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Versions: 00 01 02 04 05 06 07 draft-ietf-rtgwg-bgp-routing-large-dc

Network Working Group                                        P. Lapukhov
Internet-Draft                                     Microsoft Corporation
Intended status: Informational                                 A. Premji
Expires: January 16, 2014                                Arista Networks
                                                        J. Mitchell, Ed.
                                                   Microsoft Corporation
                                                           July 15, 2013


           Use of BGP for routing in large-scale data centers
                 draft-lapukhov-bgp-routing-large-dc-05

Abstract

   Some network operators build and operate data centers that support
   over one hundred thousand servers.  In this document, such data
   centers are referred to as "large-scale" to differentiate them from
   smaller infrastructures.  Environments of this scale have a unique
   set of network requirements with an emphasis on operational
   simplicity and network stability.  This document summarizes
   operational experience in designing and operating large-scale data
   centers using BGP as the only routing protocol.  The intent is to
   report on a proven and stable routing design that could be leveraged
   by others in the industry.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 16, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Network Design Requirements . . . . . . . . . . . . . . . . .   4
     2.1.  Bandwidth and Traffic Patterns  . . . . . . . . . . . . .   4
     2.2.  CAPEX Minimization  . . . . . . . . . . . . . . . . . . .   4
     2.3.  OPEX Minimization . . . . . . . . . . . . . . . . . . . .   5
     2.4.  Traffic Engineering . . . . . . . . . . . . . . . . . . .   5
     2.5.  Summarized Requirements . . . . . . . . . . . . . . . . .   6
   3.  Data Center Topologies Overview . . . . . . . . . . . . . . .   6
     3.1.  Traditional DC Topology . . . . . . . . . . . . . . . . .   6
     3.2.  Clos Network topology . . . . . . . . . . . . . . . . . .   7
       3.2.1.  Clos Topology Overview  . . . . . . . . . . . . . . .   7
       3.2.2.  Clos Topology Properties  . . . . . . . . . . . . . .   8
       3.2.3.  Scaling the Clos topology . . . . . . . . . . . . . .   8
       3.2.4.  Managing the Size of Clos Topology Tiers  . . . . . .   9
   4.  Data Center Routing Overview  . . . . . . . . . . . . . . . .  10
     4.1.  Layer 2 Only Designs  . . . . . . . . . . . . . . . . . .  10
     4.2.  Hybrid L2/L3 Designs  . . . . . . . . . . . . . . . . . .  11
     4.3.  Layer 3 Only Designs  . . . . . . . . . . . . . . . . . .  12
   5.  Routing Protocol Selection and Design . . . . . . . . . . . .  12
     5.1.  Choosing EBGP as the Routing Protocol . . . . . . . . . .  12
     5.2.  EBGP Configuration for Clos topology  . . . . . . . . . .  14
       5.2.1.  Example ASN Scheme  . . . . . . . . . . . . . . . . .  14
       5.2.2.  Private Use BGP ASNs  . . . . . . . . . . . . . . . .  15
       5.2.3.  Prefix Advertisement  . . . . . . . . . . . . . . . .  16
       5.2.4.  External Connectivity . . . . . . . . . . . . . . . .  16
       5.2.5.  Route Aggregation at the Edge . . . . . . . . . . . .  17
   6.  ECMP Considerations . . . . . . . . . . . . . . . . . . . . .  18
     6.1.  Basic ECMP  . . . . . . . . . . . . . . . . . . . . . . .  18
     6.2.  BGP ECMP over Multiple ASNs . . . . . . . . . . . . . . .  19
     6.3.  Weighted ECMP . . . . . . . . . . . . . . . . . . . . . .  20
   7.  BGP Convergence of Design . . . . . . . . . . . . . . . . . .  20
     7.1.  Fault Detection Timing  . . . . . . . . . . . . . . . . .  20
     7.2.  Event Propagation Timing  . . . . . . . . . . . . . . . .  21
     7.3.  Impact of Clos Topology Fan-outs  . . . . . . . . . . . .  21
     7.4.  Failure Impact Scope  . . . . . . . . . . . . . . . . . .  22
     7.5.  Routing Micro-Loops . . . . . . . . . . . . . . . . . . .  23



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   8.  Additional Options for Design . . . . . . . . . . . . . . . .  23
     8.1.  Third-party Route Injection . . . . . . . . . . . . . . .  23
     8.2.  Route Aggregation within Clos Topology  . . . . . . . . .  24
       8.2.1.  Collapsing Tier-1 Devices Layer . . . . . . . . . . .  24
       8.2.2.  Implications of Collapsing Tier-1 Devices Layer . . .  25
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  25
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  26
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  26
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  26
     12.2.  Informative References . . . . . . . . . . . . . . . . .  26
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27

1.  Introduction

   This document describes a practical routing design that can be used
   in a large-scale data center ("DC") design.  Such data centers, also
   known as hyper-scale or warehouse-scale data centers, have a unique
   attribute of supporting over a hundred thousand servers.  In order to
   accommodate networks of this scale, operators are revisiting
   networking designs and platforms to address this need.

   The design described in this document is based on operational
   experience with data centers built to support large scale Web
   infrastructure.  The primary requirements in such environments are
   operational simplicity and network stability so that a small group of
   people can effectively support a large network infrastructure.

   After experimentation and extensive testing, Microsoft choose to use
   an end to end routed network infrastructure with External BGP (EBGP)
   [RFC4271] as the only routing protocol for some of its DC
   deployments.  This is in contrast with more traditional DC designs,
   which may use more hierarchical topologies and rely on extending
   Layer 2 domains across multiple network devices.  This document
   elaborates on the requirements that led to this design choice and
   presents details of the EBGP routing design as well as explores ideas
   for further enhancements.

   This document first presents an overview of network design
   requirements and considerations for large-scale data centers.  Then
   traditional hierarchical data center network topologies are
   contrasted with Clos networks that are horizontally scaled out.
   Arguments for selecting EBGP with a Clos topology as the most
   appropriate routing protocol to meet the requirements are presented.
   Then the design is described in detail.  Finally some additional
   considerations and options are presented.





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2.  Network Design Requirements

   This section describes and summarizes network design requirements for
   large-scale data centers.

2.1.  Bandwidth and Traffic Patterns

   The primary requirement when building an interconnection network for
   large number of servers is to accommodate application bandwidth and
   latency requirements.  Until recently it was quite common to see the
   majority of traffic entering and leaving the data center (also known
   as north-south traffic).  As a result, traditional "tree" topologies
   were sufficient to accommodate such flows, even with high
   oversubscription ratios in network equipment.  If more bandwidth was
   required, it was added by "scaling up" the network elements,
   especially at the Tier-1 layer, e.g. by upgrading the device's line-
   cards or fabrics or replacing the device with one with higher port
   density.

   Today many large-scale data centers host applications generating
   significant amounts of server to server traffic, traveling between
   various Tier-2 or Tier-3 devices but without egressing the DC, also
   known as "east-west" traffic.  Examples of such applications could be
   compute clusters such as Hadoop, large amounts of replication traffic
   between clusters needed by certain applications, or virtual machine
   migrations.  Scaling up traditional tree topologies to match these
   bandwidth demands becomes either too expensive or impossible due to
   physical limitations.

2.2.  CAPEX Minimization

   The cost of the network infrastructure alone (CAPEX) constitutes
   about 10-15% of total data center expenditure (see [GREENBERG2009]).
   However, the absolute cost is significant, and there is a need to
   constantly drive down the cost of individual network elements.  This
   can be accomplished in two ways:

   o  Unifying all network elements, preferably using the same hardware
      type or even the same device.  This allows for bulk purchases with
      discounted pricing.

   o  Driving costs down using competitive pressures, by introducing
      multiple network equipment vendors.








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   In order to allow for vendor diversity, it is important to minimize
   the software feature requirements for the network elements.
   Furthermore, this strategy provides maximum flexibility of vendor
   equipment choices while enforcing interoperability using open
   standards.

2.3.  OPEX Minimization

   Operating large scale infrastructure could be expensive, provided
   that larger amount of elements will statistically fail more often.
   Having a simpler design and operating using a limited software
   feature-set minimizes software issue related failures.

   An important aspect of OPEX minimization is reducing size of failure
   domains in the network.  Ethernet networks are known to be
   susceptible to broadcast or unicast traffic storms that have dramatic
   impact on network performance and availability.  The use of a fully
   routed design significantly reduces the size of the data-plane
   failure domains (e.g. limits them to Tier-3 devices only).  However,
   such designs also introduce the problem of distributed control-plane
   failures.  This observation calls for simpler control-plane protocols
   that are expected to have less chances of network meltdown.

2.4.  Traffic Engineering

   In any data center, application load-balancing is a critical function
   performed by network devices.  Traditionally, load-balancers are
   deployed as dedicated devices in the traffic forwarding path.  The
   problem arises in scaling load-balancers under growing traffic
   demand.  A preferable solution would be able to scale load-balancing
   layer horizontally, by adding more of the uniform nodes and
   distributing incoming traffic across these nodes.

   In situation like this, an ideal choice would be to use network
   infrastructure itself to distribute traffic across a group of load-
   balancers.  The combination of Anycast prefix advertisement [RFC4786]
   and Equal Cost Multipath (ECMP) functionality can be used to
   accomplish this goal.  To allow for more granular load-distribution,
   it is beneficial for the network to support the ability to perform
   controlled per-hop traffic engineering.  For example, it is
   beneficial to directly control the ECMP next-hop set for Anycast
   prefixes at every level of network hierarchy.









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2.5.  Summarized Requirements

   This section summarizes the list of requirements outlined in the
   previous sections:

   o  REQ1: Select a topology that can be scaled "horizontally" by
      adding more links and network devices of the same type without
      requiring upgrades to the network elements themselves.

   o  REQ2: Define a narrow set of software features/protocols supported
      by a multitude of networking equipment vendors.

   o  REQ3: Choose a routing protocol that has a simple implementation
      in terms of programming code complexity and ease of operational
      support.

   o  REQ4: Minimize the failure domain of equipment or protocol issues
      as much as possible.

   o  REQ5: Allow for traffic engineering, preferably via explicit
      control of the routing prefix next-hop using built-in protocol
      mechanics.

3.  Data Center Topologies Overview

   This section provides an overview of two general types of data center
   designs - hierarchical (also known as tree based) and Clos based
   network designs.

3.1.  Traditional DC Topology

   In the networking industry, a common design choice for data centers
   typically look like a (upside-down) tree with redundant uplinks and
   three layers of hierarchy namely core, aggregation/distribution and
   access layers (see Figure 1).  To accommodate bandwidth demands, each
   higher layer, from server towards DC egress or WAN, has higher port
   density and bandwidth capacity where the core functions as the
   "trunk" of the tree based design.  To keep terminology uniform and
   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,
   Aggregation or Access layers.

                       +------+  +------+
                       |      |  |      |
                       |      |--|      |           Tier-1
                       |      |  |      |
                       +------+  +------+
                         |  |      |  |



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               +---------+  |      |  +----------+
               | +-------+--+------+--+-------+  |
               | |       |  |      |  |       |  |
             +----+     +----+    +----+     +----+
             |    |     |    |    |    |     |    |
             |    |-----|    |    |    |-----|    | Tier-2
             |    |     |    |    |    |     |    |
             +----+     +----+    +----+     +----+
                |         |          |         |
                |         |          |         |
                | +-----+ |          | +-----+ |
                +-|     |-+          +-|     |-+    Tier-3
                  +-----+              +-----+
                   | | |                | | |
               <- Servers ->        <- Servers ->


                   Figure 1: Typical DC network topology

3.2.  Clos Network topology

   This section describes a common design for horizontally scalable
   topology in large scale data centers in order to meet REQ1.

3.2.1.  Clos Topology Overview

   A common choice for a horizontally scalable topology is a folded Clos
   topology, sometimes called "fat-tree" (see, for example, [INTERCON]
   and [ALFARES2008]).  This topology features an odd number of stages
   (sometimes known as dimensions) and is commonly made of uniform
   elements, e.g. network switches with the same port count.  Therefore,
   the choice of Clos topology satisfies both REQ1 and also facilitates
   REQ2.  See Figure 2 below for an example of a folded 3-stage Clos
   topology:

          +-------+
          |       |----------------------------+
          |       |------------------+         |
          |       |--------+         |         |
          +-------+        |         |         |
          +-------+        |         |         |
          |       |--------+---------+-------+ |
          |       |--------+-------+ |       | |
          |       |------+ |       | |       | |
          +-------+      | |       | |       | |
          +-------+      | |       | |       | |
          |       |------+-+-------+-+-----+ | |
          |       |------+-+-----+ | |     | | |



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          |       |----+ | |     | | |     | | |
          +-------+    | | |     | | |   ---------> M links
           Tier-1      | | |     | | |     | | |
                     +-------+ +-------+ +-------+
                     |       | |       | |       |
                     |       | |       | |       | Tier-2
                     |       | |       | |       |
                     +-------+ +-------+ +-------+
                       | | |     | | |     | | |
                       | | |     | | |   ---------> N Links
                       | | |     | | |     | | |
                       O O O     O O O     O O O   Servers

                  Figure 2: 3-Stage Folded Clos topology

   This topology is sometimes also referred to as a "Leaf and Spine"
   network, where "Spine" is the name given to the middle stage of the
   Clos topology (Tier-1) and "Leaf" is the name of input/output stage
   (Tier-2).  For uniformity, this document will refer to these layers
   using the "Tier-n" notation.

3.2.2.  Clos Topology Properties

   The following are some key properties of the Clos topology:

   o  The topology is fully non-blocking (or more accurately: non-
      interfering) if M >= N and oversubscribed by a factor of N/M
      otherwise.  Here M and N is the uplink and downlink port count
      respectively, for a Tier-2 switch as shown in Figure 2

   o  Utilizing this topology requires an control and data plane
      supporting ECMP with the fan-out of M or more

   o  Tier-1 switches have exactly one path to every server in this
      topology

   o  Traffic flowing from server to server is load-balanced over all
      available paths using ECMP

3.2.3.  Scaling the Clos topology

   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
   illustrated in Figure 3 below:

                                Tier-1
                               +-----+
                               |     |



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                            +--|     |--+
                            |  +-----+  |
                    Tier-2  |           |   Tier-2
                   +-----+  |  +-----+  |  +-----+
     +-------------| DEV |--+--|     |--+--|     |-------------+
     |       +-----|  C  |--+  |     |  +--|     |-----+       |
     |       |     +-----+     +-----+     +-----+     |       |
     |       |                                         |       |
     |       |     +-----+     +-----+     +-----+     |       |
     | +-----+-----| DEV |--+  |     |  +--|     |-----+-----+ |
     | |     | +---|  D  |--+--|     |--+--|     |---+ |     | |
     | |     | |   +-----+  |  +-----+  |  +-----+   | |     | |
     | |     | |            |           |            | |     | |
   +-----+ +-----+          |  +-----+  |          +-----+ +-----+
   | DEV | | DEV |          +--|     |--+          |     | |     |
   |  A  | |  B  | Tier-3      |     |      Tier-3 |     | |     |
   +-----+ +-----+             +-----+             +-----+ +-----+
     | |     | |                                     | |     | |
     O O     O O                                     O O     O O
       Servers                                         Servers

                      Figure 3: 5-Stage Clos topology

   The small example topology on Figure 3 is built from devices with a
   port count of 4 and provides full bisectional bandwidth to all
   connected servers.  In this document, one set of directly connected
   Tier-2 and Tier-3 devices along with their attached servers will be
   referred to as a "cluster".  For example, DEV A, B, C, D, and the
   servers that connect to DEV A and B, on Figure 3 form a cluster.

   In practice, the Tier-3 layer of the network, which are typically top
   of rack switches (ToRs), is where oversubscription is introduced to
   allow for packaging of more servers in the data center while meeting
   the bandwidth requirements for different types of applications.  The
   main reason to limit oversubscription at a single layer of the
   network is to simplify application development that would otherwise
   need to account for multiple bandwidth pools: within rack (Tier-3),
   between racks (Tier-2), and between cluster (Tier-1).  Since
   oversubscription does not have a direct relationship to the routing
   design it is not discussed further in this document.

3.2.4.  Managing the Size of Clos Topology Tiers

   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 power 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
   devices.  If half of the ports on each of the Tier-1 devices are not



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   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
   Tier-1 device that were previously mapped to different Tier-1
   devices.  This technique maintains the same bisectional bandwidth
   while reducing the number of elements in the Tier-1 layer, thus
   saving on CAPEX.  The tradeoff, in this example, is the reduction of
   maximum DC size in terms of overall server count by half.

   In this example, Tier-2 devices will be using two parallel links to
   connect to each Tier-1 device.  If one of these links fails, the
   other will pick up all traffic of the failed link, possible resulting
   in heavy congestion and quality of service degradation if the path
   determination procedure, does not take bandwidth amount into account.
   To avoid this situation, parallel links can be grouped in link
   aggregation groups (LAGs) with widely available implementation
   settings that take the whole bundle down upon a single link failure.
   Equivalent techniques that enforce "fate sharing" on the parallel
   links can be used in place of LAGs to achieve the same effect.  As a
   result of such fate-sharing, traffic from two or more failed links
   will be re-balanced over the multitude of remaining paths that equals
   the number of Tier-1 devices.  Although this example is using 2 for
   simplicity, reduced impact of bandwidth capacity can be achieved for
   a link or device failure with a larger fan-out.

4.  Data Center Routing Overview

   This section provides an overview of three general types of data
   center protocol designs - Layer 2 only, Hybrid L2/L3 and Layer 3
   only.

4.1.  Layer 2 Only Designs

   Originally most data center protocol designs used Spanning-Tree
   Protocol (STP) for loop free topology creation, typically utilizing
   variants of the typical DC topology described in Section 3.1.  At the
   time, many DC switches either did not support Layer 3 routed
   protocols or supported it with additional licensing fees, which
   played a part in the design choice.  Although many enhancements have
   been made through the introduction of Rapid Spanning Tree Protocol
   and Multiple Spanning Tree Protocol that increase convergence,
   stability and load balancing in larger topologies many of the
   fundamentals of the protocol limit its applicability in large scale
   DC's.  STP and its newer variants use an active/standby approach to
   path selection and are therefore hard to deploy in horizontally
   scaled topologies described in Section 3.2.  Further, operators have
   had many experiences with large failures due to issues caused by
   improper cabling, misconfiguration, or flawed software on a single
   device.  These failures regularly impacted the entire spanning-tree



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   domain and were very hard to troubleshoot due to the nature of the
   protocol.  For these reasons, and since almost all DC traffic is now
   IP therefore requiring a Layer 3 routing protocol at the network edge
   for external connectivity, designs utilizing STP usually fail all of
   the requirements of large scale DC operators.

   It should be noted that building large, horizontally scalable, Layer
   2 only networks without STP is possible recently through the
   introduction of TRILL [RFC6325].  TRILL resolves many of the issues
   STP has for large scale DC design however currently the maturity of
   the protocol, limited number of implementations, and requirement for
   new equipment that supports it has limited it's applicability and
   increased the cost of such designs.

4.2.  Hybrid L2/L3 Designs

   Operators have sought to limit the impact of STP failures and build
   larger scale topologies through implementing routing protocols 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
   allowed data centers to scale up, but at the cost of complexity in
   the network managing multiple protocols.  For the following reasons,
   operators have still retained Layer 2 in either the access (Tier-3)
   or both access and aggregation (Tier-3 and Tier-2) parts of the
   network:

   o  Supporting legacy applications that may require direct Layer 2
      adjacency or use non-IP protocols

   o  Seamless mobility for virtual machines that require the
      preservation of IP addresses when a virtual machine moves to
      different access switch

   o  Simplified IP addressing = less IP subnets is required for the
      data center

   o  Application load-balancing may require direct Layer 2 reachability
      to perform certain functions such as Layer 2 Direct Server Return
      (DSR)

   o  Continued CAPEX differences between Layer-2 and Layer-3 capable
      switches









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4.3.  Layer 3 Only Designs

   Network designs that leverage IP routing down to Tier-3 of the
   network have gained popularity as well.  The main benefit of these
   designs is improved network stability and scalability, as a result of
   confining L2 broadcast domains.  Commonly an IGP such as OSPF
   [RFC2328] is used as the primary routing protocol in such a design.
   As data centers grow in scale, and server count exceeds tens of
   thousands, such fully routed designs have become more attractive.
   Many vendors pricing has also changed to support this model so that
   data center class switches often do not cost more whether running
   traditional Layer 2 or Layer 3 control plane protocols.

   Choosing a Layer 3 only design greatly simplifies the network,
   facilitating the meeting of REQ1 and REQ2, and has wide spread
   adoption in networks where large Layer 2 adjacency and larger size
   Layer 3 subnets are not as critical compared to network scalability
   and stability.  Application providers and network operators continue
   to also develop new solutions to meet some of the requirements that
   previously have driven large Layer 2 domains.

5.  Routing Protocol Selection and Design

   In this section the motivations for using External BGP (EBGP) as the
   single routing protocol for data center networks having a Layer 3
   protocol design and Clos topology are reviewed.  Then, a practical
   approach for designing an EBGP based network is provided.

5.1.  Choosing EBGP as the Routing Protocol

   REQ2 would give preference to the selection of a single routing
   protocol to reduce complexity and interdependencies.  While it is
   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
   (IBGP) throughout, this document proposes the use of an EBGP only
   design.

   Although EBGP is the protocol used for almost all inter-provider
   routing on the Internet and has wide support from both vendor and
   service provider communities, it is not generally deployed as the
   primary routing protocol within the data center for a number of
   reasons:

   o  BGP is perceived as a "WAN only protocol only" and not often
      considered for enterprise or data center applications.

   o  BGP is believed to have a "much slower" routing convergence than
      traditional IGPs.



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   o  BGP deployment within an Autonomous System typically assumes the
      presence of an IGP for next-hop resolution.

   o  BGP is perceived to require significant configuration overhead and
      does not support any form of neighbor auto-discovery.

   This document discusses some of these perceptions, especially as
   applicable to the proposed design, and highlights some of the
   advantages of using the protocol such as:

   o  BGP has less complexity within its protocol design - internal data
      structures and state-machines are simpler when compared to a link-
      state IGP such as OSPF.  For example, instead of implementing
      adjacency formation, adjacency maintenance and/or flow-control,
      BGP simply relies on TCP as the underlying transport.  This
      fulfills REQ2 and REQ3.

   o  BGP information flooding overhead is less when compared to link-
      state IGPs.  Since every BGP router calculates and propagates only
      the best-path selected, a network failure is masked as soon as the
      BGP speaker finds an alternate path, which exists when highly
      symmetric topologies, such as Clos, are coupled with EBGP only
      design.  In contrast, the event propagation scope of a link-state
      IGP is an entire area, regardless of the failure type.  This meets
      REQ3 and REQ4.  It is worth mentioning that all widely deployed
      link-state IGPs also feature periodic refreshes of routing
      information, while BGP does not expire routing state, even if this
      rarely causes significant impact to modern router control planes.

   o  BGP supports third-party (recursively resolved) next-hops.  This
      allows for manipulating multi-path to be non-ECMP based or
      forwarding based on application-defined forwarding paths, through
      establishment of a peering session with an application
      "controller" which can inject routing information into the system,
      satisfying REQ5.  OSPF provides similar functionality using
      concepts such as "Forwarding Address", but with more difficulty in
      implementation and lack of protocol simplicity.

   o  Using a well-defined BGP ASN allocation scheme and standard
      AS_PATH loop detection, "BGP path hunting" can be controlled and
      complex unwanted paths will be ignored.  See Section 5.2 for an
      example of a working scheme.  In a link-state IGP accomplishing
      the same goal would require multi-(instance/topology/processes)
      support, typically not available in all DC devices and quite
      complex to configure and troubleshoot.  Using a traditional single
      flooding domain, which most DC designs utilize, under certain
      failure conditions may pick up unwanted lengthy paths, e.g.
      traversing multiple Tier-2 devices.



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   o  EBGP configuration that is implemented with minimal routing policy
      is easier to troubleshoot for network reachability issues.  In
      most implementations, it is straightforward to view contents of
      BGP Loc-RIB and compare it to the router's RIB.  Also every BGP
      neighbor has corresponding Adj-RIB-In and Adj-RIB-Out structures
      with incoming and outgoing NRLI information that can be easily
      correlated on both sides of a BGP session.  Thus, BGP satisfies
      REQ3.

5.2.  EBGP Configuration for Clos topology

   Clos topologies that have more than 5 stages are very uncommon due to
   the large numbers of interconnects required by such a design.
   Therefore, the examples below are made with regards to the 5 stage
   Clos topology (unfolded).

5.2.1.  Example ASN Scheme

   The diagram below illustrates an example ASN allocation scheme.  The
   following is a list of guidelines that can be used:

   o  Only EBGP sessions established over direct point-to-point links
      interconnecting the network nodes.

   o  16-bit (two octet) BGP ASNs are used, since these are widely
      supported and have better vendor interoperability (e.g. no need to
      support BGP capability negotiation).

   o  Private BGP ASNs from the range 64512-65534 are used so as to
      avoid ASN conflicts.

   o  A single BGP ASN is allocated to all of the Clos topology's Tier-1
      devices

   o  Unique BGP ASN is allocated per each group of Tier-2 devices

   o  Unique BGP ASN is allocated to every Tier-3 device (e.g. ToR) in
      this topology.

                                ASN 65534
                               +---------+
                               | +-----+ |
                               | |     | |
                             +-|-|     |-|-+
                             | | +-----+ | |
                  ASN 646XX  | |         | |  ASN 646XX
                 +---------+ | |         | | +---------+
                 | +-----+ | | | +-----+ | | | +-----+ |



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     +-----------|-|     |-|-+-|-|     |-|-+-|-|     |-|-----------+
     |       +---|-|     |-|-+ | |     | | +-|-|     |-|---+       |
     |       |   | +-----+ |   | +-----+ |   | +-----+ |   |       |
     |       |   |         |   |         |   |         |   |       |
     |       |   |         |   |         |   |         |   |       |
     |       |   | +-----+ |   | +-----+ |   | +-----+ |   |       |
     | +-----+---|-|     |-|-+ | |     | | +-|-|     |-|---+-----+ |
     | |     | +-|-|     |-|-+-|-|     |-|-+-|-|     |-|-+ |     | |
     | |     | | | +-----+ | | | +-----+ | | | +-----+ | | |     | |
     | |     | | +---------+ | |         | | +---------+ | |     | |
     | |     | |             | |         | |             | |     | |
   +-----+ +-----+           | | +-----+ | |           +-----+ +-----+
   | ASN | |     |           +-|-|     |-|-+           |     | |     |
   |65YYY| | ... |             | |     | |             | ... | | ... |
   +-----+ +-----+             | +-----+ |             +-----+ +-----+
     | |     | |               +---------+               | |     | |
     O O     O O              <- Servers ->              O O     O O

                 Figure 4: BGP ASN layout for 5-stage Clos

5.2.2.  Private Use BGP ASNs

   The original range of Private Use BGP
   ASNs[I-D.ietf-idr-as-private-reservation] limited operators to 1023
   unique ASNs.  Since it is quite likely that the number of network
   devices may exceed this number, a workaround is required.  One
   approach is to re-use the ASNs assigned to the Tier-3 devices across
   different clusters.  For example, Private Use BGP ASNs 65001, 65002
   ...  65032 could be used within every individual cluster and assigned
   to Tier-3 devices.

   To avoid route suppression due to the AS_PATH loop detection
   mechanism in BGP, upstream EBGP sessions on Tier-3 devices must be
   configured with the "AllowAS In" feature that allows accepting a
   device's own ASN in received route advertisements.  Introducing this
   feature does not create an opportunity for routing loops under
   misconfiguration since the AS_PATH is always incremented when routes
   are propagated between topology tiers.  Loop protection is also in
   place at the Tier-1 device which does not accept routes with a path
   including its own ASN.

   Another solution to this problem would be using Four-Octet BGP ASNs
   [RFC6793], where there are additional Private Use ASN's available,
   see [IANA.AS].  Use of Four-Octet BGP ASNs put additional protocol
   complexity in the BGP implementation so should be considered against
   the complexity of re-use when considering REQ3 and REQ4.  Perhaps
   more importantly, they are not yet supported by all BGP
   implementations, which may limit vendor selection of DC equipment.



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5.2.3.  Prefix Advertisement

   A Clos topology features a large number of point-to-point links and
   associated prefixes.  Advertising all of these routes into BGP may
   create FIB overload conditions in the network devices.  Advertising
   these links also puts additional path computation stress on the BGP
   control plane for little benefit.  There are two possible solutions:

   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
      device, distant networks will automatically be reachable via the
      advertising EBGP peer and do not require reachability to these
      prefixes.  However this may complicate operational troubleshooting
      or monitoring systems if the addresses are not reachable.

   o  Advertise point-to-point links, but summarize them on every
      device.  This requires an address allocation scheme such as
      allocating a consecutive block of IP addresses per Tier-1 and
      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
      addressing and so forth).

   Server subnets on Tier-3 devices must be announced into BGP without
   using route aggregation on Tier-2 and Tier-1 devices.  Summarizing
   subnets in a Clos topology will result in route black-holing under a
   single link failure (e.g. between Tier-2 and Tier-3 devices) and must
   be avoided.  The use of peer links within the same tier to resolve
   the black-holing problem by providing "bypass paths" is undesirable
   due to O(N^2) complexity of the peering mesh and waste of ports on
   the devices.  In Section 8.2 a method for performing route
   summarization in Clos networks and the associated trade-offs is
   described.

5.2.4.  External Connectivity

   A dedicated cluster (or clusters) in the Clos topology could be used
   for the purpose of connecting to the Wide Area Network (WAN) edge
   devices, or WAN Routers.  Tier-3 devices in such a cluster would be
   replaced with WAN Routers, and EBGP peering would be used again,
   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 dedicated cluster will be referred to as
   "Border Routers" in this document.  These devices have to perform a
   few special functions:

   o  Hide network topology information when advertising paths to WAN
      routers, i.e. remove Private BGP ASNs from the AS_PATH attribute.



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      This is typically done to avoid ASN number collisions between
      different data centers.  An implementation specific BGP feature
      typically called "Remove Private AS" is commonly used to
      accomplish this.  Depending on implementation, the feature should
      strip a contiguous sequence of private ASNs found in AS_PATH
      attribute prior to advertising the path to a neighbor.  This
      assumes that all BGP ASN's used for intra data center numbering
      are from the private ASN range.

   o  Originate a default route to the data center devices.  This is the
      only place where default route can be originated, as route
      summarization is highly undesirable for the "scale-out" topology.
      Alternatively, Border Routers may simply relay the default route
      learned from WAN routers.  Advertising the default route from
      Border Routers requires that all Border Routers to be fully
      connected to the WAN Routers upstream, to provide resistance to a
      single-link failure causing the black holing of traffic.  To
      prevent chance of operator or implementation error that may impact
      EBGP sessions to the WAN routers simultaneously (although these
      scenarios are not planned for by many operators since they
      represents a multiple failure) it may be more desirable to take
      this approach than introducing complicated conditional default
      origination schemes provided by some implementations.

5.2.5.  Route Aggregation at the Edge

   It is often desirable to aggregate network reachability information
   prior to advertising it to the WAN network due to high amount of IP
   prefixes originated from within the data center with a fully routed
   network design.  For example, a network with 2000 Tier-3 devices will
   have at least 2000 servers subnets advertised into BGP, along with
   the infrastructure or other prefixes.  However, as discussed before,
   the proposed network design does not allow for route aggregation due
   to the lack of peer links inside every tier.

   However, it is possible to lift this restriction for the Border
   Routers, by devising a different connectivity model for these
   devices.  There are two options possible:

   o  Interconnect the Border Routers using a full-mesh of physical
      links or by using additional aggregation devices, forming hub-and-
      spoke topology.  Next, build a full-mesh of IBGP sessions between
      all Border Routers to allow for sharing of specific network
      prefixes.  Notice that in this case the interconnecting peer links
      need to be appropriately sized for the amount of traffic that will
      be present in the case of a device or link failure underneath the
      Border Routers.




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   o  Tier-1 devices may have additional physical links running toward
      the Border Routers (which are Tier-2 devices in essence).
      Specifically, if protection from a single link or node failure is
      desired, each Tier-1 devices would have to connect to at least two
      Border Routers.  This puts additional requirements on the port
      count for Tier-1 devices and Border Routers, potentially making it
      a non-uniform, larger port count, device with the other devices in
      the Clos.

   If any of the above options are implemented, it is possible to
   perform route aggregation at the Border Routers toward the WAN
   network core without risking a routing black-hole condition under a
   single link failure.  Both of the options would result in non-uniform
   topology as additional links have to be provisioned on some network
   devices.

6.  ECMP Considerations

   This section covers the Equal Cost Multipath (ECMP) functionality for
   Clos topology and discusses a few special requirements.

6.1.  Basic ECMP

   ECMP is the fundamental load-sharing mechanism used by a Clos
   topology.  Effectively, every lower-tier device will use all of its
   directly attached upper-tier devices to load-share traffic destined
   to the same IP prefix.  Number of ECMP paths between any two Tier-3
   devices in Clos topology equals to the number of the devices in the
   middle stage (Tier-1).  For example, Figure 5 illustrates the
   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
   respectively.

                                Tier-1
                               +-----+
                               | DEV |
                            +->|  1  |--+
                            |  +-----+  |
                    Tier-2  |           |   Tier-2
                   +-----+  |  +-----+  |  +-----+
     +------------>| DEV |--+->| DEV |--+--|     |-------------+
     |       +-----|  B  |--+  |  2  |  +--|     |-----+       |
     |       |     +-----+     +-----+     +-----+     |       |
     |       |                                         |       |
     |       |     +-----+     +-----+     +-----+     |       |
     | +-----+---->| DEV |--+  | DEV |  +--|     |-----+-----+ |
     | |     | +---|  C  |--+->|  3  |--+--|     |---+ |     | |
     | |     | |   +-----+  |  +-----+  |  +-----+   | |     | |



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     | |     | |            |           |            | |     | |
   +-----+ +-----+          |  +-----+  |          +-----+ +-----+
   | DEV | |     | Tier-3   +->| DEV |--+   Tier-3 |     | |     |
   |  A  | |     |             |  4  |             |     | |     |
   +-----+ +-----+             +-----+             +-----+ +-----+
     | |     | |                                     | |     | |
     O O     O O            <- Servers ->            X Y     O O

               Figure 5: ECMP fan-out tree from A to X and Y

   The ECMP requirement implies that the BGP implementation must support
   multi-path fan-out for up to the maximum number of devices directly
   attached at any point in the topology in upstream or downstream
   direction.  Normally, this number does not exceed half of the ports
   found on a device in the topology.  For example, an ECMP max-path of
   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
   able to connect to multitude of Tier-1 devices if route summarization
   at Border Router level is implemented as described in Section 5.2.5.
   If a device's hardware does not support wider ECMP, logical link-
   grouping (link-aggregation at layer 2) could be used to provide
   "hierarchical" ECMP (Layer 3 ECMP followed by Layer 2 ECMP) to
   compensate for fan-out limitations.  Such approach, however,
   increases the risk of flow polarization, as less entropy will be
   available to the second stage of ECMP.

   Most BGP implementations declare paths to be equal from ECMP
   perspective if they match up to and including step (e)
   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
   otherwise the same value across all paths and policies may be applied
   as necessary to equalize BGP attributes that vary in vendor defaults,
   as has been seen occasionally with MED and origin code.  Routing
   loops are unlikely due to the BGP best-path selection process which
   prefers shorter AS_PATH length, and longer paths through the Tier-1
   devices which don't allow their own AS in the path and have the same
   ASN are also not possible.

6.2.  BGP ECMP over Multiple ASNs












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   For application load-balancing purposes it is desirable to have the
   same prefix advertised from multiple Tier-3 devices.  From the
   perspective of other devices, such a prefix would have BGP paths with
   different AS_PATH attribute values, while having the same AS_PATH
   attribute lengths.  Therefore, BGP implementations must support load-
   sharing over above-mentioned paths.  This feature is sometimes known
   as "multipath relax" and effectively allows for ECMP to be done
   across different neighboring ASNs if all other attributes are equal
   as described in the previous section.

6.3.  Weighted ECMP

   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-
   out.  This could be helpful to compensate for failures in the network
   and send more traffic over paths that have more capacity.  The
   prefixes that require weighted ECMP would have to be injected using
   remote BGP speaker (central agent) over a multihop session as
   described further in Section 8.1.  If support in implementations is
   available, weight-distribution for multiple BGP paths could be
   signaled using the technique described in
   [I-D.ietf-idr-link-bandwidth].

7.  BGP Convergence of Design

   This section reviews routing convergence properties in the proposed
   design.  A case is made that sub-second convergence is achievable
   provided that the implementation supports fast EBGP peering session
   deactivation and timely RIB and FIB update upon failure of the
   associated link.

7.1.  Fault Detection Timing

   BGP typically relies on an IGP to route around link/node failures
   inside an AS, and implements either a polling based or an event-
   driven mechanism to obtain updates on IGP state changes.  The
   proposed routing design does not use an IGP, so the only mechanisms
   that could be used for fault detection are BGP keep-alive process (or
   any other type of keep-alive mechanism) and link-failure triggers.

   Relying solely on BGP keep-alive packets may result in high
   convergence delays, in the order of multiple seconds (on many BGP
   implementations the minimum configurable BGP hold timer value is
   three seconds).  However, many BGP implementations can shut down
   local EBGP peering sessions in response to the "link down" event for
   the outgoing interface used for BGP peering.  This feature is
   sometimes called as "fast fall-over".  Since links in modern data
   centers are often point-to-point fiber connections, a physical



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   interface failure is often detected in milliseconds and subsequently
   triggers a BGP re-convergence.

   Ethernet technologies may support failure signaling or detection
   standards such as [IEEE8021AG] and [IEEE8023AH], which may make
   failure detection more robust.  Alternatively, some platforms may
   support Bidirectional Forwarding Detection (BFD) [RFC5880] to allow
   for sub-second failure detection and fault signaling to the BGP
   process.  However use of either of these presents additional
   requirements to vendor software and possibly hardware, and may
   contradict REQ1.

7.2.  Event Propagation Timing

   In this design the impact of BGP Minimum Route Advertisement Interval
   (MRAI) timer (See section 9.2.1.1 of [RFC4271]) should be considered.
   It is required for BGP implementations to space out consecutive BGP
   UPDATE messages by at least MRAI seconds, which is often a
   configurable value.  Notice that initial BGP UPDATE messages after an
   event carrying withdrawn routes are commonly not affected by this
   timer.  The MRAI timer may present significant convergence delays if
   a BGP speaker "waits" for the new path to be learned from peers and
   has no local backup path information.

   In a Clos topology each EBGP speaker has either one path only or N
   paths for the same prefix, where N is a significantly large number,
   e.g. N=32.  Therefore, if a path fails there is either no backup at
   all, or the backup is readily available in BGP Loc-RIB.  In the first
   case, the BGP withdrawal announcement will propagate un-delayed and
   trigger re-convergence on affected devices.  In the second case, only
   the local ECMP group needs to be changed.

7.3.  Impact of Clos Topology Fan-outs

   Clos topology has large fan-outs, which may impact the "Up->Down"
   convergence in some cases, as described in this section.  In a
   situation when a link between Tier-3 and Tier-2 device fails, the
   Tier-2 device will send BGP WITHDRAW message to all upstream Tier-1
   devices, and Tier-1 devices will relay this message to all downstream
   Tier-2 devices.  A Tier-2 device other than the one originating the
   WITHDRAW should wait for ALL adjacent Tier-1 devices to send a
   WITHDRAW message before it removes the affected prefixes and sends
   WITHDRAW downstream to Tier-3 devices.  If the original Tier-2 device
   or the relaying Tier-1 devices introduce some delay into their
   announcements, the result could be WITHDRAW message "dispersion",
   that could be as much as multiple seconds.  In order to avoid such
   behavior, BGP implementations must support "update groups", where a
   BGP message is built once for a group of neighbors, which typically



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   must have the same outgoing policy, which will receive this update
   and then advertised synchronously to all neighbors.

   The impact of such "dispersion" grows with the size of topology fan-
   out and could also grow under network convergence churn.

7.4.  Failure Impact Scope

   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
   have re-calculated their RIB's and consequently FIB's.  Larger
   failure impact scope typically means slower convergence since more
   devices have to be notified, and additionally results in a less
   stable network.  In this section BGP's advantages over link-state
   routing protocols in reducing failure impact scope when implemented
   in a Clos topology are described.

   BGP is similar to a distance-vector protocol as only the best path
   from the point of view of the local router is sent to neighbors and
   routers do not maintain a full view of the topology.  As such, some
   failures are masked if the local node can immediately find a backup
   path.  In the worst case ALL devices in a data center topology have
   to either withdraw a prefix completely or update the ECMP groups in
   the FIB.  However, many failures will not result in such a wide
   impact.  There are two main failure types where impact scope is
   reduced:

   o  Failure of a link between Tier-2 and Tier-1 devices: In this case,
      a Tier-2 device will update its ECMP group, removing the failed
      link.  There is no need to send new information to downstream
      Tier-3 devices.  The affected Tier-1 device will lose the only
      path available to reach a particular cluster and will have to
      withdraw the associated prefixes.  Such prefix withdrawal process
      will only affect Tier-2 devices directly connected to the affected
      Tier-1 device.  The Tier-2 devices receiving the BGP UPDATE
      messages withdrawing prefixes will simply have to update their
      ECMP groups.  The Tier-3 devices are not involved in the re-
      convergence process.

   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
      ECMP groups for all IP prefixes from non-local cluster.  The
      Tier-3 devices are once again not involved in the re-convergence
      process.

   Even though in case of such failures multiple IP prefixes will have
   to be reprogrammed in the FIB, it is worth noting that ALL of these
   prefixes share a single ECMP group on Tier-2 device.  Therefore, in



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   the case of implementations with a hierarchical FIB, only a single
   change has to be made to the FIB.

   Even though BGP offers some failure scope reduction, reduction of the
   fault domain using summarization is not always possible with the
   proposed design, since using this technique may create routing black-
   holes as mentioned previously.  Therefore, the 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 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 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 using the "scale-
   out" Clos topology.

7.5.  Routing Micro-Loops

   When a downstream device, e.g. Tier-2 device, loses a path for a
   prefix, it normally has the default route pointing toward the
   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,
   but Tier-1 switch still has the path which results in transient
   micro-loop, since Tier-1 switch will keep passing packets to the
   affected prefix back to Tier-2 device, and Tier-2 will bounce it back
   again using the default route.  This micro-loop will last for the
   duration of time it takes the upstream device to fully update its
   forwarding tables.

   To minimize impact of the micro-loops, Tier-2 and Tier-1 switches can
   be configured with static "discard" or "null0" routes that will be
   more specific than the default route for specific prefixes missing
   during network convergence.  For Tier-2 switches, the discard route
   should be an aggregate route, covering all server subnets of the
   underlying Tier-3 devices.  For Tier-1 devices, the discard route
   should be an aggregate covering the server IP address subnet
   allocated for the whole data-center.  Those discard routes will only
   take precedence for the duration of network convergence, until the
   device learns a more specific prefix via a new path.

8.  Additional Options for Design

8.1.  Third-party Route Injection

   BGP allows for a "third-party", or not directly attached, BGP speaker
   to inject routes anywhere in the network topology, meeting REQ5.
   This can be achieved by peering using a multihop BGP session with
   some or even all devices in the topology.  Furthermore, BGP diverse
   path distribution [RFC6774] could be used to inject multiple BGP next



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   hops 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
   implementation.  Unfortunately in many implementations ADD-PATH has
   been found to only support IBGP properly due to the use cases it was
   originally optimized for.

   To implement route injection in the proposed design a third-party 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 devices.
   Those next-hops are assumed to resolve recursively via BGP, and could
   be, for example, IP addresses on Tier-3 devices.  The resulting
   forwarding table programming could provide desired traffic proportion
   distribution among different clusters.

8.2.  Route Aggregation within Clos Topology

   As mentioned previously, route aggregation is not possible within the
   proposed Clos topology since it makes the network susceptible to
   route black-holing under single link failures.  The main problem is
   the limited number of parallel paths between network elements, such
   as when there is only a single path between any pair of Tier-1 and
   Tier-3 devices.  However, some operators may find route aggregation
   desirable to improve control plane stability.

   By changing the network topology route aggregation can be allowed, if
   necessary, though the trade-off would be reduction of the total size
   of the network as well as network congestion under specific failures.
   This approach is very similar to the technique described above, which
   allows Border Routers to summarize the entire data-center address
   space.

8.2.1.  Collapsing Tier-1 Devices Layer

   In order to add more paths between Tier-1 and Tier-3 devices, group
   Tier-2 devices into pairs, and then connect the pairs to the same
   group of Tier-1 devices.  This is logically equivalent to
   "collapsing" Tier-1 devices into a group of half the size, merging
   the links on the "collapsed" devices.  The result is illustrated in
   Figure 6.  For example, in this topology DEV C and DEV D connect to
   the same set of Tier-1 devices (DEV 1 and DEV 2), whereas before they
   were connecting to different groups of Tier-1 devices.

                    Tier-2       Tier-1       Tier-2
                   +-----+      +-----+      +-----+
     +-------------| DEV |------| DEV |------|     |-------------+
     |       +-----|  C  |--++--|  1  |--++--|     |-----+       |
     |       |     +-----+  ||  +-----+  ||  +-----+     |       |
     |       |              ||           ||              |       |



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     |       |     +-----+  ||  +-----+  ||  +-----+     |       |
     | +-----+-----| DEV |--++--| DEV |--++--|     |-----+-----+ |
     | |     | +---|  D  |------|  2  |------|     |---+ |     | |
     | |     | |   +-----+      +-----+      +-----+   | |     | |
     | |     | |                                       | |     | |
   +-----+ +-----+                                   +-----+ +-----+
   | DEV | | DEV |                                   |     | |     |
   |  A  | |  B  | Tier-3                     Tier-3 |     | |     |
   +-----+ +-----+                                   +-----+ +-----+
     | |     | |                                       | |     | |
     O O     O O             <- Servers ->             O O     O O

                      Figure 6: 5-Stage Clos topology

   In this design choice, Tier-2 devices may be configured to 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 the second
   available path known to a Tier-2 switch.  It is not 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 down to this
   prefix.  It would require dual-homed servers to accomplish that.
   Also note that this design is only resilient to single link failure.
   It is possible for a double link failure to isolate a Tier-2 device
   from all paths toward a specific Tier-3 device, thus causing a
   routing black-hole.

8.2.2.  Implications of Collapsing Tier-1 Devices Layer

   As mentioned already, a result of the proposed topology modification
   would be reduction of Tier-1 devices port capacity.  This limits the
   maximum number of attached Tier-2 devices and therefore will limit
   the maximum DC network size.  A larger network would require
   different Tier-1 devices that have higher port density to implement
   this change.

   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
   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
   result in doubling of link utilization on the remaining link.

9.  Security Considerations

   The design does not introduce any additional security concerns.
   General BGP security considerations are discussed in [RFC4271] and
   [RFC4272].  Furthermore, the Generalized TTL Security Mechanism
   [RFC5082] could be used to reduce the risk of BGP session spoofing.




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10.  IANA Considerations

   This document includes no request to IANA.

11.  Acknowledgements

   This publication summarizes work of many people who participated in
   developing, testing and deploying the proposed network design, some
   of whom were George Chen, Parantap Lahiri, Dave Maltz, Edet Nkposong,
   Robert Toomey, and Lihua Yuan.  Authors would also like to thank
   Linda Dunbar and Susan Hares for reviewing the document and providing
   valuable feedback and Mary Mitchell for grammar and style
   suggestions.

12.  References

12.1.  Normative References

   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.

   [I-D.ietf-idr-as-private-reservation]
              Mitchell, J., "Autonomous System (AS) Reservation for
              Private Use", draft-ietf-idr-as-private-reservation-05
              (work in progress), May 2013.

12.2.  Informative References

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

   [RFC4272]  Murphy, S., "BGP Security Vulnerabilities Analysis", RFC
              4272, January 2006.

   [RFC4786]  Abley, J. and K. Lindqvist, "Operation of Anycast
              Services", BCP 126, RFC 4786, December 2006.

   [RFC5082]  Gill, V., Heasley, J., Meyer, D., Savola, P., and C.
              Pignataro, "The Generalized TTL Security Mechanism
              (GTSM)", RFC 5082, October 2007.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, June 2010.

   [RFC6325]  Perlman, R., Eastlake, D., Dutt, D., Gai, S., and A.
              Ghanwani, "Routing Bridges (RBridges): Base Protocol
              Specification", RFC 6325, July 2011.





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   [RFC6774]  Raszuk, R., Fernando, R., Patel, K., McPherson, D., and K.
              Kumaki, "Distribution of Diverse BGP Paths", RFC 6774,
              November 2012.

   [RFC6793]  Vohra, Q. and E. Chen, "BGP Support for Four-Octet
              Autonomous System (AS) Number Space", RFC 6793, December
              2012.

   [I-D.ietf-idr-add-paths]
              Walton, D., Retana, A., Chen, E., and J. Scudder,
              "Advertisement of Multiple Paths in BGP", draft-ietf-idr-
              add-paths-08 (work in progress), December 2012.

   [I-D.ietf-idr-link-bandwidth]
              Mohapatra, P. and R. Fernando, "BGP Link Bandwidth
              Extended Community", draft-ietf-idr-link-bandwidth-06
              (work in progress), January 2013.

   [GREENBERG2009]
              Greenberg, A., Hamilton, J., and D. Maltz, "The Cost of a
              Cloud: Research Problems in Data Center Networks", January
              2009.

   [IEEE8021AG]
              IEEE 802.1Q, ., "IEEE Standard for Local and metropolitan
              area networks - Media Access Control (MAC) Bridges and
              Virtual Bridged Local Area Networks", October 2012.

   [IEEE8023AH]
              IEEE 802.3, ., "IEEE Standard for Information technology -
              Local and metropolitan area networks - Carrier sense
              multiple access with collision detection (CSMA/CD) access
              method and physical layer specifications", December 2008.

   [INTERCON]
              Dally, W. and B. Towles, "Principles and Practices of
              Interconnection Networks", ISBN 978-0122007514, January
              2004.

   [ALFARES2008]
              Al-Fares, M., Loukissas, A., and A. Vahdat, "A Scalable,
              Commodity Data Center Network Architecture", August 2008.

   [IANA.AS]  IANA, ., "Autonomous System (AS) Numbers", July 2013,
              <http://www.iana.org/assignments/as-numbers/>.

Authors' Addresses




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   Petr Lapukhov
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   US

   Phone: +1 425 703 2723
   Email: petrlapu@microsoft.com
   URI:   http://microsoft.com/


   Ariff Premji
   Arista Networks
   5470 Great America Parkway
   Santa Clara, CA  95054
   US

   Phone: +1 408 547 5699
   Email: ariff@aristanetworks.com
   URI:   http://aristanetworks.com/


   Jon Mitchell (editor)
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   US

   Email: Jon.Mitchell@microsoft.com






















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