Use of BGP for Routing in Large-Scale Data Centers
RFC 7938

Document Type RFC - Informational (August 2016; No errata)
Last updated 2016-08-10
Replaces draft-lapukhov-bgp-routing-large-dc
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Internet Engineering Task Force (IETF)                       P. Lapukhov
Request for Comments: 7938                                      Facebook
Category: Informational                                        A. Premji
ISSN: 2070-1721                                          Arista Networks
                                                        J. Mitchell, Ed.
                                                             August 2016

           Use of BGP for Routing in Large-Scale Data Centers

Abstract

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

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7938.

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Copyright Notice

   Copyright (c) 2016 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
   Provisions Relating to IETF Documents
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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Network Design Requirements . . . . . . . . . . . . . . . . .   4
     2.1.  Bandwidth and Traffic Patterns  . . . . . . . . . . . . .   4
     2.2.  CAPEX Minimization  . . . . . . . . . . . . . . . . . . .   4
     2.3.  OPEX Minimization . . . . . . . . . . . . . . . . . . . .   5
     2.4.  Traffic Engineering . . . . . . . . . . . . . . . . . . .   5
     2.5.  Summarized Requirements . . . . . . . . . . . . . . . . .   6
   3.  Data Center Topologies Overview . . . . . . . . . . . . . . .   6
     3.1.  Traditional DC Topology . . . . . . . . . . . . . . . . .   6
     3.2.  Clos Network Topology . . . . . . . . . . . . . . . . . .   7
       3.2.1.  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  . . . . . . . . . . . . . . . .  11
     4.1.  L2-Only Designs . . . . . . . . . . . . . . . . . . . . .  11
     4.2.  Hybrid L2/L3 Designs  . . . . . . . . . . . . . . . . . .  12
     4.3.  L3-Only Designs . . . . . . . . . . . . . . . . . . . . .  12
   5.  Routing Protocol Design . . . . . . . . . . . . . . . . . . .  13
     5.1.  Choosing EBGP as the Routing Protocol . . . . . . . . . .  13
     5.2.  EBGP Configuration for Clos Topology  . . . . . . . . . .  15
       5.2.1.  EBGP Configuration Guidelines and Example ASN Scheme   15
       5.2.2.  Private Use ASNs  . . . . . . . . . . . . . . . . . .  16
       5.2.3.  Prefix Advertisement  . . . . . . . . . . . . . . . .  17
       5.2.4.  External Connectivity . . . . . . . . . . . . . . . .  18
       5.2.5.  Route Summarization at the Edge . . . . . . . . . . .  19
   6.  ECMP Considerations . . . . . . . . . . . . . . . . . . . . .  20
     6.1.  Basic ECMP  . . . . . . . . . . . . . . . . . . . . . . .  20
     6.2.  BGP ECMP over Multiple ASNs . . . . . . . . . . . . . . .  21
     6.3.  Weighted ECMP . . . . . . . . . . . . . . . . . . . . . .  21
     6.4.  Consistent Hashing  . . . . . . . . . . . . . . . . . . .  22

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   7.  Routing Convergence Properties  . . . . . . . . . . . . . . .  22
     7.1.  Fault Detection Timing  . . . . . . . . . . . . . . . . .  22
     7.2.  Event Propagation Timing  . . . . . . . . . . . . . . . .  23
     7.3.  Impact of Clos Topology Fan-Outs  . . . . . . . . . . . .  24
     7.4.  Failure Impact Scope  . . . . . . . . . . . . . . . . . .  24
     7.5.  Routing Micro-Loops . . . . . . . . . . . . . . . . . . .  26
   8.  Additional Options for Design . . . . . . . . . . . . . . . .  26
     8.1.  Third-Party Route Injection . . . . . . . . . . . . . . .  26
     8.2.  Route Summarization within Clos Topology  . . . . . . . .  27
       8.2.1.  Collapsing Tier 1 Devices Layer . . . . . . . . . . .  27
       8.2.2.  Simple Virtual Aggregation  . . . . . . . . . . . . .  29
     8.3.  ICMP Unreachable Message Masquerading . . . . . . . . . .  29
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  30
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  30
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  30
     10.2.  Informative References . . . . . . . . . . . . . . . . .  31
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  35
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  35

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.

   Experimentation and extensive testing have shown that External BGP
   (EBGP) [RFC4271] is well suited as a stand-alone routing protocol for
   these types of data center applications.  This is in contrast with
   more traditional DC designs, which may use simple tree topologies and
   rely on extending Layer 2 (L2) 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 exploring ideas for further enhancements.

   This document first presents an overview of network design
   requirements and considerations for large-scale data centers.  Then,
   traditional hierarchical data center network topologies are
   contrasted with Clos networks [CLOS1953] that are horizontally scaled

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   out.  This is followed by arguments for selecting EBGP with a Clos
   topology as the most appropriate routing protocol to meet the
   requirements and the proposed design is described in detail.
   Finally, this document reviews some additional considerations and
   design options.  A thorough understanding of BGP is assumed by a
   reader planning on deploying the design described within the
   document.

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
   a 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.  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 linecards 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 computer clusters such as Hadoop
   [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 Capital Expenditures (CAPEX) associated with the network
   infrastructure alone 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 and reduced maintenance and inventory costs.

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   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.
   This strategy provides maximum flexibility of vendor equipment
   choices while enforcing interoperability using open standards.

2.3.  OPEX Minimization

   Operating large-scale infrastructure can be expensive as a 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 Operational Expenditure (OPEX) minimization is
   reducing the size of failure domains in the network.  Ethernet
   networks are known to be susceptible to broadcast or unicast traffic
   storms that can have a 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 and less control-plane protocols to
   reduce protocol interaction issues, reducing the chance of a 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 the load-
   balancing layer horizontally, by adding more of the uniform nodes and
   distributing incoming traffic across these nodes.  In situations 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
   the network hierarchy.

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2.5.  Summarized Requirements

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

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

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

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

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

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

3.  Data Center Topologies Overview

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

3.1.  Traditional DC Topology

   In the networking industry, a common design choice for data centers
   typically looks like an (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 the 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

   Unfortunately, as noted previously, it is not possible to scale a
   tree-based design to a large enough degree for handling large-scale
   designs due to the inability to be able to acquire Tier 1 devices
   with a large enough port density to sufficiently scale Tier 2.  Also,
   continuous upgrades or replacement of the upper-tier devices are
   required as deployment size or bandwidth requirements increase, which
   is operationally complex.  For this reason, REQ1 is in place,
   eliminating this type of design from consideration.

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" (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

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   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 flow):

   +-------+
   |       |----------------------------+
   |       |------------------+         |
   |       |--------+         |         |
   +-------+        |         |         |
   +-------+        |         |         |
   |       |--------+---------+-------+ |
   |       |--------+-------+ |       | |
   |       |------+ |       | |       | |
   +-------+      | |       | |       | |
   +-------+      | |       | |       | |
   |       |------+-+-------+-+-----+ | |
   |       |------+-+-----+ | |     | | |
   |       |----+ | |     | | |     | | |
   +-------+    | | |     | | |   ---------> 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.

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   o  Utilizing this topology requires control and data-plane support
      for ECMP with a fan-out of M or more.

   o  Tier 1 switches have exactly one path to every server in this
      topology.  This is an important property that makes route
      summarization dangerous 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 by adding more stages, e.g., moving to a 5-stage
   Clos, as illustrated in Figure 3 below:

                                      Tier 1
                                     +-----+
          Cluster                    |     |
 +----------------------------+   +--|     |--+
 |                            |   |  +-----+  |
 |                    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 of topology in Figure 3 is built from devices with
   a port count of 4.  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.  The

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   concept of a cluster may also be a useful concept as a single
   deployment or maintenance unit that can be operated on at a different
   frequency than the entire topology.

   In practice, Tier 3 of the network, which is typically Top-of-Rack
   switches (ToRs), is where oversubscription is introduced to allow for
   packaging of more servers in the data center while meeting the
   bandwidth requirements for different types of applications.  The main
   reason to limit oversubscription at a single layer of the network is
   to simplify application development that would otherwise need to
   account for multiple bandwidth pools: within rack (Tier 3), between
   racks (Tier 2), and between 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 a Clos topology by a factor
   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 bandwidth while
   reducing the number of elements in Tier 1, 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, possibly resulting
   in heavy congestion and quality of service degradation if the path
   determination procedure does not take bandwidth amount into account,
   since the number of upstream Tier 1 devices is likely wider than two.
   To avoid this situation, parallel links can be grouped in link
   aggregation groups (LAGs), e.g., [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 rebalanced over the multitude
   of remaining paths that equals the number of Tier 1 devices.  This
   example is using two links for simplicity, having more links in a
   bundle will have less impact on capacity upon a member-link failure.

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4.  Data Center Routing Overview

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

4.1.  L2-Only Designs

   Originally, most data center designs used Spanning Tree Protocol
   (STP) originally defined in [IEEE8021D-1990] 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 routing protocols or supported them 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 (RSTP) in the latest revision of
   [IEEE8021D-2004] and Multiple Spanning Tree Protocol (MST) specified
   in [IEEE8021Q] that increase convergence, stability, and load-
   balancing in larger topologies, many of the fundamentals of the
   protocol limit its applicability in large-scale DCs.  STP and its
   newer variants use an active/standby approach to path selection, and
   are therefore hard to deploy in horizontally scaled topologies as
   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 downsides of this
   approach are the lack of ability to scale linearly past two in most
   implementations, lack of standards-based implementations, and the
   added failure domain risk of syncing state between the devices.

   It should be noted that building large, horizontally scalable,
   L2-only networks without STP is possible recently through the
   introduction of the Transparent Interconnection of Lots of Links
   (TRILL) protocol in [RFC6325].  TRILL resolves many of the issues STP
   has for large-scale DC design however, due to the limited number of
   implementations, and often the requirement for specific equipment
   that supports it, this has limited its applicability and increased
   the cost of such designs.

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   Finally, neither the base TRILL specification nor the M-LAG approach
   totally eliminate the problem of the shared broadcast domain that is
   so detrimental to the operations of any Layer 2, Ethernet-based
   solution.  Later TRILL extensions have been proposed to solve the
   this problem statement, primarily based on the approaches outlined in
   [RFC7067], but this even further limits the number of available
   interoperable implementations that can be used to build a fabric.
   Therefore, TRILL-based designs have issues meeting REQ2, REQ3, and
   REQ4.

4.2.  Hybrid L2/L3 Designs

   Operators have sought to limit the impact of data-plane faults and
   build large-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
   managing multiple network 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 a
      different Tier 3 switch.

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

   o  Application load balancing may require direct Layer 2 reachability
      to perform certain functions such as Layer 2 Direct Server Return
      (DSR).  See [L3DSR].

   o  Continued CAPEX differences between L2- and L3-capable switches.

4.3.  L3-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 Interior Gateway
   Protocol (IGP) such as Open Shortest Path First (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.

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   Choosing a L3-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 develop new solutions to meet some of the requirements that
   previously had driven large Layer 2 domains by using various overlay
   or tunneling techniques.

5.  Routing Protocol Design

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

5.1.  Choosing EBGP as the Routing Protocol

   REQ2 would give preference to the selection of a single routing
   protocol to reduce complexity and interdependencies.  While it is
   common to rely on an IGP in this situation, sometimes with either the
   addition of EBGP at the device bordering the WAN or Internal BGP
   (IBGP) throughout, this document proposes the use of an EBGP-only
   design.

   Although EBGP is the protocol used for almost all Inter-Domain
   Routing in 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  Large-scale BGP deployments typically utilize an IGP for BGP next-
      hop resolution as all nodes in the IBGP topology are not directly
      connected.

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

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   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 in parts of its protocol design --
      internal data structures and state machine are simpler as compared
      to most link-state IGPs 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 an EBGP-only
      design.  In contrast, the event propagation scope of a link-state
      IGP is an entire area, regardless of the failure type.  In this
      way, BGP better meets REQ3 and REQ4.  It is also worth mentioning
      that all widely deployed link-state IGPs feature periodic
      refreshes of routing information while BGP does not expire routing
      state, although this rarely impacts modern router control planes.

   o  BGP supports third-party (recursively resolved) next hops.  This
      allows for manipulating multipath to be non-ECMP-based or
      forwarding-based on application-defined paths, through
      establishment of a peering session with an application
      "controller" that 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 far less control of information propagation
      scope.

   o  Using a well-defined Autonomous System Number (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 ASN
      allocation scheme.  In a link-state IGP, accomplishing the same
      goal would require multi-(instance/topology/process) support,
      typically not available in all DC devices and quite complex to
      configure and troubleshoot.  Using a traditional single flooding
      domain, which most DC designs utilize, under certain failure
      conditions may pick up unwanted lengthy paths, e.g., traversing
      multiple Tier 2 devices.

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   o  EBGP configuration that is implemented with minimal routing policy
      is easier to troubleshoot for network reachability issues.  In
      most implementations, it is straightforward to view contents of
      the BGP Loc-RIB and compare it to the router's Routing Information
      Base (RIB).  Also, in most implementations, an operator can view
      every BGP neighbors Adj-RIB-In and Adj-RIB-Out structures, and
      therefore incoming and outgoing Network Layer Reachability
      Information (NLRI) information can be easily correlated on both
      sides of a BGP session.  Thus, BGP satisfies REQ3.

5.2.  EBGP Configuration for Clos Topology

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

5.2.1.  EBGP Configuration Guidelines and Example ASN Scheme

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

   o  EBGP single-hop sessions are established over direct point-to-
      point links interconnecting the network nodes, no multi-hop or
      loopback sessions are used, even in the case of multiple links
      between the same pair of nodes.

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

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

   o  A unique ASN is allocated to each set of Tier 2 devices in the
      same cluster.

   o  A unique ASN is allocated to every Tier 3 device (e.g., ToR) in
      this topology.

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                                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 ASNs

   The original range of Private Use 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 required.
   One approach is to re-use the ASNs assigned to the Tier 3 devices
   across different clusters.  For example, Private Use 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 [ALLOWASIN] that allows
   accepting a device's own ASN in received route advertisements.
   Although this feature is not standardized, it is widely available
   across multiple vendors implementations.  Introducing this feature
   does not make routing loops more likely in the design since the
   AS_PATH is being added to by routers at each of the topology tiers
   and AS_PATH length is an early tie breaker in the BGP path selection

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   process.  Further loop protection is still in place at the Tier 1
   device, which will not accept routes with a path including its own
   ASN.  Tier 2 devices do not have direct connectivity with each other.

   Another solution to this problem would be to use Four-Octet ASNs
   ([RFC6793]), where there are additional Private Use ASNs available,
   see [IANA.AS].  Use of Four-Octet ASNs puts additional protocol
   complexity in the BGP implementation and should be balanced 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.
   When supported, ensure that deployed implementations are able to
   remove the Private Use ASNs when external connectivity
   (Section 5.2.4) to these ASNs is required.

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 Forwarding Information Base (FIB) overload 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 operations or monitoring:
      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 allocated from Tier 1 address
      blocks and so forth).

   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 to use a simpler bypass topology, e.g., a "ring" as

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   described in [FB4POST], but such a topology adds extra hops and has
   limited bandwidth.  It may require special tweaks to make BGP routing
   work, e.g., splitting every device into an ASN of its own.  Later in
   this document, Section 8.2 introduces a less intrusive method for
   performing a limited form of route summarization in Clos networks and
   discusses its associated tradeoffs.

5.2.4.  External Connectivity

   A dedicated cluster (or clusters) in the Clos topology could be used
   for the purpose of connecting to the Wide Area Network (WAN) edge
   devices, or WAN Routers.  Tier 3 devices in such a cluster would be
   replaced with WAN routers, and EBGP peering would be used again,
   though WAN routers are likely to belong to a public ASN if Internet
   connectivity is required in the design.  The Tier 2 devices in such a
   dedicated cluster will be referred to as "Border Routers" in this
   document.  These devices have to perform a few special functions:

   o  Hide network topology information when advertising paths to WAN
      routers, i.e., remove Private Use ASNs [RFC6996] from the AS_PATH
      attribute.  This is typically done to avoid ASN number collisions
      between different data centers and also to provide a uniform
      AS_PATH length to the WAN for purposes of WAN ECMP to anycast
      prefixes originated in the topology.  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 Use ASNs found in an
      AS_PATH attribute prior to advertising the path to a neighbor.
      This assumes that all ASNs used for intra data center numbering
      are from the Private Use ranges.  The process for stripping the
      Private Use ASNs is not currently standardized, see [REMOVAL].
      However, most implementations at least follow the logic described
      in this vendor's document [VENDOR-REMOVE-PRIVATE-AS], which is
      enough for the design specified.

   o  Originate a default route to the data center devices.  This is the
      only place where a default route can be originated, as route
      summarization is risky for the unmodified Clos 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 be fully connected
      to the WAN Routers upstream, to provide resistance to a single-
      link failure causing the black-holing of traffic.  To prevent
      black-holing in the situation when all of the EBGP sessions to the
      WAN routers fail simultaneously on a given device, it is more
      desirable to readvertise the default route rather than originating
      the default route via complicated conditional route origination
      schemes provided by some implementations [CONDITIONALROUTE].

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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 the 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 prefixes.  However, as discussed in Section 5.2.3,
   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 in the mesh connecting 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 device 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 nonuniform, larger port count,
      device compared 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 nonuniform
   topology as additional links have to be provisioned on some network
   devices.

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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.  The number of ECMP paths between any two Tier
   3 devices in Clos topology is equal to the number of the devices in
   the middle stage (Tier 1).  For example, Figure 5 illustrates a
   topology where Tier 3 device A has four paths to reach servers X and
   Y, via Tier 2 devices B and C and then Tier 1 devices 1, 2, 3, and 4,
   respectively.

                                Tier 1
                               +-----+
                               | DEV |
                            +->|  1  |--+
                            |  +-----+  |
                    Tier 2  |           |   Tier 2
                   +-----+  |  +-----+  |  +-----+
     +------------>| DEV |--+->| DEV |--+--|     |-------------+
     |       +-----|  B  |--+  |  2  |  +--|     |-----+       |
     |       |     +-----+     +-----+     +-----+     |       |
     |       |                                         |       |
     |       |     +-----+     +-----+     +-----+     |       |
     | +-----+---->| DEV |--+  | DEV |  +--|     |-----+-----+ |
     | |     | +---|  C  |--+->|  3  |--+--|     |---+ |     | |
     | |     | |   +-----+  |  +-----+  |  +-----+   | |     | |
     | |     | |            |           |            | |     | |
   +-----+ +-----+          |  +-----+  |          +-----+ +-----+
   | 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
   multipath fan-out for up to the maximum number of devices directly
   attached at any point in the topology in the upstream or downstream
   direction.  Normally, this number does not exceed half of the ports
   found on a device in the topology.  For example, an ECMP fan-out of
   32 would be required when building a Clos network using 64-port

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   devices.  The Border Routers may need to have wider fan-out to be
   able to connect to a 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 coupled with Layer 2 ECMP)
   to compensate for fan-out limitations.  However, this approach
   increases the risk of flow polarization, as less entropy will be
   available at the second stage of ECMP.

   Most BGP implementations declare paths to be equal from an 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 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,
   such as the MULTI_EXIT_DISC (MED) attribute and origin code.  For
   historical reasons, it is also useful to not use 0 as the equalized
   MED value; this and some other useful BGP information is available in
   [RFC4277].  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 ASN in
   the path) are 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 the above-mentioned paths.  This feature is sometimes
   known as "multipath relax" or "multipath multiple-AS" and effectively
   allows for ECMP to be done across different neighboring ASNs if all
   other attributes are equal as already 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 multi-hop session as
   described further in Section 8.1.  If support in implementations is
   available, weight distribution for multiple BGP paths could be
   signaled using the technique described in [LINK].

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6.4.  Consistent Hashing

   It is often desirable to have the hashing function used for ECMP to
   be consistent (see [CONS-HASH]), to minimize the impact on flow to
   next-hop affinity changes when a next hop is added or removed to an
   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 a detrimental
   effect on 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.
   The downside of consistent hashing is increased load on hardware
   resource utilization, as typically more resources (e.g., Ternary
   Content-Addressable Memory (TCAM) space) are required to implement a
   consistent-hashing function.

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 updates 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 remaining
   mechanisms that could be used for fault detection are BGP keep-alive
   time-out (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, on 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 "fast fallover".  Since links in modern data centers
   are predominantly point-to-point fiber connections, a physical
   interface failure is often detected in milliseconds and subsequently
   triggers a BGP reconvergence.

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   Ethernet links may support failure signaling or detection standards
   such as Connectivity Fault Management (CFM) as described in
   [IEEE8021Q]; this 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, the use of either
   of these presents additional requirements to vendor software and
   possibly hardware, and may contradict REQ1.  Until recently with
   [RFC7130], BFD also did not allow detection of a single member link
   failure on a LAG, which would have limited its usefulness in some
   designs.

7.2.  Event Propagation Timing

   In the proposed design, the impact of the BGP
   MinRouteAdvertisementIntervalTimer (MRAI timer), as specified in
   Section 9.2.1.1 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 typically has either one path
   (Tier 2 devices don't accept paths from other Tier 2 in the same
   cluster due to same ASN) or N paths for the same prefix, where N is a
   significantly large number, e.g., N=32 (the ECMP fan-out to the next
   tier).  Therefore, if a link fails to another device from which a
   path is received there is either no backup path at all (e.g., from
   the perspective of a Tier 2 switch losing the link to a Tier 3
   device), or the backup is readily available in BGP Loc-RIB (e.g.,
   from the perspective of a Tier 2 device losing the link to a Tier 1
   switch).  In the former case, the BGP withdrawal announcement will
   propagate without delay and trigger reconvergence 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 will be
   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.

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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 UPDATE messages to all upstream Tier 1
   devices, withdrawing the affected prefixes.  The Tier 1 devices, in
   turn, will relay these messages to all downstream Tier 2 devices
   (except for the originator).  Tier 2 devices other than the one
   originating the UPDATE should then wait for ALL upstream Tier 1
   devices to send an UPDATE message before removing the affected
   prefixes and sending corresponding UPDATE downstream to connected
   Tier 3 devices.  If the original Tier 2 device or the relaying Tier 1
   devices introduce some delay into their UPDATE message announcements,
   the result could be UPDATE message "dispersion", that could be as
   long as multiple seconds.  In order to avoid such a 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.  Some
   operators may be tempted to introduce "route flap dampening" type
   features that vendors include to reduce the control-plane impact of
   rapidly flapping prefixes.  However, due to issues described with
   false positives in these implementations especially under such
   "dispersion" events, it is not recommended to enable this feature in
   this design.  More background and issues with "route flap dampening"
   and possible implementation changes that could affect this are well
   described in [RFC7196].

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 recalculated their RIBs and consequently updated their FIBs.
   Larger failure impact scope typically means slower convergence since
   more devices have to be notified, and 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 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

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   topology have to either withdraw a prefix completely or update the
   ECMP groups in their FIBs.  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 and this 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 a 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 reconvergence 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 a non-local cluster.  The
      Tier 3 devices are once again not involved in the reconvergence
      process, but may receive "implicit withdraws" as described above.

   Even in the case of such failures where 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 a 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
   stores pointers to the respective forwarding information.  See
   [BGP-PIC] for discussion of FIB hierarchies and fast convergence.

   Even though BGP offers reduced failure scope for some cases, further
   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 failure impact scope on the control plane is the network as a
   whole -- for instance, in the case of a link failure between Tier 2
   and Tier 3 devices.  The amount of impacted prefixes in this case
   would be much 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 Clos topology.

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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 where a Tier 2 switch loses a
   prefix, but a Tier 1 switch still has the path pointing to the Tier 2
   device; this results in a transient micro-loop, since the Tier 1
   switch will keep passing packets to the affected prefix back to the
   Tier 2 device, and the Tier 2 will bounce them back again using the
   default route.  This micro-loop will last for the time it takes the
   upstream device to fully update its forwarding tables.

   To minimize impact of such 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 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 subnets allocated for the
   whole data center.  Those discard routes will only take precedence
   for the duration of network convergence, until the device learns a
   more specific prefix via a new path.

8.  Additional Options for Design

8.1.  Third-Party Route Injection

   BGP allows for a "third-party", i.e., a directly attached BGP
   speaker, to inject routes anywhere in the network topology, meeting
   REQ5.  This can be achieved by peering via a multi-hop 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 [RFC7911] if supported by the
   implementation.  Unfortunately, in many implementations, ADD-PATH has
   been found to only support IBGP properly in the use cases for which
   it was originally optimized; this limits the "third-party" peering to
   IBGP only.

   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.

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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 redundant 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.

   If any technique to summarize within the topology is planned,
   modeling of the routing behavior and potential for black-holing
   should be done not only for single or multiple link failures, but
   also for fiber pathway failures or optical domain failures when the
   topology extends beyond a physical location.  Simple modeling can be
   done by checking the reachability on devices doing summarization
   under the condition of a link or pathway failure between a set of
   devices in every tier as well as to the WAN routers when external
   connectivity is present.

   Route summarization would be possible with a small modification to
   the network topology, though the tradeoff would be reduction of the
   total size of the network as well as network congestion under
   specific failures.  This approach is very similar to the technique
   described above, which allows Border Routers to summarize the entire
   data center address space.

8.2.1.  Collapsing Tier 1 Devices Layer

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

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                    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 still 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 failures.  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 a reduction
   of the port capacity of Tier 1 devices.  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 rebalancing under link failures.  Since
   there are two paths from Tier 1 to Tier 3, a failure of the link
   between Tier 1 and Tier 2 switch would result in all traffic that was
   taking the failed link to switch to the remaining path.  This will
   result in doubling the 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 size, 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, from the perspective of Tier 3 devices,
   all routes learned from upstream Tier 2 devices, including the
   default route, will share the same set of BGP next hops, provided
   that there are no failures in the network.  This makes it possible to
   use the technique similar to that 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 needs to be
   programmed into the 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 detection of mismatches in the next-hop
   sets if a particular link fails, thus changing the next-hop set for a
   specific prefix.

   Restating once again, this technique does not reduce the amount of
   control-plane state (i.e., BGP UPDATEs, BGP Loc-RIB size), but only
   allows for more efficient FIB utilization, by detecting more specific
   prefixes that share their next-hop set with a subsuming less specific
   prefix.

8.3.  ICMP Unreachable Message Masquerading

   This section discusses some operational aspects of not advertising
   point-to-point link subnets into BGP, as previously identified 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
   complicated.

   One way to overcome this limitation is by using the DNS subsystem to
   create the "reverse" entries for these point-to-point IP addresses
   pointing to the same name as the loopback address.  The connectivity
   then can be made by resolving this name to the "primary" IP address

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   of the device, e.g., its Loopback interface, which is always
   advertised into BGP.  However, this creates a dependency on the DNS
   subsystem, which may 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 by the device with the "primary" IP
   address of the device.  Specifically, the ICMP Destination
   Unreachable Message (type 3) code 3 (port unreachable) and ICMP Time
   Exceeded (type 11) code 0 are required for correct operation 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.  This has the downside of hiding
   the address of the "entry point" into the device.  If the devices
   support [RFC5837], this may allow the best of both worlds by
   providing the information about the incoming interface even if the
   return address is the "primary" IP address.

9.  Security Considerations

   The design does not introduce any additional security concerns.
   General BGP security considerations are discussed in [RFC4271] and
   [RFC4272].  Since a DC is a single-operator domain, this document
   assumes that edge filtering is in place to prevent attacks against
   the BGP sessions themselves from outside the perimeter of the DC.
   This may be a more feasible option for most deployments than having
   to deal with key management for TCP MD5 as described in [RFC2385] or
   dealing with the lack of implementations of the TCP Authentication
   Option [RFC5925] available at the time of publication of this
   document.  The Generalized TTL Security Mechanism [RFC5082] could
   also be used to further reduce the risk of BGP session spoofing.

10.  References

10.1.  Normative References

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <http://www.rfc-editor.org/info/rfc4271>.

   [RFC6996]  Mitchell, J., "Autonomous System (AS) Reservation for
              Private Use", BCP 6, RFC 6996, DOI 10.17487/RFC6996, July
              2013, <http://www.rfc-editor.org/info/rfc6996>.

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10.2.  Informative References

   [ALFARES2008]
              Al-Fares, M., Loukissas, A., and A. Vahdat, "A Scalable,
              Commodity Data Center Network Architecture",
              DOI 10.1145/1402958.1402967, August 2008,
              <http://dl.acm.org/citation.cfm?id=1402967>.

   [ALLOWASIN]
              Cisco Systems, "Allowas-in Feature in BGP Configuration
              Example", February 2015,
              <http://www.cisco.com/c/en/us/support/docs/ip/
              border-gateway-protocol-bgp/112236-allowas-in-bgp-config-
              example.html>.

   [BGP-PIC]  Bashandy, A., Ed., Filsfils, C., and P. Mohapatra, "BGP
              Prefix Independent Convergence", Work in Progress,
              draft-ietf-rtgwg-bgp-pic-02, August 2016.

   [CLOS1953] Clos, C., "A Study of Non-Blocking Switching Networks",
              The Bell System Technical Journal, Vol. 32(2),
              DOI 10.1002/j.1538-7305.1953.tb01433.x, March 1953.

   [CONDITIONALROUTE]
              Cisco Systems, "Configuring and Verifying the BGP
              Conditional Advertisement Feature", August 2005,
              <http://www.cisco.com/c/en/us/support/docs/ip/
              border-gateway-protocol-bgp/16137-cond-adv.html>.

   [CONS-HASH]
              Wikipedia, "Consistent Hashing", July 2016,
              <https://en.wikipedia.org/w/
              index.php?title=Consistent_hashing&oldid=728825684>.

   [FB4POST]  Farrington, N. and A. Andreyev, "Facebook's Data Center
              Network Architecture", May 2013,
              <http://nathanfarrington.com/papers/facebook-oic13.pdf>.

   [GREENBERG2009]
              Greenberg, A., Hamilton, J., and D. Maltz, "The Cost of a
              Cloud: Research Problems in Data Center Networks",
              DOI 10.1145/1496091.1496103, January 2009,
              <http://dl.acm.org/citation.cfm?id=1496103>.

   [HADOOP]   Apache, "Apache Hadoop", April 2016,
              <https://hadoop.apache.org/>.

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RFC 7938               BGP Routing in Data Centers           August 2016

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

   [IEEE8021D-1990]
              IEEE, "IEEE Standard for Local and Metropolitan Area
              Networks: Media Access Control (MAC) Bridges", IEEE
              Std 802.1D, DOI 10.1109/IEEESTD.1991.101050, 1991,
              <http://ieeexplore.ieee.org/servlet/opac?punumber=2255>.

   [IEEE8021D-2004]
              IEEE, "IEEE Standard for Local and Metropolitan Area
              Networks: Media Access Control (MAC) Bridges", IEEE
              Std 802.1D, DOI 10.1109/IEEESTD.2004.94569, June 2004,
              <http://ieeexplore.ieee.org/servlet/opac?punumber=9155>.

   [IEEE8021Q]
              IEEE, "IEEE Standard for Local and Metropolitan Area
              Networks: Bridges and Bridged Networks", IEEE Std 802.1Q,
              DOI 10.1109/IEEESTD.2014.6991462,
              <http://ieeexplore.ieee.org/servlet/
              opac?punumber=6991460>.

   [IEEE8023AD]
              IEEE, "Amendment to Carrier Sense Multiple Access With
              Collision Detection (CSMA/CD) Access Method and Physical
              Layer Specifications - Aggregation of Multiple Link
              Segments", IEEE Std 802.3ad,
              DOI 10.1109/IEEESTD.2000.91610, October 2000,
              <http://ieeexplore.ieee.org/servlet/opac?punumber=6867>.

   [INTERCON] Dally, W. and B. Towles, "Principles and Practices of
              Interconnection Networks", ISBN 978-0122007514, January
              2004, <http://dl.acm.org/citation.cfm?id=995703>.

   [JAKMA2008]
              Jakma, P., "BGP Path Hunting", 2008,
              <https://blogs.oracle.com/paulj/entry/bgp_path_hunting>.

   [L3DSR]    Schaumann, J., "L3DSR - Overcoming Layer 2 Limitations of
              Direct Server Return Load Balancing", 2011,
              <https://www.nanog.org/meetings/nanog51/presentations/
              Monday/NANOG51.Talk45.nanog51-Schaumann.pdf>.

   [LINK]     Mohapatra, P. and R. Fernando, "BGP Link Bandwidth
              Extended Community", Work in Progress, draft-ietf-idr-
              link-bandwidth-06, January 2013.

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RFC 7938               BGP Routing in Data Centers           August 2016

   [REMOVAL]  Mitchell, J., Rao, D., and R. Raszuk, "Private Autonomous
              System (AS) Removal Requirements", Work in Progress,
              draft-mitchell-grow-remove-private-as-04, April 2015.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,
              <http://www.rfc-editor.org/info/rfc2328>.

   [RFC2385]  Heffernan, A., "Protection of BGP Sessions via the TCP MD5
              Signature Option", RFC 2385, DOI 10.17487/RFC2385, August
              1998, <http://www.rfc-editor.org/info/rfc2385>.

   [RFC2992]  Hopps, C., "Analysis of an Equal-Cost Multi-Path
              Algorithm", RFC 2992, DOI 10.17487/RFC2992, November 2000,
              <http://www.rfc-editor.org/info/rfc2992>.

   [RFC4272]  Murphy, S., "BGP Security Vulnerabilities Analysis",
              RFC 4272, DOI 10.17487/RFC4272, January 2006,
              <http://www.rfc-editor.org/info/rfc4272>.

   [RFC4277]  McPherson, D. and K. Patel, "Experience with the BGP-4
              Protocol", RFC 4277, DOI 10.17487/RFC4277, January 2006,
              <http://www.rfc-editor.org/info/rfc4277>.

   [RFC4786]  Abley, J. and K. Lindqvist, "Operation of Anycast
              Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786,
              December 2006, <http://www.rfc-editor.org/info/rfc4786>.

   [RFC5082]  Gill, V., Heasley, J., Meyer, D., Savola, P., Ed., and C.
              Pignataro, "The Generalized TTL Security Mechanism
              (GTSM)", RFC 5082, DOI 10.17487/RFC5082, October 2007,
              <http://www.rfc-editor.org/info/rfc5082>.

   [RFC5837]  Atlas, A., Ed., Bonica, R., Ed., Pignataro, C., Ed., Shen,
              N., and JR. Rivers, "Extending ICMP for Interface and
              Next-Hop Identification", RFC 5837, DOI 10.17487/RFC5837,
              April 2010, <http://www.rfc-editor.org/info/rfc5837>.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
              <http://www.rfc-editor.org/info/rfc5880>.

   [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
              June 2010, <http://www.rfc-editor.org/info/rfc5925>.

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RFC 7938               BGP Routing in Data Centers           August 2016

   [RFC6325]  Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A.
              Ghanwani, "Routing Bridges (RBridges): Base Protocol
              Specification", RFC 6325, DOI 10.17487/RFC6325, July 2011,
              <http://www.rfc-editor.org/info/rfc6325>.

   [RFC6769]  Raszuk, R., Heitz, J., Lo, A., Zhang, L., and X. Xu,
              "Simple Virtual Aggregation (S-VA)", RFC 6769,
              DOI 10.17487/RFC6769, October 2012,
              <http://www.rfc-editor.org/info/rfc6769>.

   [RFC6774]  Raszuk, R., Ed., Fernando, R., Patel, K., McPherson, D.,
              and K. Kumaki, "Distribution of Diverse BGP Paths",
              RFC 6774, DOI 10.17487/RFC6774, November 2012,
              <http://www.rfc-editor.org/info/rfc6774>.

   [RFC6793]  Vohra, Q. and E. Chen, "BGP Support for Four-Octet
              Autonomous System (AS) Number Space", RFC 6793,
              DOI 10.17487/RFC6793, December 2012,
              <http://www.rfc-editor.org/info/rfc6793>.

   [RFC7067]  Dunbar, L., Eastlake 3rd, D., Perlman, R., and I.
              Gashinsky, "Directory Assistance Problem and High-Level
              Design Proposal", RFC 7067, DOI 10.17487/RFC7067, November
              2013, <http://www.rfc-editor.org/info/rfc7067>.

   [RFC7130]  Bhatia, M., Ed., Chen, M., Ed., Boutros, S., Ed.,
              Binderberger, M., Ed., and J. Haas, Ed., "Bidirectional
              Forwarding Detection (BFD) on Link Aggregation Group (LAG)
              Interfaces", RFC 7130, DOI 10.17487/RFC7130, February
              2014, <http://www.rfc-editor.org/info/rfc7130>.

   [RFC7196]  Pelsser, C., Bush, R., Patel, K., Mohapatra, P., and O.
              Maennel, "Making Route Flap Damping Usable", RFC 7196,
              DOI 10.17487/RFC7196, May 2014,
              <http://www.rfc-editor.org/info/rfc7196>.

   [RFC7911]  Walton, D., Retana, A., Chen, E., and J. Scudder,
              "Advertisement of Multiple Paths in BGP", RFC 7911,
              DOI 10.17487/RFC7911, July 2016,
              <http://www.rfc-editor.org/info/rfc7911>.

   [VENDOR-REMOVE-PRIVATE-AS]
              Cisco Systems, "Removing Private Autonomous System Numbers
              in BGP", August 2005,
              <http://www.cisco.com/en/US/tech/tk365/
              technologies_tech_note09186a0080093f27.shtml>.

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RFC 7938               BGP Routing in Data Centers           August 2016

Acknowledgements

   This publication summarizes the 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.  The authors would also like
   to thank Linda Dunbar, Anoop Ghanwani, Susan Hares, Danny McPherson,
   Robert Raszuk, and Russ White for reviewing this document and
   providing valuable feedback, and Mary Mitchell for initial grammar
   and style suggestions.

Authors' Addresses

   Petr Lapukhov
   Facebook
   1 Hacker Way
   Menlo Park, CA  94025
   United States of America

   Email: petr@fb.com

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

   Email: ariff@arista.com
   URI:   http://arista.com/

   Jon Mitchell (editor)

   Email: jrmitche@puck.nether.net

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