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.
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 (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Network Design Requirements . . . . . . . . . . . . . . . . . 4 2.1. Bandwidth and Traffic Patterns . . . . . . . . . . . . . 4 2.2. CAPEX Minimization . . . . . . . . . . . . . . . . . . . 4 2.3. OPEX Minimization . . . . . . . . . . . . . . . . . . . . 5 2.4. Traffic Engineering . . . . . . . . . . . . . . . . . . . 5 2.5. Summarized Requirements . . . . . . . . . . . . . . . . . 6 3. Data Center Topologies Overview . . . . . . . . . . . . . . . 6 3.1. Traditional DC Topology . . . . . . . . . . . . . . . . . 6 3.2. Clos Network Topology . . . . . . . . . . . . . . . . . . 7 3.2.1. 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
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
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.
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.
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.
+------+ +------+ | | | | | |--| | 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
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.
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
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.
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.
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.
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.
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.
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.
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
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
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].
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.