RFC 4456
BGP Route Reflection: An Alternative to Full Mesh Internal BGP (IBGP)
RFC 4456 💅 BGP Route Reflection: An Alternative to Full Mesh Internal BGP (IBGP)
Okay bestie, so this is RFC 4456 🎀 (Network Working Group), and it's a Standards Track doc, which means it's an actual Internet standards track protocol ✨. It was published in April 2006 and written by T. Bates and E. Chen from Cisco Systems, plus R. Chandra from Sonoa Systems. It obsoletes RFC 2796 and RFC 1966 💀, so those two are retired and this one takes their place. Let's get into it 🦋
Status of This Memo
This doc specifies an Internet standards track protocol for the Internet community, and it's asking for discussion and suggestions for improvements 🌸 For the standardization state and status of this protocol, check the current edition of the "Internet Official Protocol Standards" (STD 1). Distribution of this memo is unlimited 🫶
Copyright Notice
Copyright (C) The Internet Society (2006). 💜
Abstract
Okay so the Border Gateway Protocol (BGP) is an inter-autonomous system routing protocol made for TCP/IP internets 🌈 Typically, every BGP speaker inside a single AS must be fully meshed, so any external routing information has to be re-distributed to all the other routers in that Autonomous System (AS). Honestly this is a serious scaling problem, and it's been well documented, with several alternatives proposed 😭
This doc describes the use and design of a method called "route reflection," which helps get rid of the need for "full mesh" Internal BGP (IBGP) ✨
This document obsoletes RFC 2796 and RFC 1966. 👑
Table of Contents
1. Introduction ....................................................2
2. Specification of Requirements ...................................2
3. Design Criteria .................................................3
4. Route Reflection ................................................3
5. Terminology and Concepts ........................................4
6. Operation .......................................................5
7. Redundant RRs ...................................................6
8. Avoiding Routing Information Loops ..............................6
9. Impact on Route Selection .......................................7
10. Implementation Considerations ..................................7
11. Configuration and Deployment Considerations ....................7
12. Security Considerations ........................................8
13. Acknowledgements ...............................................9
14. References .....................................................9
14.1. Normative References ......................................9
14.2. Informative References ....................................9
Appendix A: Comparison with RFC 2796 ..............................10
Appendix B: Comparison with RFC 1966 ..............................101. Introduction 🎀
Okay so the big problem here: typically, every BGP speaker inside a single AS must be fully meshed, and any external routing information must be re-distributed to all the other routers in that AS. For n BGP speakers within an AS, you need n*(n-1)/2 unique Internal BGP (IBGP) sessions 🙈 This "full mesh" requirement clearly does not scale when there are a large number of IBGP speakers each exchanging a big volume of routing information, which is super common in today's networks.
This scaling problem has been well documented, and a bunch of proposals have been made to fix it [2,3]. This doc is another alternative for getting rid of the need for a "full mesh," and it's called "route reflection" 🦋 This approach lets a BGP speaker (known as a "route reflector") advertise IBGP learned routes to certain IBGP peers. It's a change to the commonly understood concept of IBGP, and it adds two new optional non-transitive BGP attributes to prevent loops in routing updates ✨
This document obsoletes RFC 2796 [6] and RFC 1966 [4]. 💅
2. Specification of Requirements 💜
Here's the key word situation: the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [7] 🫶
3. Design Criteria 🧚
Route reflection was designed to hit the following criteria, and honestly they're iconic 👑
- Simplicity ✨
Any alternative must be simple to configure and easy to understand.
- Easy Transition 🌸
It must be possible to transition from a full-mesh configuration without the need to change either topology or AS. This is an unfortunate management overhead of the technique proposed in [3].
- Compatibility 🌈
It must be possible for noncompliant IBGP peers to continue to be part of the original AS or domain without any loss of BGP routing information.
These criteria were motivated by operational experiences of a very large and topology-rich network with many external connections. That's giving real-world main character energy 💀
4. Route Reflection 🦋
The basic idea of route reflection is very simple, bestie 🍓 Let's look at the simple example in Figure 1 below.
+-------+ +-------+
| | IBGP | |
| RTR-A |--------| RTR-B |
| | | |
+-------+ +-------+
\ /
IBGP \ ASX / IBGP
\ /
+-------+
| |
| RTR-C |
| |
+-------+
Figure 1: Full-Mesh IBGPIn ASX there are three IBGP speakers (routers RTR-A, RTR-B, and RTR-C). With the existing BGP model, if RTR-A receives an external route and it gets selected as the best path, it has to advertise the external route to both RTR-B and RTR-C. RTR-B and RTR-C (as IBGP speakers) will not re-advertise these IBGP learned routes to other IBGP speakers 🙃
Now, if this rule is relaxed and RTR-C is allowed to advertise IBGP learned routes to IBGP peers, then it could re-advertise (or reflect) the IBGP routes learned from RTR-A to RTR-B, and vice versa. This would get rid of the need for the IBGP session between RTR-A and RTR-B, as shown in Figure 2 below 🧠
+-------+ +-------+
| | | |
| RTR-A | | RTR-B |
| | | |
+-------+ +-------+
\ /
IBGP \ ASX / IBGP
\ /
+-------+
| |
| RTR-C |
| |
+-------+
Figure 2: Route Reflection IBGPThe route reflection scheme is built on this basic principle ✨
5. Terminology and Concepts 💅
We use the term "route reflection" to describe the operation of a BGP speaker advertising an IBGP learned route to another IBGP peer. A BGP speaker like that is said to be a "route reflector" (RR), and the route itself is said to be a reflected route 🦋
The internal peers of an RR are divided into two groups:
- Client peers
- Non-Client peers
An RR reflects routes between these groups, and it may reflect routes among client peers. An RR along with its client peers form a cluster 🫧 The Non-Client peer must be fully meshed, but the Client peers need not be fully meshed. Figure 3 shows a simple example that outlines the basic RR components using the terminology above 🌸
/ - - - - - - - - - - - - - -
| Cluster |
+-------+ +-------+
| | | | | |
| RTR-A | | RTR-B |
| |Client | |Client | |
+-------+ +-------+
| \ / |
IBGP \ / IBGP
| \ / |
+-------+
| | | |
| RTR-C |
| | RR | |
+-------+
| / \ |
- - - - - /- - -\- - - - - - /
IBGP / \ IBGP
+-------+ +-------+
| RTR-D | IBGP | RTR-E |
| Non- |---------| Non- |
|Client | |Client |
+-------+ +-------+
Figure 3: RR Components6. Operation 🌈
When an RR receives a route from an IBGP peer, it selects the best path based on its path selection rule. Once the best path is selected, it must do the following, depending on the type of peer it's receiving the best path from 👑
- A route from a Non-Client IBGP peer:
Reflect to all the Clients.
- A route from a Client peer:
Reflect to all the Non-Client peers and also to the Client peers. (Hence the Client peers are not required to be fully meshed.) 🫶
An Autonomous System could have many RRs. An RR treats other RRs just like any other internal BGP speakers. An RR could be configured to have other RRs in a Client group or a Non-client group 🧚
In a simple configuration, the backbone could be divided into many clusters. Each RR would be configured with the other RRs as Non-Client peers (so all the RRs will be fully meshed). The Clients will be configured to maintain IBGP session only with the RR in their cluster. Because of route reflection, all the IBGP speakers will receive reflected routing information 🍓
It's possible in an Autonomous System to have BGP speakers that don't understand the concept of route reflectors (let's call them conventional BGP speakers). The route reflector scheme allows those conventional BGP speakers to coexist 🙈 Conventional BGP speakers could be members of either a Non-Client group or a Client group. This allows for an easy and gradual migration from the current IBGP model to the route reflection model. You could start creating clusters by configuring a single router as the designated RR and configuring other RRs and their clients as normal IBGP peers. Additional clusters can be created gradually ✨
7. Redundant RRs 💖
Usually, a cluster of clients will have a single RR. In that case, the cluster is identified by the BGP Identifier of the RR. Honestly though, this is a single point of failure, so to make it possible to have multiple RRs in the same cluster, all RRs in the same cluster can be configured with a 4-byte CLUSTER_ID. That way an RR can discard routes from other RRs in the same cluster 🔥
8. Avoiding Routing Information Loops ✨
Okay bestie, so when a route gets reflected, a little misconfiguration can accidentally create route re-distribution loops 😭 That's a major disaster, so the route reflection method defines the following attributes to detect and avoid routing information loops 🛡️
ORIGINATOR_ID
ORIGINATOR_ID is a new optional, non-transitive BGP attribute of Type code 9 🎀 It's 4 bytes long, and an RR creates it when it reflects a route. It carries the BGP Identifier of the originator of the route in the local AS. A BGP speaker SHOULD NOT create an ORIGINATOR_ID attribute if one already exists 🙅♀️ And a router that recognizes the ORIGINATOR_ID attribute SHOULD ignore a route received with its BGP Identifier as the ORIGINATOR_ID. Not me ignoring my own route coming back around 💀
CLUSTER_LIST
CLUSTER_LIST is a new, optional, non-transitive BGP attribute of Type code 10 🦋 It's a sequence of CLUSTER_ID values that shows the reflection path the route has passed. Main character energy for a route, honestly 💅
When an RR reflects a route, it MUST prepend the local CLUSTER_ID to the CLUSTER_LIST. If the CLUSTER_LIST is empty, it MUST create a new one 🌸 With this attribute, an RR can identify if the routing information has looped back to the same cluster because of misconfiguration. If the local CLUSTER_ID is found in the CLUSTER_LIST, the advertisement received SHOULD be ignored. Loop caught, loop tossed 🗑️✨
9. Impact on Route Selection ✨
Okay so the BGP Decision Process Tie Breaking rules (Sect. 9.1.2.2, [1]) get a little glow-up here, and they're modified as follows 💖
If a route carries the ORIGINATOR_ID attribute, then in Step f) the ORIGINATOR_ID SHOULD be treated as the BGP Identifier of the BGP speaker that has advertised the route 🫶
In addition, the following rule SHOULD be inserted between Steps f) and g): a BGP Speaker SHOULD prefer a route with the shorter CLUSTER_LIST length. The CLUSTER_LIST length is zero if a route does not carry the CLUSTER_LIST attribute. Shorter path, more iconic 👑
10. Implementation Considerations ✨
Care should be taken to make sure that none of the BGP path attributes defined above can be modified through configuration when exchanging internal routing information between RRs and Clients and Non-Clients. Their modification could potentially result in routing loops 😬
In addition, when a RR reflects a route, it SHOULD NOT modify the following path attributes: NEXT_HOP, AS_PATH, LOCAL_PREF, and MED. Their modification could potentially result in routing loops. Please don't touch these, they're load-bearing 🚫💅
11. Configuration and Deployment Considerations ✨
The BGP protocol provides no way for a Client to identify itself dynamically as a Client of an RR. The simplest way to achieve this is by manual configuration 🛠️
One of the key components of the route reflection approach for addressing the scaling issue is that the RR summarizes routing information and only reflects its best path. Less drama, more scale 📈
Both Multi-Exit Discriminators (MEDs) and Interior Gateway Protocol (IGP) metrics may impact the BGP route selection. Because MEDs are not always comparable, and the IGP metric may differ for each router, with certain route reflection topologies the route reflection approach may not yield the same route selection result as that of the full IBGP mesh approach 🥺 A way to make route selection the same as it would be with the full IBGP mesh approach is to make sure that route reflectors are never forced to perform the BGP route selection based on IGP metrics that are significantly different from the IGP metrics of their clients, or based on incomparable MEDs. The former can be achieved by configuring the intra-cluster IGP metrics to be better than the inter-cluster IGP metrics, and maintaining full mesh within the cluster. The latter can be achieved by:
- setting the local preference of a route at the border router to reflect the MED values, or
- making sure the AS-path lengths from different ASes are different when the AS-path length is used as a route selection criteria, or
- configuring community-based policies to influence the route selection.
One could argue, though, that the latter requirement is overly restrictive, and perhaps impractical in some cases 🤔 One could further argue that as long as there are no routing loops, there are no compelling reasons to force route selection with route reflectors to be the same as it would be with the full IBGP mesh approach. Honestly, a valid point 🌈
To prevent routing loops and maintain a consistent routing view, it is essential that the network topology be carefully considered when designing a route reflection topology. In general, the route reflection topology should be congruent with the network topology when there exist multiple paths for a prefix 🗺️ One commonly used approach is reflection based on Point of Presence (POP), in which each POP maintains its own route reflectors serving clients in the POP, and all route reflectors are fully meshed. In addition, clients of the reflectors in each POP are often fully meshed for the purpose of optimal intra-POP routing, and the intra-POP IGP metrics are configured to be better than the inter-POP IGP metrics. Organized network, organized life 🧚
12. Security Considerations ✨
This extension to BGP does not change the underlying security issues inherent in the existing IBGP [1, 5]. Dramatic? Maybe. But the security stuff is still serious, so don't skip past it 🔐😭
13. Acknowledgements ✨
The authors would like to thank Dennis Ferguson, John Scudder, Paul Traina, and Tony Li for the many discussions resulting in this work 🫶 This idea was developed from an earlier discussion between Tony Li and Dimitri Haskin.
In addition, the authors would like to acknowledge valuable review and suggestions from Yakov Rekhter on this document, and helpful comments from Tony Li, Rohit Dube, John Scudder, and Bruce Cole 💜
14. References ✨
14.1. Normative References
[1] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway Protocol 4 (BGP-4)", RFC 4271, January 2006.
14.2. Informative References
[2] Savola, P., "Reclassification of RFC 1863 to Historic", RFC 4223, October 2005.
[3] Traina, P., McPherson, D., and J. Scudder, "Autonomous System Confederations for BGP", RFC 3065, February 2001.
[4] Bates, T. and R. Chandra, "BGP Route Reflection An alternative to full mesh IBGP", RFC 1966, June 1996.
[5] Heffernan, A., "Protection of BGP Sessions via the TCP MD5 Signature Option", RFC 2385, August 1998.
[6] Bates, T., Chandra, R., and E. Chen, "BGP Route Reflection - An Alternative to Full Mesh IBGP", RFC 2796, April 2000.
[7] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
Appendix A: Comparison with RFC 2796 🎀
The impact on route selection is added 👑
The pictorial description of the encoding of the CLUSTER_LIST attribute is removed, as the description is redundant to the BGP specification. The attribute length field was also inadvertently described as one octet, which got fixed 🙈
Appendix B: Comparison with RFC 1966 🎀
All the changes listed in Appendix A, plus the following 💅
Several terminologies related to route reflection are clarified, and the reference to EBGP routes/peers is removed ✨
The handling of a routing information loop (due to route reflection) by a receiver is clarified and made more consistent 🫶
The addition of a CLUSTER_ID to the CLUSTER_LIST has changed from "append" to "prepend" to reflect the deployed code 🔥
The section on "Configuration and Deployment Considerations" has been expanded to address several operational issues 🌸
Authors' Addresses
Tony Bates
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
EMail: tbates@cisco.com
Ravi Chandra
Sonoa Systems, Inc.
3255-7 Scott Blvd.
Santa Clara, CA 95054
EMail: rchandra@sonoasystems.com
Enke Chen
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
EMail: enkechen@cisco.comFull Copyright Statement
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