Network Working Group                                        P. Lapukhov
Internet-Draft                                                  Facebook
Intended status: Informational                                 A. Premji
Expires: August 10, 2014                                 Arista Networks
                                                        J. Mitchell, Ed.
                                                   Microsoft Corporation
                                                        February 6, 2014

           Use of BGP for routing in large-scale data centers


   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

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Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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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 . . . . . . . . . . . . . . . . .   5
   3.  Data Center Topologies Overview . . . . . . . . . . . . . . .   6
     3.1.  Traditional DC Topology . . . . . . . . . . . . . . . . .   6
     3.2.  Clos Network topology . . . . . . . . . . . . . . . . . .   7
       3.2.1.  Overview  . . . . . . . . . . . . . . . . . . . . . .   7
       3.2.2.  Clos Topology Properties  . . . . . . . . . . . . . .   8
       3.2.3.  Scaling the Clos topology . . . . . . . . . . . . . .   9
       3.2.4.  Managing the Size of Clos Topology Tiers  . . . . . .  10
   4.  Data Center Routing Overview  . . . . . . . . . . . . . . . .  10
     4.1.  Layer 2 Only Designs  . . . . . . . . . . . . . . . . . .  11
     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 . . . . . . . . . .  13
     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 . . . . . . . . . . . . . . . .  17
       5.2.5.  Route Summarization at the Edge . . . . . . . . . . .  18
   6.  ECMP Considerations . . . . . . . . . . . . . . . . . . . . .  19
     6.1.  Basic ECMP  . . . . . . . . . . . . . . . . . . . . . . .  19
     6.2.  BGP ECMP over Multiple ASNs . . . . . . . . . . . . . . .  20
     6.3.  Weighted ECMP . . . . . . . . . . . . . . . . . . . . . .  20
     6.4.  Consistent Hashing  . . . . . . . . . . . . . . . . . . .  21
   7.  Routing Convergence Properties  . . . . . . . . . . . . . . .  21
     7.1.  Fault Detection Timing  . . . . . . . . . . . . . . . . .  21
     7.2.  Event Propagation Timing  . . . . . . . . . . . . . . . .  22
     7.3.  Impact of Clos Topology Fan-outs  . . . . . . . . . . . .  22
     7.4.  Failure Impact Scope  . . . . . . . . . . . . . . . . . .  23

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     7.5.  Routing Micro-Loops . . . . . . . . . . . . . . . . . . .  24
   8.  Additional Options for Design . . . . . . . . . . . . . . . .  25
     8.1.  Third-party Route Injection . . . . . . . . . . . . . . .  25
     8.2.  Route Summarization within Clos Topology  . . . . . . . .  25
       8.2.1.  Collapsing Tier-1 Devices Layer . . . . . . . . . . .  25
       8.2.2.  Simple Virtual Aggregation  . . . . . . . . . . . . .  27
     8.3.  ICMP Unreachable Message Masquerading . . . . . . . . . .  27
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  28
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  28
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  28
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  28
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  28
     12.2.  Informative References . . . . . . . . . . . . . . . . .  29
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  31

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 presented in this document is based on operational
   experience with data centers built to support large scale distributed
   software infrastructure, such as a Web search engine.  The primary
   requirements in such an environment are operational simplicity and
   network stability so that a small group of people can effectively
   support a significantly sized network.

   After experimentation and extensive testing, Microsoft chose 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 simple tree 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

   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.  This
   is followed by arguments for selecting EBGP with a Clos topology as
   the most appropriate routing protocol to meet the requirements and

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   the proposed design is described in detail.  Finally, the document
   reviews some additional considerations and design options.

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, commonly
   referred to as "north-south" traffic.  As a result, traditional
   "tree" topologies were sufficient to accommodate such flows, even
   with high oversubscription ratios between the layers of the network.
   If more bandwidth was required, it was added by "scaling up" the
   network elements, 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, which does not
   egress the DC, commonly referred to as "east-west" traffic.  Examples
   of such applications could be compute clusters such as Hadoop,
   massive data replication between clusters needed by certain
   applications, or virtual machine migrations.  Scaling traditional
   tree topologies to match these bandwidth demands becomes either too
   expensive or impossible due to physical limitations, e.g. port
   density in a switch.

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 hence 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 volume pricing on
      bulk purchases.

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

   In order to allow for good vendor diversity it is important to
   minimize the software feature requirements for the network elements.

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   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 - i.e. limits them to the lowest level in the network
   hierarchy.  However, such designs 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.  Minimizing software feature
   requirements as described in the CAPEX section above also reduces
   testing and training requirements.

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.

2.5.  Summarized Requirements

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

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   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

   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

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.

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             +------+  +------+
             |      |  |      |
             |      |--|      |           Tier-1
             |      |  |      |
             +------+  +------+
               |  |      |  |
     +---------+  |      |  +----------+
     | +-------+--+------+--+-------+  |
     | |       |  |      |  |       |  |
   +----+     +----+    +----+     +----+
   |    |     |    |    |    |     |    |
   |    |-----|    |    |    |-----|    | 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.  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 folded Clos topology satisfies REQ1 and facilitates
   REQ2.  See Figure 2 below for an example of a folded 3-stage Clos
   topology (3 stages counting Tier-2 stage twice, when tracing a packet

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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 often 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 control and data plane supporting
      ECMP with the fan-out of M or more.

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   o  Tier-1 switches have exactly one path to every server in this
      topology.  This is an important property that makes route
      summarization impossible in this topology (see Section 8.2 below).

   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-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

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   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 clusters (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
   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, such as [IEEE8023AD]) 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.  This
   example is using two links for simplicity it should be noted, that
   having more links in a bundle will have less impact on capacity upon
   a member-link failure.

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

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4.1.  Layer 2 Only Designs

   Originally most data center designs used Spanning-Tree Protocol (STP)
   for loop free topology creation, typically utilizing variants of the
   traditional 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 affected the entire spanning-tree 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.  Various enhancements
   to link-aggregation protocols such as [IEEE8023AD], generally known
   as Multi-Chassis Link-Aggregation (M-LAG) made it possible to use
   Layer 2 designs with active-active network paths while relying on STP
   as the backup for loop prevention.  The major downside of this
   approach is proprietary nature of such extensions.

   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 its applicability and
   increased the cost of such designs.

   Finally, neither TRILL nor M-LAG approach eliminate the fundamental
   problem of the shared broadcast domain, that is so detrimental to the
   operations of any Layer 2, Ethernet based solutions.

4.2.  Hybrid L2/L3 Designs

   Operators have sought to limit the impact of data-plane faults 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

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   the network managing multiple protocols.  For the following reasons,
   operators have retained Layer 2 in either the access (Tier-3) or both
   access and aggregation (Tier-3 and Tier-2) parts of the network:

   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 Tier-3 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

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

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.

   Choosing a Layer 3 only design greatly simplifies the network,
   facilitating the meeting of REQ1 and REQ2, and has widespread
   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.

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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

   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 (some of which are interrelated):

   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
      compared to IGPs.

   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 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

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      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" (see [JAKMA2008]) can
      be controlled and complex unwanted paths will be ignored.  See
      Section 5.2 for an example of a working BGP ASN allocation 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

   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

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 reference to the 5-stage
   Clos topology (5 stages in unfolded state).

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.

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   o  16-bit (two octet) BGP ASNs are used, since these are widely
      supported and have better vendor interoperability.

   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

   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
                 +---------+ | |         | | +---------+
                 | +-----+ | | | +-----+ | | | +-----+ |
     +-----------|-|     |-|-+-|-|     |-|-+-|-|     |-|-----------+
     |       +---|-|     |-|-+ | |     | | +-|-|     |-|---+       |
     |       |   | +-----+ |   | +-----+ |   | +-----+ |   |       |
     |       |   |         |   |         |   |         |   |       |
     |       |   |         |   |         |   |         |   |       |
     |       |   | +-----+ |   | +-----+ |   | +-----+ |   |       |
     | +-----+---|-|     |-|-+ | |     | | +-|-|     |-|---+-----+ |
     | |     | +-|-|     |-|-+-|-|     |-|-+-|-|     |-|-+ |     | |
     | |     | | | +-----+ | | | +-----+ | | | +-----+ | | |     | |
     | |     | | +---------+ | |         | | +---------+ | |     | |
     | |     | |             | |         | |             | |     | |
   +-----+ +-----+           | | +-----+ | |           +-----+ +-----+
   | 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 [RFC6996] limited
   operators to 1023 unique ASNs.  Since it is quite likely that the
   number of network devices may exceed this number, a workaround is

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   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.

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: e.g. using the popular "traceroute" tool will display
      IP addresses that 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).

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   Server subnets on Tier-3 devices must be announced into BGP without
   using route summarization on Tier-2 and Tier-1 devices.  Summarizing
   subnets in a Clos topology results in route black-holing under a
   single link failure (e.g. between Tier-2 and Tier-3 devices) and
   hence 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.  An alternative to the full-mesh of peer-links
   would be using a simpler bypass topology, e.g. a "ring" as described
   in [FB4POST], but such a topology adds extra hops and has very
   limited bisection bandwidth, in addition requiring special tweaks to
   make BGP routing work - such as possibly splitting every device into
   an ASN on its own.  In Section 8.2 another, less intrusive, method
   for performing a limited form route summarization in Clos networks
   and the associated trade-offs are 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 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.
      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.  The process for stripping the
      private ASNs is not currently standardized, but most
      implementations commonly follow the logic described in
      [REMOVE-PRIVATE-AS] vendor's document.

   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 risky 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

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      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 is more desirable to take this
      approach rather than introducing complicated conditional default
      origination schemes provided by some implementations.

5.2.5.  Route Summarization at the Edge

   It is often desirable to summarize network reachability information
   prior to advertising it to the WAN network due to high amount of IP
   prefixes originated from within the data center in a fully routed
   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 summarization
   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 using any other "peer-mesh" topology, such as ring or
      hub-and-spoke.  Configure BGP accordingly on all Border Leafs to
      exchange network reachability information - e.g. by adding a mesh
      of iBGP sessions.  The interconnecting peer links need to be
      appropriately sized for traffic that will be present in the case
      of a device or link failure underneath the Border Routers.

   o  Tier-1 devices may have additional physical links provisioned
      toward the Border Routers (which are Tier-2 devices from the
      perspective of Tier-1).  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.  This also reduces the
      number of ports available to "regular" Tier-2 switches and hence
      the number of clusters that could be interconnected via Tier-1

   If any of the above options are implemented, it is possible to
   perform route summarization 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

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   topology as additional links have to be provisioned on some network

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

                               | 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 in upstream or downstream

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   direction.  Normally, this number does not exceed half of the ports
   found on a device in the topology.  For example, an ECMP fan-out of
   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 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

   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

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   signaled using the technique described in

6.4.  Consistent Hashing

   It is often desirable to have the hashing function used to ECMP to be
   consistent (see [CONS-HASH]), to minimizing the impact on flow to
   next-hop affinity changes when a next-hop is added or removed to ECMP
   group.  This could be used if the network device is used as a load-
   balancer, mapping flows toward multiple destinations - in this case,
   losing or adding a destination will not have detrimental effect of
   currently established flows.  One particular recommendation on
   implementing consistent hashing is provided in [RFC2992], though
   other implementations are possible.  This functionality could be
   naturally combined with weighted ECMP, with the impact of the next-
   hop changes being proportional to the weight of the given next-hop.
   Notice that the usual downside of consistent hashing is increased
   load on hardware resource utilization, as typically more space is
   required to implement a consistent-hashing region.

7.  Routing Convergence Properties

   This section reviews routing convergence properties in the proposed
   design.  A case is made that sub-second convergence is achievable if
   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 fallover".  Since links in modern data
   centers are often point-to-point fiber connections, a physical
   interface failure is often detected in milliseconds and subsequently
   triggers a BGP re-convergence.

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   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.  Until recently with [I-D.ietf-bfd-on-lags], BFD
   also did not allow detection of a single member link failure on a
   LAG, which would limit's it's usefulness in some designs.

7.2.  Event Propagation Timing

   In this design the impact of BGP Minimum Route Advertisement Interval
   (MRAI) timer (See section of [RFC4271]) should be considered.
   Per the standard it is required for BGP implementations to space out
   consecutive BGP UPDATE messages by at least MRAI seconds, which is
   often a configurable value.  The 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
   when a BGP speaker "waits" for the new path to be learned from its
   peers and has no local backup path information.

   In a Clos topology each EBGP speaker has either one path or N paths
   for the same prefix, where N is a significantly large number, e.g.
   N=32 (the ECMP fan-out).  Therefore, if a path fails there is either
   no backup path at all, or the backup is readily available in BGP Loc-
   RIB.  In the former case, the BGP withdrawal announcement will
   propagate un-delayed and trigger re-convergence on affected devices.
   In the latter case, the best-path will be re-evaluated and the local
   ECMP group corresponding to the new next-hop set changed.  If the BGP
   path was the best-path selected previously, an "implicit withdraw"
   will be sent via a BGP UPDATE message as described as option b in
   Section 3.1 of [RFC4271] due to the BGP AS_PATH attribute changing.

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 (except for the originator).  Tier-2 devices other
   than the one originating the WITHDRAW should then wait for ALL
   adjacent Tier-1 devices to send a WITHDRAW message before it removes
   the affected prefixes and sends corresponding WITHDRAW downstream to
   connected Tier-3 devices.  If the original Tier-2 device or the
   relaying Tier-1 devices introduce some delay into their

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   announcements, the result could be WITHDRAW message "dispersion",
   that could be as long as multiple seconds.  In order to avoid such
   behavior, BGP implementations must support "update groups".  The
   "update group" is defined as a collection of neighbors sharing the
   same outbound policy - the local speaker will send BGP updates to the
   members of the group synchronously.

   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 updated their 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 we describe BGP's advantages over
   link-state routing protocols in reducing failure impact scope for a
   Clos topology.

   BGP is behaves like a distance-vector protocol in the sense that only
   the best path from the point of view of the local router is sent to
   neighbors.  As such, some failures are masked if the local node can
   immediately find a backup path and does not have to send any updates
   further.  Notice that 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 the affected ECMP groups, removing the
      failed link.  There is no need to send new information to
      downstream Tier-3 devices, unless the path was selected as best by
      the BGP process, in which case only an "implicit withdraw" needs
      to be sent, which should not affect forwarding.  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

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      Tier-3 devices are once again not involved in the re-convergence
      process, but may receive "implicit withdraws" as described above.

   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
   the case of implementations with a hierarchical FIB, only a single
   change has to be made to the FIB.  Hierarchical FIB here means FIB
   structure where the next-hop forwarding information is stored
   separately from the prefix lookup table, and the latter only store
   pointers to the respective forwarding information.

   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 all paths 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 pointing to the Tier-2 device,
   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 "null" 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 a summary route, covering all server subnets of the
   underlying Tier-3 devices.  For Tier-1 devices, the discard route
   should be a summary 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.

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8.  Additional Options for Design

8.1.  Third-party Route Injection

   BGP allows for a "third-party", i.e. directly attached, BGP speaker
   to inject routes anywhere in the network topology, meeting REQ5.
   This can be achieved by peering via 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 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, which limits the "third-party" peering to
   iBGP only, if the feature is used.

   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 Summarization within Clos Topology

   As mentioned previously, route summarization 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, e.g.
   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.

   Route summarization would be possible with a small modification to
   the network topology, 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

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

   Having this design in place, Tier-2 devices may be configured to
   advertise only a default route down to Tier-3 devices.  If a link
   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.

   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.

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8.2.2.  Simple Virtual Aggregation

   A completely different approach to route summarization is possible,
   provided that the main goal is to reduce the FIB pressure, while
   allowing the control plane to disseminate full routing information.
   Firstly, it could be easily noted that in many cases multiple
   prefixes, some of which are less specific, share the same set of the
   next-hops (same ECMP group).  For example, looking from the
   perspective of a Tier-3 devices, all routes learned from upstream
   Tier-2's, including the default route, will share the same set of BGP
   next-hops, provided that there is no failures in the network.  This
   makes it possible to use the technique similar to described in
   [RFC6769] and only install the least specific route in the FIB,
   ignoring more specific routes if they share the same next-hop set.
   For example, under normal network conditions, only the default route
   need to be programmed into FIB.

   Furthermore, if the Tier-2 devices are configured with summary
   prefixes covering all of their attached Tier-3 device's prefixes the
   same logic could be applied in Tier-1 devices as well, and, by
   induction to Tier-2/Tier-3 switches in different clusters.  These
   summary routes should still allow for more specific prefixes to leak
   to Tier-1 devices, to enable for detection of mismatches in the next-
   hop sets if a particular link fails, changing the next-hop set for a
   specific prefix.

   Re-stating once again, this technique does not reduce the amount of
   control plane state (i.e. BGP UPDATEs/BGP LocRIB sizing), but only
   allows for more efficient FIB utilization, by spotting more specific
   prefixes that share their next-hops with less specifics.

8.3.  ICMP Unreachable Message Masquerading

   This section discusses some operational aspects of not advertising
   point-to-point link subnets into BGP, as previously outlined as an
   option in Section 5.2.3.  The operational impact of this decision
   could be seen when using the well-known "traceroute" tool.
   Specifically, IP addresses displayed by the tool will be the link's
   point-to-point addresses, and hence will be unreachable for
   management connectivity.  This makes some troubleshooting more

   One way to overcome this limitation is by using the DNS subsystem to
   create the "reverse" entries for the IP addresses of the same device
   pointing to the same name.  The connectivity then can be made by
   resolving this name to the "primary" IP address of the devices, e.g.
   its Loopback interface, which is always advertised into BGP.

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   However, this create dependency on DNS subsystem, which may happen to
   be unavailable during an outage.

   Another option is to make the network device perform IP address
   masquerading, that is rewriting the source IP addresses of the
   appropriate ICMP messages sent off of the device with the "primary"
   IP address of the device.  Specifically, the ICMP Destination
   Unreachable Message (type 3) codes 3 (port unreachable) and ICMP Time
   Exceeded (type 11) code 0, which are involved in proper working of
   the "traceroute" tool.  With this modification, the "traceroute"
   probes sent to the devices will always be sent back with the
   "primary" IP address as the source, allowing the operator to discover
   the "reachable" IP address of the box.

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.

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, Susan Hares, Russ White and Robert Raszuk 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.

   [RFC6996]  Mitchell, J., "Autonomous System (AS) Reservation for
              Private Use", BCP 6, RFC 6996, July 2013.

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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.

   [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

   [RFC2992]  Hopps, C., "Analysis of an Equal-Cost Multi-Path
              Algorithm", RFC 2992, November 2000.

   [RFC6769]  Raszuk, R., Heitz, J., Lo, A., Zhang, L., and X. Xu,
              "Simple Virtual Aggregation (S-VA)", RFC 6769, October

              Walton, D., Retana, A., Chen, E., and J. Scudder,
              "Advertisement of Multiple Paths in BGP", draft-ietf-idr-
              add-paths-09 (work in progress), October 2013.

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

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              Bhatia, M., Chen, M., Boutros, S., Binderberger, M., and
              J. Haas, "Bidirectional Forwarding Detection (BFD) on Link
              Aggregation Group (LAG) Interfaces", draft-ietf-bfd-on-
              lags-04 (work in progress), December 2013.

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

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

              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.

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

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

   [IANA.AS]  IANA, , "Autonomous System (AS) Numbers", January 2014,

              IEEE 802.3ad, , "IEEE Standard for Link aggregation for
              parallel links", October 2000.

              Cisco Systems, , "Removing Private Autonomous System
              Numbers in BGP", August 2005, <

   [FB4POST]  Farrington, N. and A. Andreyev, "Facebook's Data Center
              Network Architecture", May 2013,

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              Jakma, P., "BGP Path Hunting", 2008, <https://

              Wikipedia, , "Consistent Hashing",

Authors' Addresses

   Petr Lapukhov
   1 Hacker Way
   Menlo Park, CA  94025


   Ariff Premji
   Arista Networks
   5453 Great America Parkway
   Santa Clara, CA  95054


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


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