.5. Routing Micro-Loops . . . . . . . . . . . . . . . . . . . 25 7.5. Routing Micro-Loops . . . . . . . . . . . . . . . . . . . 25
8. Additional Options for Design . . . . . . . . . . . . . . . . 26 8. Additional Options for Design . . . . . . . . . . . . . . . . 26
8.1. Third-party Route Injection . . . . . . . . . . . . . . . 26 8.1. Third-party Route Injection . . . . . . . . . . . . . . . 26
8.2. Route Summarization within Clos Topology . . . . . . . . 26 8.2. Route Summarization within Clos Topology . . . . . . . . 26
8.2.1. Collapsing Tier-1 Devices Layer . . . . . . . . . . . 27 8.2.1. Collapsing Tier-1 Devices Layer . . . . . . . . . . . 27
8.2.2. Simple Virtual Aggregation . . . . . . . . . . . . . 28 8.2.2. Simple Virtual Aggregation . . . . . . . . . . . . . 28
8.3. ICMP Unreachable Message Masquerading . . . . . . . . . . 29 8.3. ICMP Unreachable Message Masquerading . . . . . . . . . . 29
9. Security Considerations . . . . . . . . . . . . . . . . . . . 29 9. Security Considerations . . . . . . . . . . . . . . . . . . . 29
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 30 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 30
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 30 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
12.1. Normative References . . . . . . . . . . . . . . . . . . 30 12.1. Normative References . . . . . . . . . . . . . . . . . . 30
12.2. Informative References . . . . . . . . . . . . . . . . . 30 12.2. Informative References . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34
1. Introduction 1. Introduction
This document describes a practical routing design that can be used This document describes a practical routing design that can be used
in a large-scale data center ("DC") design. Such data centers, also in a large-scale data center ("DC") design. Such data centers, also
known as hyper-scale or warehouse-scale data centers, have a unique known as hyper-scale or warehouse-scale data centers, have a unique
attribute of supporting over a hundred thousand servers. In order to attribute of supporting over a hundred thousand servers. In order to
accommodate networks of this scale, operators are revisiting accommodate networks of this scale, operators are revisiting
networking designs and platforms to address this need. networking designs and platforms to address this need.
The design presented in this document is based on operational The design presented in this document is based on operational
experience with data centers built to support large-scale distributed experience with data centers built to support large-scale distributed
software infrastructure, such as a Web search engine. The primary software infrastructure, such as a Web search engine. The primary
requirements in such an environment are operational simplicity and requirements in such an environment are operational simplicity and
network stability so that a small group of people can effectively network stability so that a small group of people can effectively
support a significantly sized network. support a significantly sized network.
Experimentation and extensive testing has shown that External BGP Experimentation and extensive testing have shown that External BGP
(EBGP) [RFC4271] is well suited as a stand-alone routing protocol for (EBGP) [RFC4271] is well suited as a stand-alone routing protocol for
these type of data center applications. This is in contrast with these type of data center applications. This is in contrast with
more traditional DC designs, which may use simple tree topologies and more traditional DC designs, which may use simple tree topologies and
rely on extending Layer 2 domains across multiple network devices. rely on extending Layer 2 domains across multiple network devices.
This document elaborates on the requirements that led to this design This document elaborates on the requirements that led to this design
choice and presents details of the EBGP routing design as well as choice and presents details of the EBGP routing design as well as
explores ideas for further enhancements. explores ideas for further enhancements.
This document first presents an overview of network design This document first presents an overview of network design
requirements and considerations for large-scale data centers. Then requirements and considerations for large-scale data centers. Then
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design options. design options.
2. Network Design Requirements 2. Network Design Requirements
This section describes and summarizes network design requirements for This section describes and summarizes network design requirements for
large-scale data centers. large-scale data centers.
2.1. Bandwidth and Traffic Patterns 2.1. Bandwidth and Traffic Patterns
The primary requirement when building an interconnection network for The primary requirement when building an interconnection network for
large number of servers is to accommodate application bandwidth and a large number of servers is to accommodate application bandwidth and
latency requirements. Until recently it was quite common to see the latency requirements. Until recently it was quite common to see the
majority of traffic entering and leaving the data center, commonly majority of traffic entering and leaving the data center, commonly
referred to as "north-south" traffic. Traditional "tree" topologies referred to as "north-south" traffic. Traditional "tree" topologies
were sufficient to accommodate such flows, even with high were sufficient to accommodate such flows, even with high
oversubscription ratios between the layers of the network. If more oversubscription ratios between the layers of the network. If more
bandwidth was required, it was added by "scaling up" the network bandwidth was required, it was added by "scaling up" the network
elements, e.g. by upgrading the device's linecards or fabrics or elements, e.g., by upgrading the device's linecards or fabrics or
replacing the device with one with higher port density. replacing the device with one with higher port density.
Today many large-scale data centers host applications generating Today many large-scale data centers host applications generating
significant amounts of server-to-server traffic, which does not significant amounts of server-to-server traffic, which does not
egress the DC, commonly referred to as "east-west" traffic. Examples egress the DC, commonly referred to as "east-west" traffic. Examples
of such applications could be compute clusters such as Hadoop of such applications could be compute clusters such as Hadoop
[HADOOP], massive data replication between clusters needed by certain [HADOOP], massive data replication between clusters needed by certain
applications, or virtual machine migrations. Scaling traditional applications, or virtual machine migrations. Scaling traditional
tree topologies to match these bandwidth demands becomes either too tree topologies to match these bandwidth demands becomes either too
expensive or impossible due to physical limitations, e.g. port expensive or impossible due to physical limitations, e.g., port
density in a switch. density in a switch.
2.2. CAPEX Minimization 2.2. CAPEX Minimization
The Capital Expenditures (CAPEX) associated with the network The Capital Expenditures (CAPEX) associated with the network
infrastructure alone constitutes about 10-15% of total data center infrastructure alone constitutes about 10-15% of total data center
expenditure (see [GREENBERG2009]). However, the absolute cost is expenditure (see [GREENBERG2009]). However, the absolute cost is
significant, and hence there is a need to constantly drive down the significant, and hence there is a need to constantly drive down the
cost of individual network elements. This can be accomplished in two cost of individual network elements. This can be accomplished in two
ways: ways:
o Unifying all network elements, preferably using the same hardware o Unifying all network elements, preferably using the same hardware
type or even the same device. This allows for volume pricing on type or even the same device. This allows for volume pricing on
bulk purchases and reduced maintenance and sparing costs. bulk purchases and reduced maintenance and inventory costs.
o Driving costs down using competitive pressures, by introducing o Driving costs down using competitive pressures, by introducing
multiple network equipment vendors. multiple network equipment vendors.
In order to allow for good vendor diversity it is important to In order to allow for good vendor diversity it is important to
minimize the software feature requirements for the network elements. minimize the software feature requirements for the network elements.
This strategy provides maximum flexibility of vendor equipment This strategy provides maximum flexibility of vendor equipment
choices while enforcing interoperability using open standards. choices while enforcing interoperability using open standards.
2.3. OPEX Minimization 2.3. OPEX Minimization
Operating large-scale infrastructure can be expensive as a larger Operating large-scale infrastructure can be expensive as a larger
amount of elements will statistically fail more often. Having a amount of elements will statistically fail more often. Having a
simpler design and operating using a limited software feature set simpler design and operating using a limited software feature set
minimizes software issue-related failures. minimizes software issue-related failures.
An important aspect of Operational Expenditure (OPEX) minimization is An important aspect of Operational Expenditure (OPEX) minimization is
reducing size of failure domains in the network. Ethernet networks reducing the size of failure domains in the network. Ethernet
are known to be susceptible to broadcast or unicast traffic storms networks are known to be susceptible to broadcast or unicast traffic
that can have a dramatic impact on network performance and storms that can have a dramatic impact on network performance and
availability. The use of a fully routed design significantly reduces availability. The use of a fully routed design significantly reduces
the size of the data plane failure domains - i.e. limits them to the the size of the data plane failure domains, i.e., limits them to the
lowest level in the network hierarchy. However, such designs lowest level in the network hierarchy. However, such designs
introduce the problem of distributed control plane failures. This introduce the problem of distributed control plane failures. This
observation calls for simpler and less control plane protocols to observation calls for simpler and less control plane protocols to
reduce protocol interaction issues, reducing the chance of a network reduce protocol interaction issues, reducing the chance of a network
meltdown. Minimizing software feature requirements as described in meltdown. Minimizing software feature requirements as described in
the CAPEX section above also reduces testing and training the CAPEX section above also reduces testing and training
requirements. requirements.
2.4. Traffic Engineering 2.4. Traffic Engineering
In any data center, application load balancing is a critical function In any data center, application load balancing is a critical function
performed by network devices. Traditionally, load balancers are performed by network devices. Traditionally, load balancers are
deployed as dedicated devices in the traffic forwarding path. The deployed as dedicated devices in the traffic forwarding path. The
problem arises in scaling load balancers under growing traffic problem arises in scaling load balancers under growing traffic
demand. A preferable solution would be able to scale load balancing demand. A preferable solution would be able to scale the load
layer horizontally, by adding more of the uniform nodes and balancing layer horizontally, by adding more of the uniform nodes and
distributing incoming traffic across these nodes. In situations like distributing incoming traffic across these nodes. In situations like
this, an ideal choice would be to use network infrastructure itself this, an ideal choice would be to use network infrastructure itself
to distribute traffic across a group of load balancers. The to distribute traffic across a group of load balancers. The
combination of Anycast prefix advertisement [RFC4786] and Equal Cost combination of Anycast prefix advertisement [RFC4786] and Equal Cost
Multipath (ECMP) functionality can be used to accomplish this goal. Multipath (ECMP) functionality can be used to accomplish this goal.
To allow for more granular load distribution, it is beneficial for To allow for more granular load distribution, it is beneficial for
the network to support the ability to perform controlled per-hop the network to support the ability to perform controlled per-hop
traffic engineering. For example, it is beneficial to directly traffic engineering. For example, it is beneficial to directly
control the ECMP next-hop set for Anycast prefixes at every level of control the ECMP next-hop set for Anycast prefixes at every level of
network hierarchy. network hierarchy.
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3. Data Center Topologies Overview 3. Data Center Topologies Overview
This section provides an overview of two general types of data center This section provides an overview of two general types of data center
designs - hierarchical (also known as tree based) and Clos based designs - hierarchical (also known as tree based) and Clos based
network designs. network designs.
3.1. Traditional DC Topology 3.1. Traditional DC Topology
In the networking industry, a common design choice for data centers In the networking industry, a common design choice for data centers
typically look like a (upside down) tree with redundant uplinks and typically look like an (upside down) tree with redundant uplinks and
three layers of hierarchy namely; core, aggregation/distribution and three layers of hierarchy namely; core, aggregation/distribution and
access layers (see Figure 1). To accommodate bandwidth demands, each access layers (see Figure 1). To accommodate bandwidth demands, each
higher layer, from server towards DC egress or WAN, has higher port higher layer, from server towards DC egress or WAN, has higher port
density and bandwidth capacity where the core functions as the density and bandwidth capacity where the core functions as the
"trunk" of the tree based design. To keep terminology uniform and "trunk" of the tree based design. To keep terminology uniform and
for comparison with other designs, in this document these layers will for comparison with other designs, in this document these layers will
be referred to as Tier-1, Tier-2 and Tier-3 "tiers", instead of Core, be referred to as Tier-1, Tier-2 and Tier-3 "tiers", instead of Core,
Aggregation or Access layers. Aggregation or Access layers.
+------+ +------+ +------+ +------+
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This section describes a common design for horizontally scalable This section describes a common design for horizontally scalable
topology in large-scale data centers in order to meet REQ1. topology in large-scale data centers in order to meet REQ1.
3.2.1. Overview 3.2.1. Overview
A common choice for a horizontally scalable topology is a folded Clos A common choice for a horizontally scalable topology is a folded Clos
topology, sometimes called "fat-tree" (see, for example, [INTERCON] topology, sometimes called "fat-tree" (see, for example, [INTERCON]
and [ALFARES2008]). This topology features an odd number of stages and [ALFARES2008]). This topology features an odd number of stages
(sometimes known as dimensions) and is commonly made of uniform (sometimes known as dimensions) and is commonly made of uniform
elements, e.g. network switches with the same port count. Therefore, elements, e.g., network switches with the same port count.
the choice of folded Clos topology satisfies REQ1 and facilitates Therefore, the choice of folded Clos topology satisfies REQ1 and
REQ2. See Figure 2 below for an example of a folded 3-stage Clos facilitates REQ2. See Figure 2 below for an example of a folded
topology (3 stages counting Tier-2 stage twice, when tracing a packet 3-stage Clos topology (3 stages counting Tier-2 stage twice, when
flow): tracing a packet flow):
+-------+ +-------+
| |----------------------------+ | |----------------------------+
| |------------------+ | | |------------------+ |
| |--------+ | | | |--------+ | |
+-------+ | | | +-------+ | | |
+-------+ | | | +-------+ | | |
| |--------+---------+-------+ | | |--------+---------+-------+ |
| |--------+-------+ | | | | |--------+-------+ | | |
| |------+ | | | | | | |------+ | | | | |
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o Tier-1 switches have exactly one path to every server in this o Tier-1 switches have exactly one path to every server in this
topology. This is an important property that makes route topology. This is an important property that makes route
summarization dangerous in this topology (see Section 8.2 below). summarization dangerous in this topology (see Section 8.2 below).
o Traffic flowing from server to server is load balanced over all o Traffic flowing from server to server is load balanced over all
available paths using ECMP. available paths using ECMP.
3.2.3. Scaling the Clos topology 3.2.3. Scaling the Clos topology
A Clos topology can be scaled either by increasing network element A Clos topology can be scaled either by increasing network element
port density or adding more stages, e.g. moving to a 5-stage Clos, as port density or adding more stages, e.g., moving to a 5-stage Clos,
illustrated in Figure 3 below: as illustrated in Figure 3 below:
Tier-1 Tier-1
+-----+ +-----+
Cluster | | Cluster | |
+----------------------------+ +--| |--+ +----------------------------+ +--| |--+
| | | +-----+ | | | | +-----+ |
| Tier-2 | | | Tier-2 | Tier-2 | | | Tier-2
| +-----+ | | +-----+ | +-----+ | +-----+ | | +-----+ | +-----+
| +-------------| DEV |------+--| |--+--| |-------------+ | +-------------| DEV |------+--| |--+--| |-------------+
| | +-----| C |------+ | | +--| |-----+ | | | +-----| C |------+ | | +--| |-----+ |
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main reason to limit oversubscription at a single layer of the main reason to limit oversubscription at a single layer of the
network is to simplify application development that would otherwise network is to simplify application development that would otherwise
need to account for multiple bandwidth pools: within rack (Tier-3), need to account for multiple bandwidth pools: within rack (Tier-3),
between racks (Tier-2), and between clusters (Tier-1). Since between racks (Tier-2), and between clusters (Tier-1). Since
oversubscription does not have a direct relationship to the routing oversubscription does not have a direct relationship to the routing
design it is not discussed further in this document. design it is not discussed further in this document.
3.2.4. Managing the Size of Clos Topology Tiers 3.2.4. Managing the Size of Clos Topology Tiers
If a data center network size is small, it is possible to reduce the If a data center network size is small, it is possible to reduce the
number of switches in Tier-1 or Tier-2 of Clos topology by a factor number of switches in Tier-1 or Tier-2 of a Clos topology by a factor
of two. To understand how this could be done, take Tier-1 as an of two. To understand how this could be done, take Tier-1 as an
example. Every Tier-2 device connects to a single group of Tier-1 example. Every Tier-2 device connects to a single group of Tier-1
devices. If half of the ports on each of the Tier-1 devices are not devices. If half of the ports on each of the Tier-1 devices are not
being used then it is possible to reduce the number of Tier-1 devices being used then it is possible to reduce the number of Tier-1 devices
by half and simply map two uplinks from a Tier-2 device to the same by half and simply map two uplinks from a Tier-2 device to the same
Tier-1 device that were previously mapped to different Tier-1 Tier-1 device that were previously mapped to different Tier-1
devices. This technique maintains the same bisectional bandwidth devices. This technique maintains the same bisectional bandwidth
while reducing the number of elements in the Tier-1 layer, thus while reducing the number of elements in the Tier-1 layer, thus
saving on CAPEX. The tradeoff, in this example, is the reduction of saving on CAPEX. The tradeoff, in this example, is the reduction of
maximum DC size in terms of overall server count by half. maximum DC size in terms of overall server count by half.
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This section provides an overview of three general types of data This section provides an overview of three general types of data
center protocol designs - Layer 2 only, Hybrid L2/L3 and Layer 3 center protocol designs - Layer 2 only, Hybrid L2/L3 and Layer 3
only. only.
4.1. Layer 2 Only Designs 4.1. Layer 2 Only Designs
Originally most data center designs used Spanning-Tree Protocol (STP) Originally most data center designs used Spanning-Tree Protocol (STP)
originally defined in [IEEE8021D-1990] for loop free topology originally defined in [IEEE8021D-1990] for loop free topology
creation, typically utilizing variants of the traditional DC topology creation, typically utilizing variants of the traditional DC topology
described in Section 3.1. At the time, many DC switches either did described in Section 3.1. At the time, many DC switches either did
not support Layer 3 routed protocols or supported it with additional not support Layer 3 routing protocols or supported them with
licensing fees, which played a part in the design choice. Although additional licensing fees, which played a part in the design choice.
many enhancements have been made through the introduction of Rapid Although many enhancements have been made through the introduction of
Spanning Tree Protocol (RSTP) in the latest revision of Rapid Spanning Tree Protocol (RSTP) in the latest revision of
[IEEE8021D-2004] and Multiple Spanning Tree Protocol (MST) specified [IEEE8021D-2004] and Multiple Spanning Tree Protocol (MST) specified
in [IEEE8021Q] that increase convergence, stability and load in [IEEE8021Q] that increase convergence, stability and load
balancing in larger topologies, many of the fundamentals of the balancing in larger topologies, many of the fundamentals of the
protocol limit its applicability in large-scale DCs. STP and its protocol limit its applicability in large-scale DCs. STP and its
newer variants use an active/standby approach to path selection and newer variants use an active/standby approach to path selection and
are therefore hard to deploy in horizontally-scaled topologies as are therefore hard to deploy in horizontally-scaled topologies as
described in Section 3.2. Further, operators have had many described in Section 3.2. Further, operators have had many
experiences with large failures due to issues caused by improper experiences with large failures due to issues caused by improper
cabling, misconfiguration, or flawed software on a single device. cabling, misconfiguration, or flawed software on a single device.
These failures regularly affected the entire spanning-tree domain and These failures regularly affected the entire spanning-tree domain and
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For these reasons, and since almost all DC traffic is now IP, For these reasons, and since almost all DC traffic is now IP,
therefore requiring a Layer 3 routing protocol at the network edge therefore requiring a Layer 3 routing protocol at the network edge
for external connectivity, designs utilizing STP usually fail all of for external connectivity, designs utilizing STP usually fail all of
the requirements of large-scale DC operators. Various enhancements the requirements of large-scale DC operators. Various enhancements
to link-aggregation protocols such as [IEEE8023AD], generally known to link-aggregation protocols such as [IEEE8023AD], generally known
as Multi-Chassis Link-Aggregation (M-LAG) made it possible to use as Multi-Chassis Link-Aggregation (M-LAG) made it possible to use
Layer 2 designs with active-active network paths while relying on STP Layer 2 designs with active-active network paths while relying on STP
as the backup for loop prevention. The major downsides of this as the backup for loop prevention. The major downsides of this
approach are the lack of ability to scale linearly past two in most approach are the lack of ability to scale linearly past two in most
implementations, lack of standards based implementations, and added implementations, lack of standards based implementations, and added
failure domain risk of keeping state between the devices. the failure domain risk of syncing state between the devices.
It should be noted that building large, horizontally scalable, Layer It should be noted that building large, horizontally scalable, Layer
2 only networks without STP is possible recently through the 2 only networks without STP is possible recently through the
introduction of the TRILL protocol in [RFC6325]. TRILL resolves many introduction of the TRILL protocol in [RFC6325]. TRILL resolves many
of the issues STP has for large-scale DC design however due to the of the issues STP has for large-scale DC design however due to the
limited number of implementations, and often the requirement for limited number of implementations, and often the requirement for
specific equipment that supports it, this has limited its specific equipment that supports it, this has limited its
applicability and increased the cost of such designs. applicability and increased the cost of such designs.
Finally, neither the base TRILL specification nor the M-LAG approach Finally, neither the base TRILL specification nor the M-LAG approach
totally eliminate the problem of the shared broadcast domain, that is totally eliminate the problem of the shared broadcast domain, that is
so detrimental to the operations of any Layer 2, Ethernet based so detrimental to the operations of any Layer 2, Ethernet based
solutions. Later TRILL extensions have been proposed to solve the solution. Later TRILL extensions have been proposed to solve the
this problem statement primarily based on the approaches outlined in this problem statement primarily based on the approaches outlined in
[RFC7067], but this even further limits the number of available [RFC7067], but this even further limits the number of available
interoperable implementations that can be used to build a fabric, interoperable implementations that can be used to build a fabric.
therefore TRILL based designs have issues meeting REQ2, REQ3, and Therefore, TRILL based designs have issues meeting REQ2, REQ3, and
REQ4. REQ4.
4.2. Hybrid L2/L3 Designs 4.2. Hybrid L2/L3 Designs
Operators have sought to limit the impact of data plane faults and Operators have sought to limit the impact of data plane faults and
build large-scale topologies through implementing routing protocols build large-scale topologies through implementing routing protocols
in either the Tier-1 or Tier-2 parts of the network and dividing the in either the Tier-1 or Tier-2 parts of the network and dividing the
Layer 2 domain into numerous, smaller domains. This design has Layer 2 domain into numerous, smaller domains. This design has
allowed data centers to scale up, but at the cost of complexity in allowed data centers to scale up, but at the cost of complexity in
the network managing multiple protocols. For the following reasons, managing multiple network protocols. For the following reasons,
operators have retained Layer 2 in either the access (Tier-3) or both operators have retained Layer 2 in either the access (Tier-3) or both
access and aggregation (Tier-3 and Tier-2) parts of the network: access and aggregation (Tier-3 and Tier-2) parts of the network:
o Supporting legacy applications that may require direct Layer 2 o Supporting legacy applications that may require direct Layer 2
adjacency or use non-IP protocols. adjacency or use non-IP protocols.
o Seamless mobility for virtual machines that require the o Seamless mobility for virtual machines that require the
preservation of IP addresses when a virtual machine moves to preservation of IP addresses when a virtual machine moves to a
different Tier-3 switch. different Tier-3 switch.
o Simplified IP addressing = less IP subnets are required for the o Simplified IP addressing = less IP subnets are required for the
data center. data center.
o Application load balancing may require direct Layer 2 reachability o Application load balancing may require direct Layer 2 reachability
to perform certain functions such as Layer 2 Direct Server Return to perform certain functions such as Layer 2 Direct Server Return
(DSR). (DSR, see [L3DSR]).
o Continued CAPEX differences between Layer 2 and Layer 3 capable o Continued CAPEX differences between Layer 2 and Layer 3 capable
switches. switches.
4.3. Layer 3 Only Designs 4.3. Layer 3 Only Designs
Network designs that leverage IP routing down to Tier-3 of the Network designs that leverage IP routing down to Tier-3 of the
network have gained popularity as well. The main benefit of these network have gained popularity as well. The main benefit of these
designs is improved network stability and scalability, as a result of designs is improved network stability and scalability, as a result of
confining L2 broadcast domains. Commonly an Interior Gateway confining L2 broadcast domains. Commonly an Interior Gateway
Protocol (IGP) such as OSPF [RFC2328] is used as the primary routing Protocol (IGP) such as OSPF [RFC2328] is used as the primary routing
protocol in such a design. As data centers grow in scale, and server protocol in such a design. As data centers grow in scale, and server
count exceeds tens of thousands, such fully routed designs have count exceeds tens of thousands, such fully routed designs have
become more attractive. become more attractive.
Choosing a Layer 3 only design greatly simplifies the network, Choosing a Layer 3 only design greatly simplifies the network,
facilitating the meeting of REQ1 and REQ2, and has widespread facilitating the meeting of REQ1 and REQ2, and has widespread
adoption in networks where large Layer 2 adjacency and larger size adoption in networks where large Layer 2 adjacency and larger size
Layer 3 subnets are not as critical compared to network scalability Layer 3 subnets are not as critical compared to network scalability
and stability. Application providers and network operators continue and stability. Application providers and network operators continue
to also develop new solutions to meet some of the requirements that to develop new solutions to meet some of the requirements that
previously have driven large Layer 2 domains by using various overlay previously had driven large Layer 2 domains by using various overlay
or tunneling techniques. or tunneling techniques.
5. Routing Protocol Selection and Design 5. Routing Protocol Selection and Design
In this section the motivations for using External BGP (EBGP) as the In this section the motivations for using External BGP (EBGP) as the
single routing protocol for data center networks having a Layer 3 single routing protocol for data center networks having a Layer 3
protocol design and Clos topology are reviewed. Then, a practical protocol design and Clos topology are reviewed. Then, a practical
approach for designing an EBGP based network is provided. approach for designing an EBGP based network is provided.
5.1. Choosing EBGP as the Routing Protocol 5.1. Choosing EBGP as the Routing Protocol
REQ2 would give preference to the selection of a single routing REQ2 would give preference to the selection of a single routing
protocol to reduce complexity and interdependencies. While it is protocol to reduce complexity and interdependencies. While it is
common to rely on an IGP in this situation, sometimes with either the common to rely on an IGP in this situation, sometimes with either the
addition of EBGP at the device bordering the WAN or Internal BGP addition of EBGP at the device bordering the WAN or Internal BGP
(IBGP) throughout, this document proposes the use of an EBGP only (IBGP) throughout, this document proposes the use of an EBGP only
design. design.
Although EBGP is the protocol used for almost all inter-domain Although EBGP is the protocol used for almost all inter-domain
routing on the Internet and has wide support from both vendor and routing in the Internet and has wide support from both vendor and
service provider communities, it is not generally deployed as the service provider communities, it is not generally deployed as the
primary routing protocol within the data center for a number of primary routing protocol within the data center for a number of
reasons (some of which are interrelated): reasons (some of which are interrelated):
o BGP is perceived as a "WAN only protocol only" and not often o BGP is perceived as a "WAN only protocol only" and not often
considered for enterprise or data center applications. considered for enterprise or data center applications.
o BGP is believed to have a "much slower" routing convergence o BGP is believed to have a "much slower" routing convergence
compared to IGPs. compared to IGPs.
skipping to change at page 14, line 16 skipping to change at page 14, line 16
data structures and state machine are simpler as compared to most data structures and state machine are simpler as compared to most
link-state IGPs such as OSPF. For example, instead of link-state IGPs such as OSPF. For example, instead of
implementing adjacency formation, adjacency maintenance and/or implementing adjacency formation, adjacency maintenance and/or
flow-control, BGP simply relies on TCP as the underlying flow-control, BGP simply relies on TCP as the underlying
transport. This fulfills REQ2 and REQ3. transport. This fulfills REQ2 and REQ3.
o BGP information flooding overhead is less when compared to link- o BGP information flooding overhead is less when compared to link-
state IGPs. Since every BGP router calculates and propagates only state IGPs. Since every BGP router calculates and propagates only
the best-path selected, a network failure is masked as soon as the the best-path selected, a network failure is masked as soon as the
BGP speaker finds an alternate path, which exists when highly BGP speaker finds an alternate path, which exists when highly
symmetric topologies, such as Clos, are coupled with EBGP only symmetric topologies, such as Clos, are coupled with an EBGP only
design. In contrast, the event propagation scope of a link-state design. In contrast, the event propagation scope of a link-state
IGP is an entire area, regardless of the failure type. In this IGP is an entire area, regardless of the failure type. In this
way, BGP better meets REQ3 and REQ4. It is also worth mentioning way, BGP better meets REQ3 and REQ4. It is also worth mentioning
that all widely deployed link-state IGPs feature periodic that all widely deployed link-state IGPs feature periodic
refreshes of routing information, even if this rarely causes refreshes of routing information while BGP does not expire routing
impact to modern router control planes, while BGP does not expire state, although this rarely impacts modern router control planes.
routing state.
o BGP supports third-party (recursively resolved) next-hops. This o BGP supports third-party (recursively resolved) next-hops. This
allows for manipulating multipath to be non-ECMP based or allows for manipulating multipath to be non-ECMP based or
forwarding based on application-defined paths, through forwarding based on application-defined paths, through
establishment of a peering session with an application establishment of a peering session with an application
"controller" which can inject routing information into the system, "controller" which can inject routing information into the system,
satisfying REQ5. OSPF provides similar functionality using satisfying REQ5. OSPF provides similar functionality using
concepts such as "Forwarding Address", but with more difficulty in concepts such as "Forwarding Address", but with more difficulty in
implementation and far less control of information propagation implementation and far less control of information propagation
scope. scope.
o Using a well-defined ASN allocation scheme and standard AS_PATH o Using a well-defined ASN allocation scheme and standard AS_PATH
loop detection, "BGP path hunting" (see [JAKMA2008]) can be loop detection, "BGP path hunting" (see [JAKMA2008]) can be
controlled and complex unwanted paths will be ignored. See controlled and complex unwanted paths will be ignored. See
Section 5.2 for an example of a working ASN allocation scheme. In Section 5.2 for an example of a working ASN allocation scheme. In
a link-state IGP accomplishing the same goal would require multi- a link-state IGP accomplishing the same goal would require multi-
(instance/topology/processes) support, typically not available in (instance/topology/process) support, typically not available in
all DC devices and quite complex to configure and troubleshoot. all DC devices and quite complex to configure and troubleshoot.
Using a traditional single flooding domain, which most DC designs Using a traditional single flooding domain, which most DC designs
utilize, under certain failure conditions may pick up unwanted utilize, under certain failure conditions may pick up unwanted
lengthy paths, e.g. traversing multiple Tier-2 devices. lengthy paths, e.g., traversing multiple Tier-2 devices.
o EBGP configuration that is implemented with minimal routing policy o EBGP configuration that is implemented with minimal routing policy
is easier to troubleshoot for network reachability issues. In is easier to troubleshoot for network reachability issues. In
most implementations, it is straightforward to view contents of most implementations, it is straightforward to view contents of
BGP Loc-RIB and compare it to the router's RIB. Also, in most BGP Loc-RIB and compare it to the router's RIB. Also, in most
implementations an operator can view every BGP neighbors Adj-RIB- implementations an operator can view every BGP neighbors Adj-RIB-
In and Adj-RIB-Out structures and therefore incoming and outgoing In and Adj-RIB-Out structures and therefore incoming and outgoing
NLRI information can be easily correlated on both sides of a BGP NLRI information can be easily correlated on both sides of a BGP
session. Thus, BGP satisfies REQ3. session. Thus, BGP satisfies REQ3.
skipping to change at page 15, line 24 skipping to change at page 15, line 22
5.2.1. EBGP Configuration Guidelines and Example ASN Scheme 5.2.1. EBGP Configuration Guidelines and Example ASN Scheme
The diagram below illustrates an example ASN allocation scheme. The The diagram below illustrates an example ASN allocation scheme. The
following is a list of guidelines that can be used: following is a list of guidelines that can be used:
o EBGP single-hop sessions are established over direct point-to- o EBGP single-hop sessions are established over direct point-to-
point links interconnecting the network nodes, no multi-hop or point links interconnecting the network nodes, no multi-hop or
loopback sessions are used even in the case of multiple links loopback sessions are used even in the case of multiple links
between the same pair of nodes. between the same pair of nodes.
o Private Use ASNs from the range 64512-65534 are used so as to o Private Use ASNs from the range 64512-65534 are used to avoid ASN
avoid ASN conflicts. conflicts.
o A single ASN is allocated to all of the Clos topology's Tier-1 o A single ASN is allocated to all of the Clos topology's Tier-1
devices. devices.
o A unique ASN is allocated to each set of Tier-2 devices in the o A unique ASN is allocated to each set of Tier-2 devices in the
same cluster. same cluster.
o A unique ASN is allocated to every Tier-3 device (e.g. ToR) in o A unique ASN is allocated to every Tier-3 device (e.g., ToR) in
this topology. this topology.
ASN 65534 ASN 65534
+---------+ +---------+
| +-----+ | | +-----+ |
| | | | | | | |
+-|-| |-|-+ +-|-| |-|-+
| | +-----+ | | | | +-----+ | |
ASN 646XX | | | | ASN 646XX ASN 646XX | | | | ASN 646XX
+---------+ | | | | +---------+ +---------+ | | | | +---------+
skipping to change at page 17, line 9 skipping to change at page 17, line 9
Introducing this feature does not make it likely for routing loops in Introducing this feature does not make it likely for routing loops in
the design since the AS_PATH is being added to by routers at each of the design since the AS_PATH is being added to by routers at each of
the topology tiers and AS_PATH length is an early tie breaker in the the topology tiers and AS_PATH length is an early tie breaker in the
BGP path selection process. Further loop protection is still in BGP path selection process. Further loop protection is still in
place at the Tier-1 device, which will not accept routes with a path place at the Tier-1 device, which will not accept routes with a path
including its own ASN and Tier-2 devices do not have direct including its own ASN and Tier-2 devices do not have direct
connectivity with each other. connectivity with each other.
Another solution to this problem would be using Four-Octet ASNs Another solution to this problem would be using Four-Octet ASNs
([RFC6793]), where there are additional Private Use ASNs available, ([RFC6793]), where there are additional Private Use ASNs available,
see [IANA.AS]. Use of Four-Octet ASNs put additional protocol see [IANA.AS]. Use of Four-Octet ASNs puts additional protocol
complexity in the BGP implementation so should be considered against complexity in the BGP implementation and should be balanced against
the complexity of re-use when considering REQ3 and REQ4. Perhaps the complexity of re-use when considering REQ3 and REQ4. Perhaps
more importantly, they are not yet supported by all BGP more importantly, they are not yet supported by all BGP
implementations, which may limit vendor selection of DC equipment. implementations, which may limit vendor selection of DC equipment.
When supported, ensure that implementations in use are able to remove When supported, ensure that deployed implementations are able to
the Private Use ASNs if required for external connectivity remove the Private Use ASNs when external connectivity
(Section 5.2.4). (Section 5.2.4) to these ASNs is required.
5.2.3. Prefix Advertisement 5.2.3. Prefix Advertisement
A Clos topology features a large number of point-to-point links and A Clos topology features a large number of point-to-point links and
associated prefixes. Advertising all of these routes into BGP may associated prefixes. Advertising all of these routes into BGP may
create FIB overload conditions in the network devices. Advertising create FIB overload in the network devices. Advertising these links
these links also puts additional path computation stress on the BGP also puts additional path computation stress on the BGP control plane
control plane for little benefit. There are two possible solutions: for little benefit. There are two possible solutions:
o Do not advertise any of the point-to-point links into BGP. Since o Do not advertise any of the point-to-point links into BGP. Since
the EBGP-based design changes the next-hop address at every the EBGP-based design changes the next-hop address at every
device, distant networks will automatically be reachable via the device, distant networks will automatically be reachable via the
advertising EBGP peer and do not require reachability to these advertising EBGP peer and do not require reachability to these
prefixes. However, this may complicate operations or monitoring: prefixes. However, this may complicate operations or monitoring:
e.g. using the popular "traceroute" tool will display IP addresses e.g., using the popular "traceroute" tool will display IP
that are not reachable. addresses that are not reachable.
o Advertise point-to-point links, but summarize them on every o Advertise point-to-point links, but summarize them on every
device. This requires an address allocation scheme such as device. This requires an address allocation scheme such as
allocating a consecutive block of IP addresses per Tier-1 and allocating a consecutive block of IP addresses per Tier-1 and
Tier-2 device to be used for point-to-point interface addressing Tier-2 device to be used for point-to-point interface addressing
to the lower layers (Tier-2 uplinks will be numbered out of Tier-1 to the lower layers (Tier-2 uplinks will be allocated from Tier-1
addressing and so forth). address blocks and so forth).
Server subnets on Tier-3 devices must be announced into BGP without Server subnets on Tier-3 devices must be announced into BGP without
using route summarization on Tier-2 and Tier-1 devices. Summarizing using route summarization on Tier-2 and Tier-1 devices. Summarizing
subnets in a Clos topology results in route black-holing under a subnets in a Clos topology results in route black-holing under a
single link failure (e.g. between Tier-2 and Tier-3 devices) and single link failure (e.g., between Tier-2 and Tier-3 devices) and
hence must be avoided. The use of peer links within the same tier to hence must be avoided. The use of peer links within the same tier to
resolve the black-holing problem by providing "bypass paths" is resolve the black-holing problem by providing "bypass paths" is
undesirable due to O(N^2) complexity of the peering mesh and waste of undesirable due to O(N^2) complexity of the peering mesh and waste of
ports on the devices. An alternative to the full-mesh of peer-links ports on the devices. An alternative to the full-mesh of peer-links
would be using a simpler bypass topology, e.g. a "ring" as described would be using a simpler bypass topology, e.g., a "ring" as described
in [FB4POST], but such a topology adds extra hops and has very in [FB4POST], but such a topology adds extra hops and has very
limited bisection bandwidth, in addition requiring special tweaks to limited bisectional bandwidth. Additionally requiring special tweaks
make BGP routing work - such as possibly splitting every device into to make BGP routing work - such as possibly splitting every device
an ASN on its own. Later in this document, Section 8.2 introduces a into an ASN on its own. Later in this document, Section 8.2
less intrusive method for performing a limited form route introduces a less intrusive method for performing a limited form of
summarization in Clos networks and discusses it's associated trade- route summarization in Clos networks and discusses its associated
offs. trade-offs.
5.2.4. External Connectivity 5.2.4. External Connectivity
A dedicated cluster (or clusters) in the Clos topology could be used A dedicated cluster (or clusters) in the Clos topology could be used
for the purpose of connecting to the Wide Area Network (WAN) edge for the purpose of connecting to the Wide Area Network (WAN) edge
devices, or WAN Routers. Tier-3 devices in such cluster would be devices, or WAN Routers. Tier-3 devices in such cluster would be
replaced with WAN routers, and EBGP peering would be used again, replaced with WAN routers, and EBGP peering would be used again,
though WAN routers are likely to belong to a public ASN if Internet though WAN routers are likely to belong to a public ASN if Internet
connectivity is required in the design. The Tier-2 devices in such a connectivity is required in the design. The Tier-2 devices in such a
dedicated cluster will be referred to as "Border Routers" in this dedicated cluster will be referred to as "Border Routers" in this
document. These devices have to perform a few special functions: document. These devices have to perform a few special functions:
o Hide network topology information when advertising paths to WAN o Hide network topology information when advertising paths to WAN
routers, i.e. remove Private Use ASNs [RFC6996] from the AS_PATH routers, i.e., remove Private Use ASNs [RFC6996] from the AS_PATH
attribute. This is typically done to avoid ASN number collisions attribute. This is typically done to avoid ASN number collisions
between different data centers and also to provide a uniform between different data centers and also to provide a uniform
AS_PATH length to the WAN for purposes of WAN ECMP to Anycast AS_PATH length to the WAN for purposes of WAN ECMP to Anycast
prefixes originated in the topology. An implementation specific prefixes originated in the topology. An implementation specific
BGP feature typically called "Remove Private AS" is commonly used BGP feature typically called "Remove Private AS" is commonly used
to accomplish this. Depending on implementation, the feature to accomplish this. Depending on implementation, the feature
should strip a contiguous sequence of Private Use ASNs found in should strip a contiguous sequence of Private Use ASNs found in an
AS_PATH attribute prior to advertising the path to a neighbor. AS_PATH attribute prior to advertising the path to a neighbor.
This assumes that all ASNs used for intra data center numbering This assumes that all ASNs used for intra data center numbering
are from the Private Use ranges. The process for stripping the are from the Private Use ranges. The process for stripping the
Private Use ASNs is not currently standardized, see Private Use ASNs is not currently standardized, see
[I-D.mitchell-grow-remove-private-as]. However most [I-D.mitchell-grow-remove-private-as]. However most
implementations at least follow the logic described in this implementations at least follow the logic described in this
vendor's document [VENDOR-REMOVE-PRIVATE-AS], which is enough for vendor's document [VENDOR-REMOVE-PRIVATE-AS], which is enough for
the design specified. the design specified.
o Originate a default route to the data center devices. This is the o Originate a default route to the data center devices. This is the
only place where default route can be originated, as route only place where a default route can be originated, as route
summarization is risky for the "scale-out" topology. summarization is risky for the unmodified Clos topology.
Alternatively, Border Routers may simply relay the default route Alternatively, Border Routers may simply relay the default route
learned from WAN routers. Advertising the default route from learned from WAN routers. Advertising the default route from
Border Routers requires that all Border Routers be fully connected Border Routers requires that all Border Routers be fully connected
to the WAN Routers upstream, to provide resistance to a single- to the WAN Routers upstream, to provide resistance to a single-
link failure causing the black-holing of traffic. To prevent link failure causing the black-holing of traffic. To prevent
black-holing in the situation when all of the EBGP sessions to the black-holing in the situation when all of the EBGP sessions to the
WAN routers fail simultaneously on a given device it is more WAN routers fail simultaneously on a given device, it is more
desirable to take the "relaying" approach rather than introducing desirable to readvertise the default route rather than originating
the default route via complicated conditional route origination the default route via complicated conditional route origination
schemes provided by some implementations [CONDITIONALROUTE]. schemes provided by some implementations [CONDITIONALROUTE].
5.2.5. Route Summarization at the Edge 5.2.5. Route Summarization at the Edge
It is often desirable to summarize network reachability information It is often desirable to summarize network reachability information
prior to advertising it to the WAN network due to high amount of IP prior to advertising it to the WAN network due to high amount of IP
prefixes originated from within the data center in a fully routed prefixes originated from within the data center in a fully routed
network design. For example, a network with 2000 Tier-3 devices will network design. For example, a network with 2000 Tier-3 devices will
have at least 2000 servers subnets advertised into BGP, along with have at least 2000 servers subnets advertised into BGP, along with
the infrastructure or other prefixes. However, as discussed before, the infrastructure prefixes. However, as discussed before in
the proposed network design does not allow for route summarization Section 5.2.3, the proposed network design does not allow for route
due to the lack of peer links inside every tier. summarization due to the lack of peer links inside every tier.
However, it is possible to lift this restriction for the Border However, it is possible to lift this restriction for the Border
Routers, by devising a different connectivity model for these Routers, by devising a different connectivity model for these
devices. There are two options possible: devices. There are two options possible:
o Interconnect the Border Routers using a full-mesh of physical o Interconnect the Border Routers using a full-mesh of physical
links or using any other "peer-mesh" topology, such as ring or links or using any other "peer-mesh" topology, such as ring or
hub-and-spoke. Configure BGP accordingly on all Border Leafs to hub-and-spoke. Configure BGP accordingly on all Border Leafs to
exchange network reachability information - e.g. by adding a mesh exchange network reachability information, e.g., by adding a mesh
of IBGP sessions. The interconnecting peer links need to be of IBGP sessions. The interconnecting peer links need to be
appropriately sized for traffic that will be present in the case appropriately sized for traffic that will be present in the case
of a device or link failure underneath the Border Routers. of a device or link failure in the mesh connecting the Border
Routers.
o Tier-1 devices may have additional physical links provisioned o Tier-1 devices may have additional physical links provisioned
toward the Border Routers (which are Tier-2 devices from the toward the Border Routers (which are Tier-2 devices from the
perspective of Tier-1). Specifically, if protection from a single perspective of Tier-1). Specifically, if protection from a single
link or node failure is desired, each Tier-1 devices would have to link or node failure is desired, each Tier-1 devices would have to
connect to at least two Border Routers. This puts additional connect to at least two Border Routers. This puts additional
requirements on the port count for Tier-1 devices and Border requirements on the port count for Tier-1 devices and Border
Routers, potentially making it a non-uniform, larger port count, Routers, potentially making it a non-uniform, larger port count,
device compared with the other devices in the Clos. This also device compared with the other devices in the Clos. This also
reduces the number of ports available to "regular" Tier-2 switches reduces the number of ports available to "regular" Tier-2 switches
and hence the number of clusters that could be interconnected via and hence the number of clusters that could be interconnected via
Tier-1 layer. the Tier-1 layer.
If any of the above options are implemented, it is possible to If any of the above options are implemented, it is possible to
perform route summarization at the Border Routers toward the WAN perform route summarization at the Border Routers toward the WAN
network core without risking a routing black-hole condition under a network core without risking a routing black-hole condition under a
single link failure. Both of the options would result in non-uniform single link failure. Both of the options would result in non-uniform
topology as additional links have to be provisioned on some network topology as additional links have to be provisioned on some network
devices. devices.
6. ECMP Considerations 6. ECMP Considerations
This section covers the Equal Cost Multipath (ECMP) functionality for This section covers the Equal Cost Multipath (ECMP) functionality for
Clos topology and discusses a few special requirements. Clos topology and discusses a few special requirements.
6.1. Basic ECMP 6.1. Basic ECMP
ECMP is the fundamental load sharing mechanism used by a Clos ECMP is the fundamental load sharing mechanism used by a Clos
topology. Effectively, every lower-tier device will use all of its topology. Effectively, every lower-tier device will use all of its
directly attached upper-tier devices to load share traffic destined directly attached upper-tier devices to load share traffic destined
to the same IP prefix. Number of ECMP paths between any two Tier-3 to the same IP prefix. The number of ECMP paths between any two
devices in Clos topology equals to the number of the devices in the Tier-3 devices in Clos topology is equal to the number of the devices
middle stage (Tier-1). For example, Figure 5 illustrates the in the middle stage (Tier-1). For example, Figure 5 illustrates a
topology where Tier-3 device A has four paths to reach servers X and topology where Tier-3 device A has four paths to reach servers X and
Y, via Tier-2 devices B and C and then Tier-1 devices 1, 2, 3, and 4 Y, via Tier-2 devices B and C and then Tier-1 devices 1, 2, 3, and 4
respectively. respectively.
Tier-1 Tier-1
+-----+ +-----+
| DEV | | DEV |
+->| 1 |--+ +->| 1 |--+
| +-----+ | | +-----+ |
Tier-2 | | Tier-2 Tier-2 | | Tier-2
skipping to change at page 20, line 44 skipping to change at page 20, line 44
| DEV | | | Tier-3 +->| DEV |--+ Tier-3 | | | | | DEV | | | Tier-3 +->| DEV |--+ Tier-3 | | | |
| A | | | | 4 | | | | | | A | | | | 4 | | | | |
+-----+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+
| | | | | | | | | | | | | | | |
O O O O <- Servers -> X Y O O O O O O <- Servers -> X Y O O
Figure 5: ECMP fan-out tree from A to X and Y Figure 5: ECMP fan-out tree from A to X and Y
The ECMP requirement implies that the BGP implementation must support The ECMP requirement implies that the BGP implementation must support
multipath fan-out for up to the maximum number of devices directly multipath fan-out for up to the maximum number of devices directly
attached at any point in the topology in upstream or downstream attached at any point in the topology in the upstream or downstream
direction. Normally, this number does not exceed half of the ports direction. Normally, this number does not exceed half of the ports
found on a device in the topology. For example, an ECMP fan-out of found on a device in the topology. For example, an ECMP fan-out of
32 would be required when building a Clos network using 64-port 32 would be required when building a Clos network using 64-port
devices. The Border Routers may need to have wider fan-out to be devices. The Border Routers may need to have wider fan-out to be
able to connect to multitude of Tier-1 devices if route summarization able to connect to a multitude of Tier-1 devices if route
at Border Router level is implemented as described in Section 5.2.5. summarization at Border Router level is implemented as described in
If a device's hardware does not support wider ECMP, logical link- Section 5.2.5. If a device's hardware does not support wider ECMP,
grouping (link-aggregation at layer 2) could be used to provide logical link-grouping (link-aggregation at layer 2) could be used to
"hierarchical" ECMP (Layer 3 ECMP followed by Layer 2 ECMP) to provide "hierarchical" ECMP (Layer 3 ECMP coupled with Layer 2 ECMP)
compensate for fan-out limitations. Such approach, however, to compensate for fan-out limitations. However, this approach
increases the risk of flow polarization, as less entropy will be increases the risk of flow polarization, as less entropy will be
available to the second stage of ECMP. available at the second stage of ECMP.
Most BGP implementations declare paths to be equal from an ECMP Most BGP implementations declare paths to be equal from an ECMP
perspective if they match up to and including step (e) in perspective if they match up to and including step (e) in
Section 9.1.2.2 of [RFC4271]. In the proposed network design there Section 9.1.2.2 of [RFC4271]. In the proposed network design there
is no underlying IGP, so all IGP costs are assumed to be zero or is no underlying IGP, so all IGP costs are assumed to be zero or
otherwise the same value across all paths and policies may be applied otherwise the same value across all paths and policies may be applied
as necessary to equalize BGP attributes that vary in vendor defaults, as necessary to equalize BGP attributes that vary in vendor defaults,
such as MED and origin code. For historical reasons it is also such as MED and origin code. For historical reasons it is also
useful to not use 0 as the equalized MED value; this and some other useful to not use 0 as the equalized MED value; this and some other
useful BGP information is available in [RFC4277] . Routing loops are useful BGP information is available in [RFC4277] . Routing loops are
skipping to change at page 21, line 30 skipping to change at page 21, line 30
which don't allow their own ASN in the path and have the same ASN are which don't allow their own ASN in the path and have the same ASN are
also not possible. also not possible.
6.2. BGP ECMP over Multiple ASNs 6.2. BGP ECMP over Multiple ASNs
For application load balancing purposes it is desirable to have the For application load balancing purposes it is desirable to have the
same prefix advertised from multiple Tier-3 devices. From the same prefix advertised from multiple Tier-3 devices. From the
perspective of other devices, such a prefix would have BGP paths with perspective of other devices, such a prefix would have BGP paths with
different AS_PATH attribute values, while having the same AS_PATH different AS_PATH attribute values, while having the same AS_PATH
attribute lengths. Therefore, BGP implementations must support load attribute lengths. Therefore, BGP implementations must support load
sharing over above-mentioned paths. This feature is sometimes known sharing over the above-mentioned paths. This feature is sometimes
as "multipath relax" or "multipath multiple-as" and effectively known as "multipath relax" or "multipath multiple-as" and effectively
allows for ECMP to be done across different neighboring ASNs if all allows for ECMP to be done across different neighboring ASNs if all
other attributes are equal as already described in the previous other attributes are equal as already described in the previous
section. section.
6.3. Weighted ECMP 6.3. Weighted ECMP
It may be desirable for the network devices to implement "weighted" It may be desirable for the network devices to implement "weighted"
ECMP, to be able to send more traffic over some paths in ECMP fan- ECMP, to be able to send more traffic over some paths in ECMP fan-
out. This could be helpful to compensate for failures in the network out. This could be helpful to compensate for failures in the network
and send more traffic over paths that have more capacity. The and send more traffic over paths that have more capacity. The
skipping to change at page 22, line 9 skipping to change at page 22, line 9
remote BGP speaker (central agent) over a multihop session as remote BGP speaker (central agent) over a multihop session as
described further in Section 8.1. If support in implementations is described further in Section 8.1. If support in implementations is
available, weight-distribution for multiple BGP paths could be available, weight-distribution for multiple BGP paths could be
signaled using the technique described in signaled using the technique described in
[I-D.ietf-idr-link-bandwidth]. [I-D.ietf-idr-link-bandwidth].
6.4. Consistent Hashing 6.4. Consistent Hashing
It is often desirable to have the hashing function used for ECMP to It is often desirable to have the hashing function used for ECMP to
be consistent (see [CONS-HASH]), to minimize the impact on flow to be consistent (see [CONS-HASH]), to minimize the impact on flow to
next-hop affinity changes when a next-hop is added or removed to ECMP next-hop affinity changes when a next-hop is added or removed to an
group. This could be used if the network device is used as a load ECMP group. This could be used if the network device is used as a
balancer, mapping flows toward multiple destinations - in this case, load balancer, mapping flows toward multiple destinations - in this
losing or adding a destination will not have detrimental effect of case, losing or adding a destination will not have a detrimental
currently established flows. One particular recommendation on effect on currently established flows. One particular recommendation
implementing consistent hashing is provided in [RFC2992], though on implementing consistent hashing is provided in [RFC2992], though
other implementations are possible. This functionality could be other implementations are possible. This functionality could be
naturally combined with weighted ECMP, with the impact of the next- naturally combined with weighted ECMP, with the impact of the next-
hop changes being proportional to the weight of the given next-hop. hop changes being proportional to the weight of the given next-hop.
The downside of consistent hashing is increased load on hardware The downside of consistent hashing is increased load on hardware
resource utilization, as typically more space is required to resource utilization, as typically more resources (e.g., TCAM space)
implement a consistent-hashing region. are required to implement a consistent-hashing function.
7. Routing Convergence Properties 7. Routing Convergence Properties
This section reviews routing convergence properties in the proposed This section reviews routing convergence properties in the proposed
design. A case is made that sub-second convergence is achievable if design. A case is made that sub-second convergence is achievable if
the implementation supports fast EBGP peering session deactivation the implementation supports fast EBGP peering session deactivation
and timely RIB and FIB update upon failure of the associated link. and timely RIB and FIB update upon failure of the associated link.
7.1. Fault Detection Timing 7.1. Fault Detection Timing
BGP typically relies on an IGP to route around link/node failures BGP typically relies on an IGP to route around link/node failures
inside an AS, and implements either a polling based or an event- inside an AS, and implements either a polling based or an event-
driven mechanism to obtain updates on IGP state changes. The driven mechanism to obtain updates on IGP state changes. The
proposed routing design does not use an IGP, so the remaining proposed routing design does not use an IGP, so the remaining
mechanisms that could be used for fault detection are BGP keep-alive mechanisms that could be used for fault detection are BGP keep-alive
process (or any other type of keep-alive mechanism) and link-failure time-out (or any other type of keep-alive mechanism) and link-failure
triggers. triggers.
Relying solely on BGP keep-alive packets may result in high Relying solely on BGP keep-alive packets may result in high
convergence delays, in the order of multiple seconds (on many BGP convergence delays, on the order of multiple seconds (on many BGP
implementations the minimum configurable BGP hold timer value is implementations the minimum configurable BGP hold timer value is
three seconds). However, many BGP implementations can shut down three seconds). However, many BGP implementations can shut down
local EBGP peering sessions in response to the "link down" event for local EBGP peering sessions in response to the "link down" event for
the outgoing interface used for BGP peering. This feature is the outgoing interface used for BGP peering. This feature is
sometimes called as "fast fallover". Since links in modern data sometimes called "fast fallover". Since links in modern data centers
centers are predominantly point-to-point fiber connections, a are predominantly point-to-point fiber connections, a physical
physical interface failure is often detected in milliseconds and interface failure is often detected in milliseconds and subsequently
subsequently triggers a BGP re-convergence. triggers a BGP re-convergence.
Ethernet links may support failure signaling or detection standards Ethernet links may support failure signaling or detection standards
such as Connectivity Fault Management (CFM) as described in such as Connectivity Fault Management (CFM) as described in
[IEEE8021Q], which may make failure detection more robust. [IEEE8021Q], which may make failure detection more robust.
Alternatively, some platforms may support Bidirectional Forwarding Alternatively, some platforms may support Bidirectional Forwarding
Detection (BFD) [RFC5880] to allow for sub-second failure detection Detection (BFD) [RFC5880] to allow for sub-second failure detection
and fault signaling to the BGP process. However, use of either of and fault signaling to the BGP process. However, the use of either
these presents additional requirements to vendor software and of these presents additional requirements to vendor software and
possibly hardware, and may contradict REQ1. Until recently with possibly hardware, and may contradict REQ1. Until recently with
[RFC7130], BFD also did not allow detection of a single member link [RFC7130], BFD also did not allow detection of a single member link
failure on a LAG, which would have limited its usefulness in some failure on a LAG, which would have limited its usefulness in some
designs. designs.
7.2. Event Propagation Timing 7.2. Event Propagation Timing
In the proposed design the impact of BGP Minimum Route Advertisement In the proposed design the impact of the BGP Minimum Route
Interval (MRAI) timer (See section 9.2.1.1 of [RFC4271]) should be Advertisement Interval (MRAI) timer (See section 9.2.1.1 of
considered. Per the standard it is required for BGP implementations [RFC4271]) should be considered. Per the standard it is required for
to space out consecutive BGP UPDATE messages by at least MRAI BGP implementations to space out consecutive BGP UPDATE messages by
seconds, which is often a configurable value. The initial BGP UPDATE at least MRAI seconds, which is often a configurable value. The
messages after an event carrying withdrawn routes are commonly not initial BGP UPDATE messages after an event carrying withdrawn routes
affected by this timer. The MRAI timer may present significant are commonly not affected by this timer. The MRAI timer may present
convergence delays when a BGP speaker "waits" for the new path to be significant convergence delays when a BGP speaker "waits" for the new
learned from its peers and has no local backup path information. path to be learned from its peers and has no local backup path
information.
In a Clos topology each EBGP speaker typically has either one path In a Clos topology each EBGP speaker typically has either one path
(Tier-2 devices don't accept paths from other Tier-2 in the same (Tier-2 devices don't accept paths from other Tier-2 in the same
cluster due to same ASN) or N paths for the same prefix, where N is a cluster due to same ASN) or N paths for the same prefix, where N is a
significantly large number, e.g. N=32 (the ECMP fan-out to the next significantly large number, e.g., N=32 (the ECMP fan-out to the next
Tier). Therefore, if a link fails to another device from which a Tier). Therefore, if a link fails to another device from which a
path is received there is either no backup path at all (e.g. from path is received there is either no backup path at all (e.g., from
perspective of a Tier-2 switch losing link to a Tier-3 device), or perspective of a Tier-2 switch losing the link to a Tier-3 device),
the backup is readily available in BGP Loc-RIB (e.g. from perspective or the backup is readily available in BGP Loc-RIB (e.g., from the
of a Tier-2 device losing link to a Tier-1 switch). In the former perspective of a Tier-2 device losing the link to a Tier-1 switch).
case, the BGP withdrawal announcement will propagate un-delayed and In the former case, the BGP withdrawal announcement will propagate
trigger re-convergence on affected devices. In the latter case, the without delay and trigger re-convergence on affected devices. In the
best-path will be re-evaluated and the local ECMP group corresponding latter case, the best-path will be re-evaluated and the local ECMP
to the new next-hop set changed. If the BGP path was the best-path group corresponding to the new next-hop set changed. If the BGP path
selected previously, an "implicit withdraw" will be sent via a BGP was the best-path selected previously, an "implicit withdraw" will be
UPDATE message as described as Option b in Section 3.1 of [RFC4271] sent via a BGP UPDATE message as described as Option b in Section 3.1
due to the BGP AS_PATH attribute changing. of [RFC4271] due to the BGP AS_PATH attribute changing.
7.3. Impact of Clos Topology Fan-outs 7.3. Impact of Clos Topology Fan-outs
Clos topology has large fan-outs, which may impact the "Up->Down" Clos topology has large fan-outs, which may impact the "Up->Down"
convergence in some cases, as described in this section. In a convergence in some cases, as described in this section. In a
situation when a link between Tier-3 and Tier-2 device fails, the situation when a link between Tier-3 and Tier-2 device fails, the
Tier-2 device will send BGP UPDATE messages to all upstream Tier-1 Tier-2 device will send BGP UPDATE messages to all upstream Tier-1
devices, withdrawing the affected prefixes. The Tier-1 devices, in devices, withdrawing the affected prefixes. The Tier-1 devices, in
turn, will relay those messages to all downstream Tier-2 devices turn, will relay these messages to all downstream Tier-2 devices
(except for the originator). Tier-2 devices other than the one (except for the originator). Tier-2 devices other than the one
originating the UPDATE should then wait for ALL upstream Tier-1 originating the UPDATE should then wait for ALL upstream Tier-1
devices to send an UPDATE message before removing the affected devices to send an UPDATE message before removing the affected
prefixes and sending corresponding UPDATE downstream to connected prefixes and sending corresponding UPDATE downstream to connected
Tier-3 devices. If the original Tier-2 device or the relaying Tier-1 Tier-3 devices. If the original Tier-2 device or the relaying Tier-1
devices introduce some delay into their UPDATE message announcements, devices introduce some delay into their UPDATE message announcements,
the result could be UPDATE message "dispersion", that could be as the result could be UPDATE message "dispersion", that could be as
long as multiple seconds. In order to avoid such a behavior, BGP long as multiple seconds. In order to avoid such a behavior, BGP
implementations must support "update groups". The "update group" is implementations must support "update groups". The "update group" is
defined as a collection of neighbors sharing the same outbound policy defined as a collection of neighbors sharing the same outbound policy
- the local speaker will send BGP updates to the members of the group - the local speaker will send BGP updates to the members of the group
synchronously. synchronously.
The impact of such "dispersion" grows with the size of topology fan- The impact of such "dispersion" grows with the size of topology fan-
out and could also grow under network convergence churn. Some out and could also grow under network convergence churn. Some
operators may be tempted to introduce "route flap dampening" type operators may be tempted to introduce "route flap dampening" type
features that vendors include to reduce the control plane impact of features that vendors include to reduce the control plane impact of
rapidly flapping prefixes. However, due to issues described with rapidly flapping prefixes. However, due to issues described with
false positives in these implementations especially under such false positives in these implementations especially under such
"dispersion" events, it is not recommended to turn this feature on in "dispersion" events, it is not recommended to enable this feature in
this design. More background and issues with "route flap dampening" this design. More background and issues with "route flap dampening"
and possible implementation changes that could affect this are well and possible implementation changes that could affect this are well
described in [RFC7196]. described in [RFC7196].
7.4. Failure Impact Scope 7.4. Failure Impact Scope
A network is declared to converge in response to a failure once all A network is declared to converge in response to a failure once all
devices within the failure impact scope are notified of the event and devices within the failure impact scope are notified of the event and
have re-calculated their RIB's and consequently updated their FIB's. have re-calculated their RIBs and consequently updated their FIBs.
Larger failure impact scope typically means slower convergence since Larger failure impact scope typically means slower convergence since
more devices have to be notified, and additionally results in a less more devices have to be notified, and results in a less stable
stable network. In this section we describe BGP's advantages over network. In this section we describe BGP's advantages over link-
link-state routing protocols in reducing failure impact scope for a state routing protocols in reducing failure impact scope for a Clos
Clos topology. topology.
BGP behaves like a distance-vector protocol in the sense that only BGP behaves like a distance-vector protocol in the sense that only
the best path from the point of view of the local router is sent to the best path from the point of view of the local router is sent to
neighbors. As such, some failures are masked if the local node can neighbors. As such, some failures are masked if the local node can
immediately find a backup path and does not have to send any updates immediately find a backup path and does not have to send any updates
further. Notice that in the worst case ALL devices in a data center further. Notice that in the worst case, all devices in a data center
topology have to either withdraw a prefix completely or update the topology have to either withdraw a prefix completely or update the
ECMP groups in the FIB. However, many failures will not result in ECMP groups in their FIBs. However, many failures will not result in
such a wide impact. There are two main failure types where impact such a wide impact. There are two main failure types where impact
scope is reduced: scope is reduced:
o Failure of a link between Tier-2 and Tier-1 devices: In this case, o Failure of a link between Tier-2 and Tier-1 devices: In this case,
a Tier-2 device will update the affected ECMP groups, removing the a Tier-2 device will update the affected ECMP groups, removing the
failed link. There is no need to send new information to failed link. There is no need to send new information to
downstream Tier-3 devices, unless the path was selected as best by downstream Tier-3 devices, unless the path was selected as best by
the BGP process, in which case only an "implicit withdraw" needs the BGP process, in which case only an "implicit withdraw" needs
to be sent, which should not affect forwarding. The affected to be sent, which should not affect forwarding. The affected
Tier-1 device will lose the only path available to reach a Tier-1 device will lose the only path available to reach a
particular cluster and will have to withdraw the associated particular cluster and will have to withdraw the associated
prefixes. Such prefix withdrawal process will only affect Tier-2 prefixes. Such prefix withdrawal process will only affect Tier-2
devices directly connected to the affected Tier-1 device. The devices directly connected to the affected Tier-1 device. The
Tier-2 devices receiving the BGP UPDATE messages withdrawing Tier-2 devices receiving the BGP UPDATE messages withdrawing
prefixes will simply have to update their ECMP groups. The Tier-3 prefixes will simply have to update their ECMP groups. The Tier-3
devices are not involved in the re-convergence process. devices are not involved in the re-convergence process.
o Failure of a Tier-1 device: In this case, all Tier-2 devices o Failure of a Tier-1 device: In this case, all Tier-2 devices
directly attached to the failed node will have to update their directly attached to the failed node will have to update their
ECMP groups for all IP prefixes from non-local cluster. The ECMP groups for all IP prefixes from a non-local cluster. The
Tier-3 devices are once again not involved in the re-convergence Tier-3 devices are once again not involved in the re-convergence
process, but may receive "implicit withdraws" as described above. process, but may receive "implicit withdraws" as described above.
Even though in case of such failures multiple IP prefixes will have Even in the case of such failures where multiple IP prefixes will
to be reprogrammed in the FIB, it is worth noting that ALL of these have to be reprogrammed in the FIB, it is worth noting that all of
prefixes share a single ECMP group on Tier-2 device. Therefore, in these prefixes share a single ECMP group on Tier-2 device.
the case of implementations with a hierarchical FIB, only a single Therefore, in the case of implementations with a hierarchical FIB,
change has to be made to the FIB. Hierarchical FIB here means FIB only a single change has to be made to the FIB. Hierarchical FIB
structure where the next-hop forwarding information is stored here means FIB structure where the next-hop forwarding information is
separately from the prefix lookup table, and the latter only stores stored separately from the prefix lookup table, and the latter only
pointers to the respective forwarding information. stores pointers to the respective forwarding information. See
[I-D.ietf-rtgwg-bgp-pic] for discussion of FIB hierarchies and fast
convergence.
Even though BGP offers reduced failure scope for some cases, further Even though BGP offers reduced failure scope for some cases, further
reduction of the fault domain using summarization is not always reduction of the fault domain using summarization is not always
possible with the proposed design, since using this technique may possible with the proposed design, since using this technique may
create routing black-holes as mentioned previously. Therefore, the create routing black-holes as mentioned previously. Therefore, the
worst control plane failure impact scope is the network as a whole, worst control plane failure impact scope is the network as a whole,
for instance in a case of a link failure between Tier-2 and Tier-3 for instance in the case of a link failure between Tier-2 and Tier-3
devices. The amount of impacted prefixes in this case would be much devices. The amount of impacted prefixes in this case would be much
less than in the case of a failure in the upper layers of a Clos less than in the case of a failure in the upper layers of a Clos
network topology. The property of having such large failure scope is network topology. The property of having such large failure scope is
not a result of choosing EBGP in the design but rather a result of not a result of choosing EBGP in the design but rather a result of
using the "scale-out" Clos topology. using the Clos topology.
7.5. Routing Micro-Loops 7.5. Routing Micro-Loops
When a downstream device, e.g. Tier-2 device, loses all paths for a When a downstream device, e.g., Tier-2 device, loses all paths for a
prefix, it normally has the default route pointing toward the prefix, it normally has the default route pointing toward the
upstream device, in this case the Tier-1 device. As a result, it is upstream device, in this case the Tier-1 device. As a result, it is
possible to get in the situation when Tier-2 switch loses a prefix, possible to get in the situation where a Tier-2 switch loses a
but Tier-1 switch still has the path pointing to the Tier-2 device, prefix, but a Tier-1 switch still has the path pointing to the Tier-2
which results in transient micro-loop, since Tier-1 switch will keep device, which results in a transient micro-loop, since the Tier-1
passing packets to the affected prefix back to Tier-2 device, and switch will keep passing packets to the affected prefix back to the
Tier-2 will bounce it back again using the default route. This Tier-2 device, and the Tier-2 will bounce them back again using the
micro-loop will last for the duration of time it takes the upstream default route. This micro-loop will last for the duration of time it
device to fully update its forwarding tables. takes the upstream device to fully update its forwarding tables.
To minimize impact of the micro-loops, Tier-2 and Tier-1 switches can To minimize impact of such micro-loops, Tier-2 and Tier-1 switches
be configured with static "discard" or "null" routes that will be can be configured with static "discard" or "null" routes that will be
more specific than the default route for prefixes missing during more specific than the default route for prefixes missing during
network convergence. For Tier-2 switches, the discard route should network convergence. For Tier-2 switches, the discard route should
be a summary route, covering all server subnets of the underlying be a summary route, covering all server subnets of the underlying
Tier-3 devices. For Tier-1 devices, the discard route should be a Tier-3 devices. For Tier-1 devices, the discard route should be a
summary covering the server IP address subnets allocated for the summary covering the server IP address subnets allocated for the
whole data center. Those discard routes will only take precedence whole data center. Those discard routes will only take precedence
for the duration of network convergence, until the device learns a for the duration of network convergence, until the device learns a
more specific prefix via a new path. more specific prefix via a new path.
8. Additional Options for Design 8. Additional Options for Design
8.1. Third-party Route Injection 8.1. Third-party Route Injection
BGP allows for a "third-party", i.e. directly attached, BGP speaker BGP allows for a "third-party", i.e., directly attached, BGP speaker
to inject routes anywhere in the network topology, meeting REQ5. to inject routes anywhere in the network topology, meeting REQ5.
This can be achieved by peering via a multihop BGP session with some This can be achieved by peering via a multihop BGP session with some
or even all devices in the topology. Furthermore, BGP diverse path or even all devices in the topology. Furthermore, BGP diverse path
distribution [RFC6774] could be used to inject multiple BGP next hops distribution [RFC6774] could be used to inject multiple BGP next hops
for the same prefix to facilitate load balancing, or using the BGP for the same prefix to facilitate load balancing, or using the BGP
ADD-PATH capability [I-D.ietf-idr-add-paths] if supported by the ADD-PATH capability [I-D.ietf-idr-add-paths] if supported by the
implementation. Unfortunately, in many implementations ADD-PATH has implementation. Unfortunately, in many implementations ADD-PATH has
been found to only support IBGP properly due to the use cases it was been found to only support IBGP properly due to the use cases it was
originally optimized for, which limits the "third-party" peering to originally optimized for, which limits the "third-party" peering to
IBGP only, if the feature is used. IBGP only.
To implement route injection in the proposed design, a third-party To implement route injection in the proposed design, a third-party
BGP speaker may peer with Tier-3 and Tier-1 switches, injecting the BGP speaker may peer with Tier-3 and Tier-1 switches, injecting the
same prefix, but using a special set of BGP next-hops for Tier-1 same prefix, but using a special set of BGP next-hops for Tier-1
devices. Those next-hops are assumed to resolve recursively via BGP, devices. Those next-hops are assumed to resolve recursively via BGP,
and could be, for example, IP addresses on Tier-3 devices. The and could be, for example, IP addresses on Tier-3 devices. The
resulting forwarding table programming could provide desired traffic resulting forwarding table programming could provide desired traffic
proportion distribution among different clusters. proportion distribution among different clusters.
8.2. Route Summarization within Clos Topology 8.2. Route Summarization within Clos Topology
As mentioned previously, route summarization is not possible within As mentioned previously, route summarization is not possible within
the proposed Clos topology since it makes the network susceptible to the proposed Clos topology since it makes the network susceptible to
route black-holing under single link failures. The main problem is route black-holing under single link failures. The main problem is
the limited number of redundant paths between network elements, e.g. the limited number of redundant paths between network elements, e.g.,
there is only a single path between any pair of Tier-1 and Tier-3 there is only a single path between any pair of Tier-1 and Tier-3
devices. However, some operators may find route aggregation devices. However, some operators may find route aggregation
desirable to improve control plane stability. desirable to improve control plane stability.
If planning on using any technique to summarize within the topology If any technique to summarize within the topology is planned,
modeling of the routing behavior and potential for black-holing modeling of the routing behavior and potential for black-holing
should be done not only for single or multiple link failures, but should be done not only for single or multiple link failures, but
also fiber pathway failures or optical domain failures if the also fiber pathway failures or optical domain failures when the
topology extends beyond a physical location. Simple modeling can be topology extends beyond a physical location. Simple modeling can be
done by checking the reachability on devices doing summarization done by checking the reachability on devices doing summarization
under the condition of a link or pathway failure between a set of under the condition of a link or pathway failure between a set of
devices in every tier as well as to the WAN routers if external devices in every tier as well as to the WAN routers when external
connectivity is present. connectivity is present.
Route summarization would be possible with a small modification to Route summarization would be possible with a small modification to
the network topology, though the trade-off would be reduction of the the network topology, though the trade-off would be reduction of the
total size of the network as well as network congestion under total size of the network as well as network congestion under
specific failures. This approach is very similar to the technique specific failures. This approach is very similar to the technique
described above, which allows Border Routers to summarize the entire described above, which allows Border Routers to summarize the entire
data center address space. data center address space.
8.2.1. Collapsing Tier-1 Devices Layer 8.2.1. Collapsing Tier-1 Devices Layer
skipping to change at page 28, line 9 skipping to change at page 28, line 13
Figure 6: 5-Stage Clos topology Figure 6: 5-Stage Clos topology
Having this design in place, Tier-2 devices may be configured to Having this design in place, Tier-2 devices may be configured to
advertise only a default route down to Tier-3 devices. If a link advertise only a default route down to Tier-3 devices. If a link
between Tier-2 and Tier-3 fails, the traffic will be re-routed via between Tier-2 and Tier-3 fails, the traffic will be re-routed via
the second available path known to a Tier-2 switch. It is still not the second available path known to a Tier-2 switch. It is still not
possible to advertise a summary route covering prefixes for a single possible to advertise a summary route covering prefixes for a single
cluster from Tier-2 devices since each of them has only a single path cluster from Tier-2 devices since each of them has only a single path
down to this prefix. It would require dual-homed servers to down to this prefix. It would require dual-homed servers to
accomplish that. Also note that this design is only resilient to accomplish that. Also note that this design is only resilient to
single link failure. It is possible for a double link failure to single link failures. It is possible for a double link failure to
isolate a Tier-2 device from all paths toward a specific Tier-3 isolate a Tier-2 device from all paths toward a specific Tier-3
device, thus causing a routing black-hole. device, thus causing a routing black-hole.
A result of the proposed topology modification would be reduction of A result of the proposed topology modification would be a reduction
Tier-1 devices port capacity. This limits the maximum number of of Tier-1 devices port capacity. This limits the maximum number of
attached Tier-2 devices and therefore will limit the maximum DC attached Tier-2 devices and therefore will limit the maximum DC
network size. A larger network would require different Tier-1 network size. A larger network would require different Tier-1
devices that have higher port density to implement this change. devices that have higher port density to implement this change.
Another problem is traffic re-balancing under link failures. Since Another problem is traffic re-balancing under link failures. Since
three are two paths from Tier-1 to Tier-3, a failure of the link there are two paths from Tier-1 to Tier-3, a failure of the link
between Tier-1 and Tier-2 switch would result in all traffic that was between Tier-1 and Tier-2 switch would result in all traffic that was
taking the failed link to switch to the remaining path. This will taking the failed link to switch to the remaining path. This will
result in doubling of link utilization on the remaining link. result in doubling the link utilization on the remaining link.
8.2.2. Simple Virtual Aggregation 8.2.2. Simple Virtual Aggregation
A completely different approach to route summarization is possible, A completely different approach to route summarization is possible,
provided that the main goal is to reduce the FIB pressure, while provided that the main goal is to reduce the FIB size, while allowing
allowing the control plane to disseminate full routing information. the control plane to disseminate full routing information. Firstly,
Firstly, it could be easily noted that in many cases multiple it could be easily noted that in many cases multiple prefixes, some
prefixes, some of which are less specific, share the same set of the of which are less specific, share the same set of the next-hops (same
next-hops (same ECMP group). For example, looking from the ECMP group). For example, looking from the perspective of a Tier-3
perspective of a Tier-3 devices, all routes learned from upstream devices, all routes learned from upstream Tier-2's, including the
Tier-2's, including the default route, will share the same set of BGP default route, will share the same set of BGP next-hops, provided
next-hops, provided that there are no failures in the network. This that there are no failures in the network. This makes it possible to
makes it possible to use the technique similar to described in use the technique similar to described in [RFC6769] and only install
[RFC6769] and only install the least specific route in the FIB, the least specific route in the FIB, ignoring more specific routes if
ignoring more specific routes if they share the same next-hop set. they share the same next-hop set. For example, under normal network
For example, under normal network conditions, only the default route conditions, only the default route needs to be programmed into the
need to be programmed into FIB. FIB.
Furthermore, if the Tier-2 devices are configured with summary Furthermore, if the Tier-2 devices are configured with summary
prefixes covering all of their attached Tier-3 device's prefixes the prefixes covering all of their attached Tier-3 device's prefixes, the
same logic could be applied in Tier-1 devices as well, and, by same logic could be applied in Tier-1 devices as well, and, by
induction to Tier-2/Tier-3 switches in different clusters. These induction to Tier-2/Tier-3 switches in different clusters. These
summary routes should still allow for more specific prefixes to leak summary routes should still allow for more specific prefixes to leak
to Tier-1 devices, to enable for detection of mismatches in the next- to Tier-1 devices, to enable detection of mismatches in the next-hop
hop sets if a particular link fails, changing the next-hop set for a sets if a particular link fails, changing the next-hop set for a
specific prefix. specific prefix.
Re-stating once again, this technique does not reduce the amount of Re-stating once again, this technique does not reduce the amount of
control plane state (i.e. BGP UPDATEs/BGP LocRIB sizing), but only control plane state (i.e., BGP UPDATEs/BGP LocRIB size), but only
allows for more efficient FIB utilization, by spotting more specific allows for more efficient FIB utilization, by detecting more specific
prefixes that share their next-hops with less specifics. prefixes that share their next-hop set with a subsuming less specific
prefix.
8.3. ICMP Unreachable Message Masquerading 8.3. ICMP Unreachable Message Masquerading
This section discusses some operational aspects of not advertising This section discusses some operational aspects of not advertising
point-to-point link subnets into BGP, as previously outlined as an point-to-point link subnets into BGP, as previously identified as an
option in Section 5.2.3. The operational impact of this decision option in Section 5.2.3. The operational impact of this decision
could be seen when using the well-known "traceroute" tool. could be seen when using the well-known "traceroute" tool.
Specifically, IP addresses displayed by the tool will be the link's Specifically, IP addresses displayed by the tool will be the link's
point-to-point addresses, and hence will be unreachable for point-to-point addresses, and hence will be unreachable for
management connectivity. This makes some troubleshooting more management connectivity. This makes some troubleshooting more
complicated. complicated.
One way to overcome this limitation is by using the DNS subsystem to One way to overcome this limitation is by using the DNS subsystem to
create the "reverse" entries for the IP addresses of the same device create the "reverse" entries for these point-to-point IP addresses
pointing to the same name. The connectivity then can be made by pointing to the same name as the loopback address. The connectivity
resolving this name to the "primary" IP address of the devices, e.g. then can be made by resolving this name to the "primary" IP address
its Loopback interface, which is always advertised into BGP. of the devices, e.g., its Loopback interface, which is always
However, this creates a dependency on the DNS subsystem, which may be advertised into BGP. However, this creates a dependency on the DNS
unavailable during an outage. subsystem, which may be unavailable during an outage.
Another option is to make the network device perform IP address Another option is to make the network device perform IP address
masquerading, that is rewriting the source IP addresses of the masquerading, that is rewriting the source IP addresses of the
appropriate ICMP messages sent off of the device with the "primary" appropriate ICMP messages sent by the device with the "primary" IP
IP address of the device. Specifically, the ICMP Destination address of the device. Specifically, the ICMP Destination
Unreachable Message (type 3) codes 3 (port unreachable) and ICMP Time Unreachable Message (type 3) codes 3 (port unreachable) and ICMP Time
Exceeded (type 11) code 0, which are involved in proper working of Exceeded (type 11) code 0, which are required for correct operation
the "traceroute" tool. With this modification, the "traceroute" of the "traceroute" tool. With this modification, the "traceroute"
probes sent to the devices will always be sent back with the probes sent to the devices will always be sent back with the
"primary" IP address as the source, allowing the operator to discover "primary" IP address as the source, allowing the operator to discover
the "reachable" IP address of the box. This has the downside of the "reachable" IP address of the box. This has the downside of
hiding the address of the "entry point" into the device. hiding the address of the "entry point" into the device.
9. Security Considerations 9. Security Considerations
The design does not introduce any additional security concerns. The design does not introduce any additional security concerns.
General BGP security considerations are discussed in [RFC4271] and General BGP security considerations are discussed in [RFC4271] and
[RFC4272]. Furthermore, the Generalized TTL Security Mechanism [RFC4272]. Furthermore, the Generalized TTL Security Mechanism
skipping to change at page 32, line 15 skipping to change at page 32, line 15
[I-D.ietf-idr-add-paths] [I-D.ietf-idr-add-paths]
Walton, D., Retana, A., Chen, E., and J. Scudder, Walton, D., Retana, A., Chen, E., and J. Scudder,
"Advertisement of Multiple Paths in BGP", draft-ietf-idr- "Advertisement of Multiple Paths in BGP", draft-ietf-idr-
add-paths-13 (work in progress), December 2015. add-paths-13 (work in progress), December 2015.
[I-D.ietf-idr-link-bandwidth] [I-D.ietf-idr-link-bandwidth]
Mohapatra, P. and R. Fernando, "BGP Link Bandwidth Mohapatra, P. and R. Fernando, "BGP Link Bandwidth
Extended Community", draft-ietf-idr-link-bandwidth-06 Extended Community", draft-ietf-idr-link-bandwidth-06
(work in progress), January 2013. (work in progress), January 2013.
[I-D.ietf-rtgwg-bgp-pic]
Bashandy, A., Filsfils, C., and P. Mohapatra, "Abstract",
draft-ietf-rtgwg-bgp-pic-00 (work in progress), December
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[I-D.mitchell-grow-remove-private-as] [I-D.mitchell-grow-remove-private-as]
Mitchell, J., Rao, D., and R. Raszuk, "Private Autonomous Mitchell, J., Rao, D., and R. Raszuk, "Private Autonomous
System (AS) Removal Requirements", draft-mitchell-grow- System (AS) Removal Requirements", draft-mitchell-grow-
remove-private-as-04 (work in progress), April 2015. remove-private-as-04 (work in progress), April 2015.
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Clos, C., "A Study of Non-Blocking Switching Networks: Clos, C., "A Study of Non-Blocking Switching Networks:
Bell System Technical Journal Vol. 32(2)", March 1953. Bell System Technical Journal Vol. 32(2)", March 1953.
[HADOOP] Apache, , "Apache HaDoop", August 2016, [HADOOP] Apache, , "Apache HaDoop", April 2016,
<https://hadoop.apache.org/>. <https://hadoop.apache.org/>.
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Greenberg, A., Hamilton, J., and D. Maltz, "The Cost of a Greenberg, A., Hamilton, J., and D. Maltz, "The Cost of a
Cloud: Research Problems in Data Center Networks", January Cloud: Research Problems in Data Center Networks", January
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[IEEE8021D-1990] [IEEE8021D-1990]
IEEE 802.1D, , "IEEE Standard for Local and Metropolitan IEEE 802.1D, , "IEEE Standard for Local and Metropolitan
Area Networks--Media access control (MAC) Bridges", May Area Networks--Media access control (MAC) Bridges", May
skipping to change at page 33, line 9 skipping to change at page 33, line 14
[INTERCON] [INTERCON]
Dally, W. and B. Towles, "Principles and Practices of Dally, W. and B. Towles, "Principles and Practices of
Interconnection Networks", ISBN 978-0122007514, January Interconnection Networks", ISBN 978-0122007514, January
2004. 2004.
[ALFARES2008] [ALFARES2008]
Al-Fares, M., Loukissas, A., and A. Vahdat, "A Scalable, Al-Fares, M., Loukissas, A., and A. Vahdat, "A Scalable,
Commodity Data Center Network Architecture", August 2008. Commodity Data Center Network Architecture", August 2008.
[IANA.AS] IANA, , "Autonomous System (AS) Numbers", August 2016, [IANA.AS] IANA, , "Autonomous System (AS) Numbers", April 2016,
<http://www.iana.org/assignments/as-numbers/>. <http://www.iana.org/assignments/as-numbers/>.
[IEEE8023AD] [IEEE8023AD]
IEEE 802.3ad, , "IEEE Standard for Link aggregation for IEEE 802.3ad, , "IEEE Standard for Link aggregation for
parallel links", October 2000. parallel links", October 2000.
[ALLOWASIN] [ALLOWASIN]
Cisco Systems, , "Allowas-in Feature in BGP Configuration Cisco Systems, , "Allowas-in Feature in BGP Configuration
Example", February 2016, Example", April 2016,
<http://www.cisco.com/c/en/us/support/docs/ip/border- <http://www.cisco.com/c/en/us/support/docs/ip/border-
gateway-protocol-bgp/112236-allowas-in-bgp-config- gateway-protocol-bgp/112236-allowas-in-bgp-config-
example.html>. example.html>.
[VENDOR-REMOVE-PRIVATE-AS] [VENDOR-REMOVE-PRIVATE-AS]
Cisco Systems, , "Removing Private Autonomous System Cisco Systems, , "Removing Private Autonomous System
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[JAKMA2008] [JAKMA2008]
Jakma, P., "BGP Path Hunting", 2008, Jakma, P., "BGP Path Hunting", 2008,
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[CONS-HASH] [CONS-HASH]
Wikipedia, , "Consistent Hashing", Wikipedia, , "Consistent Hashing",
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Authors' Addresses Authors' Addresses
Petr Lapukhov Petr Lapukhov
Facebook Facebook
1 Hacker Way 1 Hacker Way
Menlo Park, CA 94025 Menlo Park, CA 94025
US US
Email: petr@fb.com Email: petr@fb.com
Ariff Premji Ariff Premji
Arista Networks Arista Networks
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