--- title: IBGP Source Interface Selection Still Requires... description: FRRouting does not automatically select the correct source interface for IBGP sessions — it only appears to work when the neighbor on the other end has... canonical: https://daily.dev/posts/ibgp-source-interface-selection-still-requires-configuration-ipspace-net-blog-hiplw6wh9 twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: IBGP Source Interface Selection Still Requires Configuration « ipSpace.net blog | daily.dev og:description: FRRouting does not automatically select the correct source interface for IBGP sessions — it only appears to work when the neighbor on the other end has... og:url: https://daily.dev/posts/ibgp-source-interface-selection-still-requires-configuration-ipspace-net-blog-hiplw6wh9 og:image: https://api.daily.dev/og/posts/HIplw6wH9.png og:image:alt: IBGP Source Interface Selection Still Requires Configuration « ipSpace.net blog og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # IBGP Source Interface Selection Still Requires Configuration « ipSpace.net blog **[ipSpace.net Blog](https://daily.dev/sources/ipspace)** · 5 min read · 0 upvotes · 0 comments ## Summary FRRouting does not automatically select the correct source interface for IBGP sessions — it only appears to work when the neighbor on the other end has `neighbor update-source` configured correctly. BGP sessions between loopback addresses require explicit source IP configuration because the TCP/IP stack defaults to the outgoing interface's IP, which the remote router will reject as an unknown neighbor. The post explains the mechanics of TCP session establishment for IBGP, why loopback-based sessions are preferred, and why relying on one-sided configuration is fragile. The takeaway: always configure `neighbor update-source` on both ends of an IBGP session. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### Why does my IBGP session work even though I only configured neighbor update-source on one router? An IBGP session can establish when only one side has `neighbor update-source` configured because the correctly configured router initiates the TCP session with the right loopback source IP, and the other router accepts the incoming connection. The misconfigured router's own outbound TCP SYN is rejected, but the session still comes up via the correctly configured peer's attempt. This is luck, not automatic source selection. _Network engineers debugging one-sided BGP configs find the edge cases faster on daily.dev._ ### What happens if I don't configure neighbor update-source for IBGP sessions between loopback interfaces? Without `neighbor update-source`, the TCP/IP stack uses the outgoing interface IP as the source address for the BGP TCP session. The remote router sees a source IP it has no configured neighbor for and drops the packet or rejects the session. IBGP sessions between loopback addresses require explicit source IP configuration so the TCP SYN is sent from the correct loopback address. _Keeping up with BGP configuration gotchas like this is easier when networking content surfaces where you already read._ ## Similar posts on daily.dev - [The Never-Ending IPv6 Loopback Prefix Saga « ipSpace.net blog](https://daily.dev/posts/the-never-ending-ipv6-loopback-prefix-saga-ipspace-net-blog-4nlhgh10l) · ipSpace.net Blog · 0 upvotes · 0 comments --- Tags: [#bgp](https://daily.dev/tags/bgp) [View this post on daily.dev](https://daily.dev/posts/ibgp-source-interface-selection-still-requires-configuration-ipspace-net-blog-hiplw6wh9) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"IBGP Source Interface Selection Still Requires Configuration « ipSpace.net blog","url":"https://daily.dev/posts/ibgp-source-interface-selection-still-requires-configuration-ipspace-net-blog-hiplw6wh9","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/ibgp-source-interface-selection-still-requires-configuration-ipspace-net-blog-hiplw6wh9"},"datePublished":"2026-08-11T08:51:53.160Z","dateModified":"2026-08-11T08:54:05.457Z","description":"FRRouting does not automatically select the correct source interface for IBGP sessions — it only appears to work when the neighbor on the other end has...","image":"https://media.daily.dev/image/upload/s--ZrL_HSsR--/f_auto/v1722860399/public/Placeholder%2006","thumbnailUrl":"https://media.daily.dev/image/upload/s--ZrL_HSsR--/f_auto/v1722860399/public/Placeholder%2006","isAccessibleForFree":true,"articleSection":"ipSpace.net Blog","inLanguage":"en","publisher":{"@type":"Organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180}},"author":{"@type":"Organization","name":"ipSpace.net Blog","logo":"https://media.daily.dev/image/upload/s--xS7E82MT--/f_auto,q_auto/v1786438303/logos/ipspace?_a=BAMAMicg0","url":"https://daily.dev/sources/ipspace"},"commentCount":0,"discussionUrl":"https://daily.dev/posts/ibgp-source-interface-selection-still-requires-configuration-ipspace-net-blog-hiplw6wh9","interactionStatistic":[{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":0},{"@type":"InteractionCounter","interactionType":{"@type":"CommentAction"},"userInteractionCount":0}],"keywords":"bgp","timeRequired":"PT5M"} {"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https://daily.dev"},{"@type":"ListItem","position":2,"name":"ipSpace.net Blog","item":"https://daily.dev/sources/ipspace"},{"@type":"ListItem","position":3,"name":"IBGP Source Interface Selection Still Requires Configuration « ipSpace.net blog"}]} {"@context":"https://schema.org","@type":"FAQPage","@id":"https://daily.dev/posts/ibgp-source-interface-selection-still-requires-configuration-ipspace-net-blog-hiplw6wh9#faq","mainEntity":[{"@type":"Question","name":"Why does my IBGP session work even though I only configured neighbor update-source on one router?","acceptedAnswer":{"@type":"Answer","text":"An IBGP session can establish when only one side has `neighbor update-source` configured because the correctly configured router initiates the TCP session with the right loopback source IP, and the other router accepts the incoming connection. The misconfigured router's own outbound TCP SYN is rejected, but the session still comes up via the correctly configured peer's attempt. This is luck, not automatic source selection. Network engineers debugging one-sided BGP configs find the edge cases faster on daily.dev."}},{"@type":"Question","name":"What happens if I don't configure neighbor update-source for IBGP sessions between loopback interfaces?","acceptedAnswer":{"@type":"Answer","text":"Without `neighbor update-source`, the TCP/IP stack uses the outgoing interface IP as the source address for the BGP TCP session. The remote router sees a source IP it has no configured neighbor for and drops the packet or rejects the session. IBGP sessions between loopback addresses require explicit source IP configuration so the TCP SYN is sent from the correct loopback address. Keeping up with BGP configuration gotchas like this is easier when networking content surfaces where you already read."}}]} ```