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IDR                                                          P. Lapukhov
Internet-Draft                                           Microsoft Corp.
Intended status: Informational                                 A. Premji
Expires: January 15, 2013                                Arista Networks
                                                           July 14, 2012


           Using BGP for routing in large-scale data centers
                 draft-lapukhov-bgp-routing-large-dc-01

Abstract

   Some service providers build and operate data centers that support
   over 100,000 servers.  In this document, such data-centers are
   referred to as "large-scale" data centers to differentiate them the
   from more common smaller infrastructures.  The data centers of this
   scale have a unique set of network requirements, with emphasis on
   operational simplicity and network stability.

   This document attempts to summarize the authors' experiences in
   designing and supporting large data centers, using BGP as the only
   control-plane protocol.  The intent here is to describe 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
   Task Force (IETF).  Note that other groups may also distribute
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   This Internet-Draft will expire on January 15, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal



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   Provisions Relating to IETF Documents
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Traditional data center designs  . . . . . . . . . . . . . . .  3
     2.1.  Layer 2 Designs  . . . . . . . . . . . . . . . . . . . . .  3
     2.2.  Fully routed network designs . . . . . . . . . . . . . . .  4
   3.  Document structure . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Network design requirements  . . . . . . . . . . . . . . . . .  5
     4.1.  Traffic patterns . . . . . . . . . . . . . . . . . . . . .  5
     4.2.  CAPEX minimization . . . . . . . . . . . . . . . . . . . .  6
     4.3.  OPEX minimization  . . . . . . . . . . . . . . . . . . . .  6
     4.4.  Traffic Engineering  . . . . . . . . . . . . . . . . . . .  7
   5.  Requirement List . . . . . . . . . . . . . . . . . . . . . . .  7
   6.  Network topology . . . . . . . . . . . . . . . . . . . . . . .  7
     6.1.  Clos topology overview . . . . . . . . . . . . . . . . . .  8
     6.2.  Clos topology properties . . . . . . . . . . . . . . . . .  8
     6.3.  Scaling Clos topology  . . . . . . . . . . . . . . . . . .  9
   7.  Routing design . . . . . . . . . . . . . . . . . . . . . . . . 10
     7.1.  Choosing the routing protocol  . . . . . . . . . . . . . . 10
     7.2.  BGP configuration for Clos topology  . . . . . . . . . . . 11
       7.2.1.  BGP Autonomous System numbering layout . . . . . . . . 11
       7.2.2.  Non-unique private BGP ASN's . . . . . . . . . . . . . 12
       7.2.3.  Prefix advertisement . . . . . . . . . . . . . . . . . 13
       7.2.4.  External connectivity  . . . . . . . . . . . . . . . . 13
     7.3.  ECMP Considerations  . . . . . . . . . . . . . . . . . . . 14
       7.3.1.  Basic ECMP . . . . . . . . . . . . . . . . . . . . . . 14
       7.3.2.  BGP ECMP over multiple ASN . . . . . . . . . . . . . . 15
     7.4.  BGP convergence properties . . . . . . . . . . . . . . . . 16
       7.4.1.  Convergence timing . . . . . . . . . . . . . . . . . . 16
       7.4.2.  Failure impact scope . . . . . . . . . . . . . . . . . 16
       7.4.3.  Third-party route injection  . . . . . . . . . . . . . 17
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
   11. Informative References . . . . . . . . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19





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1.  Introduction

   This document presents a practical routing design that can be used in
   large-scale data centers.  Such data centers, also known as hyper-
   scale or warehouse scale data centers, have a unique attribute of
   supporting over a 100,000 end hosts.  In order to support networks of
   such scale, operators are revisiting networking designs and platforms
   to address this need..  Contrary to the more traditional data center
   designs, the approach presented in this document does not have any
   dependency on building a large Layer-2 domain and instead relies on
   routing at every layer in the network.  Implementing a pure Layer-3
   design using BGP further ensures broad vendor support and almost
   guarantees interoperability between vendors given that BGP is one of
   the most widely deployed protocols on the Internet.


2.  Traditional data center designs

   This section provides an overview of two types of traditional data
   center designs - Layer-2 and fully routed Layer-3 topologies.

2.1.  Layer 2 Designs

   In the networking industry, a common design choice for data centers
   is to use a mix of Ethernet-based Layer 2 technologies.  Network
   topologies typically look like a tree with redundant uplinks and
   three levels of hierarchy commonly named Core , Aggregation and
   Access layers (see Figure 1).  To accommodate bandwidth demands,
   every next level has higher port density and bandwidth capacity,
   moving upwards in the topology.  To keep terminology uniform, tn this
   document, these topology layers will be referred to as "tiers", e.g.
   Tier 1, Tier 2 and Tier 3 instead of Core, Aggregation or Access
   layers.


















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                       +------+  +------+
                       |      |  |      |
                       |      |--|      |           Tier1
                       |      |  |      |
                       +------+  +------+
                         |  |      |  |
               +---------+  |      |  +----------+
               | +-------+--+------+--+-------+  |
               | |       |  |      |  |       |  |
             +----+     +----+    +----+     +----+
             |    |     |    |    |    |     |    |
             |    |-----|    |    |    |-----|    | Tier2
             |    |     |    |    |    |     |    |
             +----+     +----+    +----+     +----+
                |         |          |         |
                |         |          |         |
                | +-----+ |          | +-----+ |
                +-|     |-+          +-|     |-+    Tier3
                  +-----+              +-----+
                   | | |                | | |
                 [Servers]            [Servers]


               Figure 1: Typical Data Center network layout

   IP routing is normally used only at the upper layers in the topology,
   e.g.  Tier 1 or Tier 2.  Some of the reasons for introducing such
   large (sometimes called stretched) layer-2 domains are:

   o  Supporting legacy applications that may require direct Layer 2
      adjacency or use non-IP protocols
   o  Seamless mobility for virtual machines, to allow the preservation
      of IP addresses when a virtual machine moves across physical hosts
   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 Level 2 Direct Server Return
      (DSR)

2.2.  Fully routed network designs

   Network designs that leverage IP routing down to the access layer
   (Tier 3) of the network have gained popularity as well.  The main
   benefit of such designs is improved network stability and
   scalability, as a result of confining L2 broadcast domains.  A common
   choice of routing protocol for data center designs would be an IGP,
   such as OSPF or ISIS.  As data centers grow in scale, and server
   count exceeds tens of thousands, such fully routed designs become



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   more attractive.

   Although BGP is the de-facto standard protocol for routing on the
   Internet, having wide support from both the vendor and service
   provider communities, it is not generally deployed in data centers
   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.
   o  BGP deployment within an Autonomous System (iBGP mesh) is assumed
      to have a dependency on the presence of an IGP, which assists with
      recursive next-hop resolution.
   o  BGP is perceived to require significant configuration overhead and
      does not support any form of neighbor auto-discovery.

   In this document we demonstrate a practical approach for using BGP as
   the single routing protocol for data center networks.


3.  Document structure

   The remaining of this document is organized as following.  First the
   design requirements for large scale data centers are presented.
   Next, the document gives an overview of Clos network topology and its
   properties.  After that, the reasons for selecting BGP as the single
   routing protocols are presented.  Finally, the document discusses the
   design in more details and covers specific BGP policy features.


4.  Network design requirements

   This section describes and summarizes network design requirement for
   a large-scale data center.

4.1.  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
   traffic flows mostly entering and leaving the data center (also known
   as north-south traffic) There were no intense, highly meshed flows or
   traffic patterns between the machines within the same tier.  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, by upgrading line-cards or switch fabrics.



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   In contrast, large-scale data centers often host applications that
   generate significant amount of server to server traffic, also known
   as "east-west" traffic.  Examples of such applications could be
   compute clusters such as Hadoop or live virtual machine migrations.
   Scaling up traditional tree topologies to match these bandwidth
   demands becomes either too expensive or impossible due to physical
   limitations.

4.2.  CAPEX minimization

   The cost of the network infrastructure alone (CAPEX) constitutes
   about 10-15% of total data center expenditure [GREENBERG2009].
   However, The absolute cost is significant, and there is a need to
   constantly drive down the cost of networking elements themselves.
   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 by introducing multiple network equipment
      vendors.

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

4.3.  OPEX minimization

   Operating large scale infrastructure could be expensive, provide that
   larger amount of elements will statistically fail more often.  Having
   a simpler design and operating using a limited software feature-set
   ensures that failures will mostly result from hardware malfunction
   and not software issues.

   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 storms.  The use of a fully
   routed design significantly reduces the size of the data-plane
   failure domains (e.g. limits to Tier-3 switches only).  However, such
   designs also introduce the problem of distributed control-plane
   failures.  This calls for simpler control-plane protocols that are
   expected to have less chances of network meltdown.







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4.4.  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 to use network
   infrastructure itself to distribute traffic across a group of load-
   balancers.  A combination of features such as Anycast prefix
   advertisement [RFC4786] along with Equal Cost Multipath (ECMP)
   functionality could be used to accomplish this.  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.


5.  Requirement List

   This section summarizes the list of requirements, based on the
   discussion so far:

   o  REQ1: Select a network topology where capacity could be scaled
      "horizontally" by adding more links and network switches of the
      same type, without requiring an upgrade 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: Among the network protocols, choose the one that has a
      simpler implementation in terms of minimal programming code
      complexity.
   o  REQ4: The network routing protocol should allow for explicit
      control of the routing prefix next-hop set on per-hop basis.


6.  Network topology

   This section outlines the most common choice for horizontally
   scalable topology in large scale data centers.







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6.1.  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 odd number of stages
   (dimensions) and is commonly made of the same uniform elements, e.g.
   switches with the same port count.  Therefore, the choice of Clos
   topology satisfies both REQ1 and REQ2.  See Figure 2 below for an
   example of folded 3-stage Clos topology:

             +-------+
             |       |----------------------------+
             |       |------------------+         |
             |       |--------+         |         |
             +-------+        |         |         |
             +-------+        |         |         |
             |       |--------+---------+-------+ |
             |       |--------+-------+ |       | |
             |       |------+ |       | |       | |
             +-------+      | |       | |       | |
             +-------+      | |       | |       | |
             |       |------+-+-------+-+-----+ | |
             |       |------+-+-----+ | |     | | |
             |       |----+ | |     | | |     | | |
             +-------+    | | |     | | |   ---------> M links
               Tier1      | | |     | | |     | | |
                        +-------+ +-------+ +-------+
                        |       | |       | |       |
                        |       | |       | |       | Tier2
                        |       | |       | |       |
                        +-------+ +-------+ +-------+
                          | | |     | | |     | | |
                          | | |     | | |   ---------> N Links
                          | | |     | | |     | | |
                          O O O     O O O     O O O   Servers

                  Figure 2: 3-Stage Folded Clos topology

   In the networking industry, a topology like this is sometimes
   referred to as "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).  However, for consistency,
   we will refer to these layers as "Tier n".

6.2.  Clos topology properties

   The following are some key properties of the Clos topology:




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   o  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 Tier 2 switch, as shown on Figure 2
   o  Implementing Clos topology requires a routing protocol supporting
      ECMP with the fan-out of M or more
   o  Every Tier 1 device has exactly one path to every end host
      (server) in this topology
   o  Traffic flowing from server to server is naturally load-balanced
      over all available paths using simple ECMP behavior

6.3.  Scaling Clos topology

   A Clos topology could be scaled either by increasing network switch
   port count or adding more stages, e.g. moving to a 5-stage Clos, as
   illustrated on Figure 3 below:

                                  Tier1
                                 +-----+
                                 |     |
                              +--|     |--+
                              |  +-----+  |
                      Tier2   |           |   Tier2
                     +-----+  |  +-----+  |  +-----+
       +-------------| DEV |--+--|     |--+--|     |-------------+
       |       +-----|  C  |--+  |     |  +--|     |-----+       |
       |       |     +-----+     +-----+     +-----+     |       |
       |       |                                         |       |
       |       |     +-----+     +-----+     +-----+     |       |
       | +-----+-----| DEV |--+  |     |  +--|     |-----+-----+ |
       | |     | +---|  D  |--+--|     |--+--|     |---+ |     | |
       | |     | |   +-----+  |  +-----+  |  +-----+   | |     | |
       | |     | |            |           |            | |     | |
     +-----+ +-----+          |  +-----+  |          +-----+ +-----+
     | DEV | | DEV |          +--|     |--+          |     | |     |
     |  A  | |  B  | Tier3       |     |       Tier3 |     | |     |
     +-----+ +-----+             +-----+             +-----+ +-----+
       | |     | |                                     | |     | |
       O O     O O            <- Servers ->            O O     O O


                      Figure 3: 5-Stage Clos topology

   The topology on Figure 3 is built from switches with port count of 4
   and provides full bisection bandwidth to all connected servers.  We
   will refer to the collection of directly connected Tier 2 and Tier 3
   switches as a "cluster" in this document.  For example, devices A, B,
   C, and D on Figure 3 form a cluster.



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   In practice, the Tier 3 level of the network (typically top of rack
   switches, or ToRs) is where oversubscription is introduced to allow
   for packaging of more servers in data center.  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 two bandwidth pools: within the same access switch (e.g. rack)
   and outside of the local switch Since oversubscription itself does
   not have any effect on routing, we will not be discussing it further
   in this document


7.  Routing design

   This section discusses the motivation for choosing BGP as the routing
   protocol and BGP configuration for routing in Clos topology.

7.1.  Choosing the routing protocol

   The set of requirements discussed earlier call for a single routing
   protocol (REQ2) to reduce complexity and interdependencies.  While it
   is common to rely on an IGP in this situation, the document proposes
   the use of BGP only.  The advantages of using BGP are discussed
   below.

   o  BGP inherently has less complexity within its protocol design -
      internal data structures and state-machines are simpler when
      compared to a link-state IGP.  For example, instead of
      implementing adjacency formation, adjacency maintenance and/or
      flow-control, BGP simply relies on TCP as the underlying
      transport.  This fulfills REQ1 and REQ2.
   o  BGP information flooding overhead is less when compared to link-
      state IGPs.  Indeed, since every BGP router normally re-calculates
      and propagates best-paths only, a network failure is masked as
      soon as the BGP speaker finds an alternate path.  In contrary, the
      event propagation scope of a link-state IGP is single flooding
      domain, regardless of the failure type.  Furthermore, all well-
      known link-state IGPs feature periodic refresh updates, while BGP
      does not expire routing state.
   o  BGP supports third-party (recursively resolved) next-hops.  This
      allows for ECMP or forwarding based on customer-defined forwarding
      paths.  This satisfied REQ4 stated above.  Some IGPs, such as
      OSPF, support similar functionality using special concepts such as
      "Forwarding Address", but do not satisfy other requirement, such
      as protocol simplicity.
   o  Vanilla BGP configuration, without routing policies, is easier to
      troubleshoot for network reachability issues.  For example, it is
      straightforward to dump contents of LocRIB and compare it to the
      router's RIB and FIB.  Furthermore, every BGP neighbor has



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      corresponding AdjRIBIn and AdjRIBOut structures with incoming/
      outgoing NRLI information that could be easily correlated on both
      sides of the BGP peering session.  Thus BGP fully satisfies REQ3.

7.2.  BGP configuration for Clos topology

   Topologies that have more than 5 stages are very uncommon due to the
   large numbers of interconnects required by such a design.

7.2.1.  BGP Autonomous System numbering layout

   The diagram below illustrates suggests BGP Autonomous System Number
   (BGP ASN) allocation scheme.  The following is a list of guidelines
   that can be used:

   o  All BGP peering sessions are external BGP (eBGP) 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 64512-64534 are used so as to
      avoid ASN conflicts.  The private ASN stripping feature can be
      leveraged as a result (see below).
   o  A single BGP ASN is allocated to the Clos middle stage ("Tier 1"),
      e.g.  ASN 64534 as shown in Figure 4
   o  Unique BGP ASN is allocated per group of "Tier 2" switches.  All
      Tier 2 switches in the same group share the BGP ASN.
   o  Unique BGP ASN is allocated to every Tier 3 switch (e.g.  ToR) in
      this topology.






















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                                ASN 64534
                               +---------+
                               | +-----+ |
                               | |     | |
                             +-|-|     |-|-+
                             | | +-----+ | |
                  ASN 64XXX  | |         | |  ASN 64XXX
                 +---------+ | |         | | +---------+
                 | +-----+ | | | +-----+ | | | +-----+ |
     +-----------|-|     |-|-+-|-|     |-|-+-|-|     |-|-----------+
     |       +---|-|     |-|-+ | |     | | +-|-|     |-|---+       |
     |       |   | +-----+ |   | +-----+ |   | +-----+ |   |       |
     |       |   |         |   |         |   |         |   |       |
     |       |   |         |   |         |   |         |   |       |
     |       |   | +-----+ |   | +-----+ |   | +-----+ |   |       |
     | +-----+---|-|     |-|-+ | |     | | +-|-|     |-|---+-----+ |
     | |     | +-|-|     |-|-+-|-|     |-|-+-|-|     |-|-+ |     | |
     | |     | | | +-----+ | | | +-----+ | | | +-----+ | | |     | |
     | |     | | +---------+ | |         | | +---------+ | |     | |
     | |     | |             | |         | |             | |     | |
   +-----+ +-----+           | | +-----+ | |           +-----+ +-----+
   | ASN | |     |           +-|-|     |-|-+           |     | |     |
   |65YYY| | ... |             | |     | |             | ... | | ... |
   +-----+ +-----+             | +-----+ |             +-----+ +-----+
     | |     | |               +---------+               | |     | |
     O O     O O              <- Servers ->              O O     O O


                 Figure 4: BGP ASN layout for 5-stage Clos

7.2.2.  Non-unique private BGP ASN's

   The use of private BGP ASNs limits to the usable range of 1022 unique
   numbers.  Since it is very likely that the number of network switches
   could exceed this number, a workaround is required.  One approach
   would be to re-use the private ASN's assigned to the Tier 3 switches
   across different clusters.  For example, private BGP ASN's 65001,
   65002 ... 65032 could be used within every individual cluster to be
   assigned to Tier 3 switches.

   To avoid route suppression due to AS PATH loop prevention, upstream
   eBGP sessions on Tier 3 switches 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 the
   opportunity for routing loops under misconfiguration since the AS
   PATH is always incremented when routes are propagated from tier to
   tier.




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   Another solution to this problem would be to using four-octet (32-
   bit) BGP ASNs.  However, there are no reserved private ASN range in
   the four-octet numbering scheme although efforts are underway to
   support this, see [I-D.mitchell-idr-as-private-reservation].  This
   will also require vendors to implement specific policy features, such
   as four-octet private AS removal from AS-PATH attribute.

7.2.3.  Prefix advertisement

   A Clos topology has a large number of point-to-point links and
   associated prefixes.  Advertising all of these routes into BGP may
   create FIB overload conditions.  There are two possible solutions
   that can help prevent FIB overload:

   o  Do not advertise any of the point-to-point links into BGP.  Since
      eBGP peering changes the next-hop address anyways at every node,
      distant networks will automatically be reachable via the
      advertising eBGP peer
   o  Advertising point-to-point links, but summarizing them on every
      advertising device.  This requires proper address allocation, for
      example allocating a consecutive block of IP addresses per Tier 1
      and Tier 2 device to be used for point-to-point interface
      addressing.

   Server facing subnets on Tier 3 switches are announced into BGP
   without using summarization on Tier 2 and Tier 1 switches.
   Summarizing subnets in the Clos topology will result in route black-
   holing under a single link failure (e.g. between Tier 2 and Tier 3
   switch) and hence must be avoided.  The use of peer links within the
   same tier to resolve the black-holing problem is undesirable due to
   O(N^2) complexity of the peering mesh and waste of ports on the
   switches.

7.2.4.  External connectivity

   A dedicate cluster (or clusters) in the Clos topology could be used
   solely for the purpose of connecting to the Wide Area Network (WAN)
   edge devices, or WAN Routers.  Tier 3 switches in such a cluster
   would be replaced with WAN Routers, but eBGP peering would be used
   again, though WAN routers are likely to belong to a public ASN.

   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.
      This is typically done to avoid BGP ASN number collisions across



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      the data centers.  A BGP policy feature called "Remove Private AS"
      is commonly used to accomplish this.  This feature strips 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 could 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.

7.3.  ECMP Considerations

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

7.3.1.  Basic ECMP

   ECMP is the fundamental load-sharing mechanism used by a Clos
   topology.  Effectively, every lower-tier switch will use all of its
   directly attached upper-tier devices to load-share traffic destined
   to the same prefix.  Number of ECMP paths between two input/output
   switches in Clos topology equals to the number of the switches 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.























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                                  Tier 1
                                 +-----+
                                 | DEV |
                              +->|  1  |--+
                              |  +-----+  |
                      Tier 2  |           |   Tier 2
                     +-----+  |  +-----+  |  +-----+
       +------------>| DEV |--+->| DEV |--+--|     |-------------+
       |       +-----|  B  |--+  |  2  |  +--|     |-----+       |
       |       |     +-----+     +-----+     +-----+     |       |
       |       |                                         |       |
       |       |     +-----+     +-----+     +-----+     |       |
       | +-----+---->| DEV |--+  | DEV |  +--|     |-----+-----+ |
       | |     | +---|  C  |--+->|  3  |--+--|     |---+ |     | |
       | |     | |   +-----+  |  +-----+  |  +-----+   | |     | |
       | |     | |            |           |            | |     | |
     +-----+ +-----+          |  +-----+  |          +-----+ +-----+
     | 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.  Normally, this number does
   not exceed half of the ports found on a switch in the topology.  For
   example, an ECMP max-path of 32 would be required when building a
   Clos network using 64-port devices.

   Most implementations declare paths to be equal from ECMP perspective
   if they match up to and including step (e) in Section 9.1.2.2 of
   [RFC4271].  In the proposed network design there is no underlying
   IGP, so all IGP costs are automatically assumed to be zero (or
   otherwise the same value across all paths).  Loop prevention is
   assumed to be handled by the BGP best-path selection process.

7.3.2.  BGP ECMP over multiple ASN

   For application load-balancing purposes we may want the same prefix
   to be advertised from multiple Tier-3 switches.  From the perspective
   of other devices, such a prefix would have BGP paths with different
   AS PATH attribute values, though having the same AS PATH attribute
   lengths.  Therefore, the BGP implementations must support load-
   sharing over above-mentioned paths.  This feature is sometimes known



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   as "AS PATH multipath relax" and effectively allows for ECMP to be
   done across different neighboring ASNs.

7.4.  BGP convergence properties

   This section reviews routing convergence properties of BGP in the
   proposed design.  A case is made that sub-second convergence is
   achievable provided that implementation supports fast BGP peering
   session shutdown upon failure of an associated link.

7.4.1.  Convergence 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 omits the use of an IGP, so the only
   mechanisms that could be used for fault detection are BGP keep-alives
   and link-failure triggers.

   Relying solely on BGP keep-alive packets may result in high
   convergence delays, in the order of multiple seconds (normally, the
   minimum recommended BGP hold time value is 3 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 fail-over".
   Since the majority of the links in modern data centers are point to
   point fiber connections, a physical interface failure if often
   detected in milliseconds and subsequently triggers a BGP re-
   convergence.

   Furthermore, popular link technologies, such as 10Gbps Ethernet, may
   support a simple form of OAM for failure signaling such as
   [FAULTSIG10GE], which makes failure detection more robust.
   Alternatively, as opposed to relying on physical layer for fault
   signaling, some platforms may support Bidirectional Forwarding
   Detection ([RFC5880]) to allow for sub-second failure detection and
   fault signaling to the BGP process.  This, however, presents
   additional requirements to vendor software and possibly hardware, and
   may contradict REQ1.

7.4.2.  Failure impact scope

   BGP is inherently a distance-vector protocol, and as such some of
   failures could be masked if the local node can immediately find a
   backup path.  The worst case is that all devices in data center
   topology would have to either withdraw a prefix completely, or
   recalculate the ECMP paths in the FIB.  Reducing the fault domain
   using summarization is not possible with the proposed design, since



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   using this technique may create route black-holing issues as
   mentioned previously.  Thus, the control-plane failure impact scope
   is the network as a whole.  It is worth pointing that such property
   is not a result of choosing BGP, but rather a result of using the
   "scale-out" Clos topology.

7.4.3.  Third-party route injection

   BGP allows for a third-party BGP speaker (not necessarily directly
   attached to the network devices) to inject routes anywhere in the
   network topology.  This could be achieved by peering an external
   speaker using an eBGP multi-hop session with some or even all devices
   in the topology.  Furthermore, BGP diverse path distribution
   [I-D.ietf-grow-diverse-bgp-path-dist] could be used to inject
   multiple next-hop for the same prefix to facilitate load-balancing.
   Using such a technique would make it possible to implement unequal-
   cost load-balancing across multiple clusters in the data-center, by
   associating the same prefix with next-hops mapped to different
   clusters.

   For example, 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 switches.  The resulting forwarding table programming could provide
   desired traffic proportion distribution among different clusters.


8.  Security Considerations

   The design does not introduce any additional security concerns.  For
   control plane security, BGP peering sessions could be authenticated
   using TCP MD5 signature extension header [RFC2385].  Furthermore, BGP
   TTL security [I-D.gill-btsh] could be used to reduce the risk of
   session spoofing and TCP SYN flooding attacks against the control
   plane.


9.  IANA Considerations

   There are no considerations associated with IANA for this document.


10.  Acknowledgements

   This publication summarizes work of many people who participated in
   developing, testing and deploying the proposed design.  Their names,
   in alphabetical order, are George Chen, Parantap Lahiri, Dave Maltz,



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   Edet Nkposong, Robert Toomey, and Lihua Yuan.  Authors would also
   like to thank Jon Mitchell, Linda Dunbar and Susan Hares for
   reviewing and providing valuable feedback on the document.


11.  Informative References

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

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

   [RFC2385]  Heffernan, A., "Protection of BGP Sessions via the TCP MD5
              Signature Option", RFC 2385, August 1998.

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

   [I-D.ietf-grow-diverse-bgp-path-dist]
              Raszuk, R., Fernando, R., Patel, K., McPherson, D., and K.
              Kumaki, "Distribution of diverse BGP paths.",
              draft-ietf-grow-diverse-bgp-path-dist-07 (work in
              progress), May 2012.

   [I-D.mitchell-idr-as-private-reservation]
              Mitchell, J., "Autonomous System (AS) Reservation for
              Private Use", draft-mitchell-idr-as-private-reservation-00
              (work in progress), June 2012.

   [I-D.gill-btsh]
              Gill, V., Heasley, J., and D. Meyer, "The BGP TTL Security
              Hack (BTSH)", draft-gill-btsh-02 (work in progress),
              May 2003.

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

   [FAULTSIG10GE]
              Frazier, H. and S. Muller, "Remote Fault & Break Link
              Proposal for 10-Gigabit Ethernet", September 2000.

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



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


Authors' Addresses

   Petr Lapukhov
   Microsoft Corp.
   One Microsfot Way
   Redmond, WA  98052
   US

   Phone: +1 425 7032723 X 32723
   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/
























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