> ## Content Index > Fetch the complete content index at: https://www.pinglabz.com/llms.txt > Use this file to discover other available public pages before exploring further. # BGP Confederations: An Alternative to Route Reflectors - URL: https://www.pinglabz.com/bgp-confederations/ - Published: 2026-03-28T06:52:00.000Z - Updated: 2026-07-04T22:56:36.000Z - Description: BGP confederations scale iBGP by dividing the AS into sub-ASes that speak confederation eBGP internally, while appearing as a single AS to the outside world. An alternative to route reflectors with tradeoffs you need to know. - Author: Jaime - Tags: BGP, #Import 2026-08-01 19:54 Route reflectors are the most common solution for scaling iBGP, but they're not the only one. BGP confederations take a different approach: instead of designating special routers that can break the split-horizon rule, confederations divide the AS into smaller sub-autonomous systems. Within each sub-AS, you run iBGP (with a full mesh or route reflectors). Between sub-ASes, you run a modified form of eBGP that preserves iBGP-like behavior for the attributes that matter. New to BGP? Start with the [full BGP guide](https://www.pinglabz.com/bgp/), then come back here for the deep dive. ## How Confederations Work A confederation makes a single AS appear as one AS to the outside world while internally being divided into multiple sub-ASes. External peers see only the confederation's real ASN - sub-AS numbers are stripped from the AS-path before routes are advertised to true eBGP peers. Between sub-ASes, BGP sessions are "confederation eBGP" - they behave like eBGP (next-hop changes, TTL is 1 by default, AS-path is modified) but with key differences: - Sub-AS numbers are prepended to a special confederation segment in the AS-path (displayed in parentheses) - LOCAL\_PREF is preserved across confederation eBGP sessions (unlike true eBGP) - MED is preserved across confederation eBGP sessions - Confederation AS-path segments don't count toward AS-path length in best path selection (step 4) ## Configuration Let's say we split AS 65001 into two sub-ASes: sub-AS 65501 (containing R1-HQ) and sub-AS 65502 (containing R2-HQ). On R1-HQ (sub-AS 65501): ``` R1-HQ(config)# router bgp 65501 R1-HQ(config-router)# bgp confederation identifier 65001 R1-HQ(config-router)# bgp confederation peers 65502 R1-HQ(config-router)# neighbor 2.2.2.2 remote-as 65502 R1-HQ(config-router)# neighbor 2.2.2.2 update-source Loopback0 R1-HQ(config-router)# neighbor 172.16.0.2 remote-as 65010 ``` On R2-HQ (sub-AS 65502): ``` R2-HQ(config)# router bgp 65502 R2-HQ(config-router)# bgp confederation identifier 65001 R2-HQ(config-router)# bgp confederation peers 65501 R2-HQ(config-router)# neighbor 1.1.1.1 remote-as 65501 R2-HQ(config-router)# neighbor 1.1.1.1 update-source Loopback0 ``` Key commands: - `router bgp 65501` \- the local sub-AS number - `bgp confederation identifier 65001` \- the real AS number seen by the outside world - `bgp confederation peers 65502` \- lists other sub-ASes in the confederation - `neighbor 2.2.2.2 remote-as 65502` \- since 65502 is listed as a confederation peer, this creates a confederation eBGP session (not a true eBGP session) External peers (ISP-A-PE1) still see AS 65001: ``` ISP-A-PE1# show ip bgp 10.1.0.0/16 ... 65001 172.16.0.1 from 172.16.0.1 (1.1.1.1) Origin IGP, valid, external, best ``` The sub-AS numbers (65501, 65502) are completely invisible to external peers. ## AS-Path with Confederations Internally, the confederation sub-AS numbers appear in parentheses: ``` R2-HQ# show ip bgp 100.64.0.0/18 ... (65501) 65010 1.1.1.1 from 1.1.1.1 (1.1.1.1) Origin IGP, localpref 100, valid, confed-external, best ``` The `(65501)` is the confederation segment - it shows the route passed through sub-AS 65501\. The real AS-path (65010) follows. For best path selection step 4 (shortest AS-path), only the real AS-path length counts - confederation segments are ignored. ## Route Reflectors vs Confederations Complexity Route Reflectors Simple - one command per client Confederations Moderate - sub-AS design, renumbering Migration Route Reflectors Easy - add RR config, clients unchanged Confederations Hard - every router changes its local AS Path selection impact Route Reflectors RR may choose suboptimal path for clients Confederations Each sub-AS makes local decisions; more natural Scalability Route Reflectors Hierarchical RRs scale very large Confederations Each sub-AS still needs internal full mesh or RRs Common usage Route Reflectors Enterprise and SP - dominant approach Confederations Some large SPs with natural regional divisions In practice, route reflectors win almost every time due to simpler migration and operation. Confederations are mainly seen in very large service providers where the network has natural regional divisions that map cleanly to sub-ASes, or in networks acquired through mergers where each entity already had its own AS. ## Verification ``` R1-HQ# show ip bgp summary BGP router identifier 1.1.1.1, local AS number 65501 BGP confederation identifier: 65001 BGP confederation peers: 65502 Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd 172.16.0.2 4 65010 286 274 18 0 0 04:42:18 4 2.2.2.2 4 65502 156 162 18 0 0 02:33:41 2 ``` ``` R1-HQ# show ip bgp neighbors 2.2.2.2 | include confed Neighbor under common administration Member of confederation AS 65001 ``` ## Troubleshooting External peer sees sub-AS numbers in AS-path Cause `bgp confederation identifier` not set, or set to wrong ASN Fix Verify `bgp confederation identifier` matches the real public ASN on all routers. Confederation eBGP peer stuck in OpenSent Cause Remote sub-AS not listed in `bgp confederation peers` Fix Add the peer's sub-AS to `bgp confederation peers` on both sides. Routes between sub-ASes losing to longer external paths Cause Confederation AS-path segments being counted in path length Fix Verify IOS XE version - confederation segments should not count. Check `show ip bgp [prefix]` for actual path length comparison. ## Key Takeaways - Confederations divide one AS into sub-ASes, using modified eBGP between them to eliminate the full mesh requirement. - Sub-AS numbers are invisible to external peers - only the confederation identifier (real ASN) appears in the AS-path. - LOCAL\_PREF and MED are preserved across confederation eBGP boundaries, unlike true eBGP. - Route reflectors are simpler to deploy and migrate to - choose confederations only when the network has a natural sub-AS structure. - Confederation AS-path segments don't count toward best path AS-path length comparison.