--- title: Three Dimensions of BGP Address Family Nerd Knobs «... description: BGP has three dimensions of address family configurability: transport sessions (TCP over IPv4/IPv6, with QUIC drafts emerging), address families enabled per... canonical: https://daily.dev/posts/three-dimensions-of-bgp-address-family-nerd-knobs-ipspace-net-blog-hc5al2eku twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Three Dimensions of BGP Address Family Nerd Knobs « ipSpace.net blog | daily.dev og:description: BGP has three dimensions of address family configurability: transport sessions (TCP over IPv4/IPv6, with QUIC drafts emerging), address families enabled per... og:url: https://daily.dev/posts/three-dimensions-of-bgp-address-family-nerd-knobs-ipspace-net-blog-hc5al2eku og:image: https://api.daily.dev/og/posts/hc5AL2EkU.png og:image:alt: Three Dimensions of BGP Address Family Nerd Knobs « 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. # Three Dimensions of BGP Address Family Nerd Knobs « ipSpace.net blog **[ipSpace.net Blog](https://daily.dev/sources/ipspace)** · 5 min read · 0 upvotes · 0 comments ## Summary BGP has three dimensions of address family configurability: transport sessions (TCP over IPv4/IPv6, with QUIC drafts emerging), address families enabled per session (AFI/SAFI combinations), and next hop address families. The post walks through well-known AFI/SAFI combinations (IPv4 unicast, IPv6 unicast, MPLS VPN, EVPN), explains how the assumption that prefixes and next hops belong to the same address family broke down with VPNv4/VPNv6, BGP-LU, and EVPN. RFC 5549 and RFC 8950 formalized IPv6 next hops for IPv4 prefixes, a requirement that becomes critical for SRv6 overlay services. The Extended Next Hop Encoding Capability (RFC 8950) uses AFI/SAFI/NH-AFI triplets to negotiate these combinations between BGP neighbors. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### What AFI/SAFI numbers are used for EVPN in BGP? EVPN uses AFI 25 (L2VPN) and SAFI 70, making the combination 25/70. EVPN with MPLS can use either IPv4 or IPv6 next hops, while EVPN with VXLAN typically requires an IPv4 next hop. _Network engineers working with EVPN deployments track AFI/SAFI details and BGP RFCs on daily.dev._ ### What RFC defines IPv6 next hops for IPv4 prefixes in BGP? RFC 8950 (successor to RFC 5549) defines how to advertise IPv4 prefixes with IPv6 next hops in BGP. This is negotiated using the Extended Next Hop Encoding Capability, which uses AFI/SAFI/NH-AFI triplets so neighbors can signal support for cross-family next hops. It is essential for SRv6 deployments where IPv4 traffic must be forwarded to an IPv6 SRv6 SID. _Engineers designing SRv6 or unnumbered EBGP topologies find relevant RFC discussions and implementation notes on daily.dev._ ### What happens to a BGP session when you change the AFI/SAFI set on it? Changing the AFI/SAFI set on a BGP session brings down the transport session, because AFI/SAFI combinations are negotiated during BGP capabilities exchange at session establishment. The exception is Multisession BGP (draft-ietf-idr-bgp-multisession), which opens a separate transport session per AFI/SAFI combination, avoiding session resets when the set changes. _Teams managing BGP policy changes in production can follow operational BGP topics on daily.dev._ ## Similar posts on daily.dev - [BGP-Free Network Core with SRv6 « ipSpace.net blog](https://daily.dev/posts/bgp-free-network-core-with-srv6-ipspace-net-blog-9bbn88wub) · ipSpace.net Blog · 0 upvotes · 0 comments - [The world in which IPv6 was a good design](https://daily.dev/posts/the-world-in-which-ipv6-was-a-good-design-tj3i8ov0s) · Hacker News · 0 upvotes · 0 comments - [UDP Broadcasting And The Brave New World Of IPv6](https://daily.dev/posts/udp-broadcasting-and-the-brave-new-world-of-ipv6-kbr2qsudb) · Hackaday · 0 upvotes · 0 comments - [IPng Networks](https://daily.dev/posts/ipng-networks-u37jx9af5) · IPng Networks · 0 upvotes · 0 comments - [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: [#networking](https://daily.dev/tags/networking), [#bgp](https://daily.dev/tags/bgp) [View this post on daily.dev](https://daily.dev/posts/three-dimensions-of-bgp-address-family-nerd-knobs-ipspace-net-blog-hc5al2eku) ```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":"Three Dimensions of BGP Address Family Nerd Knobs « ipSpace.net blog","url":"https://daily.dev/posts/three-dimensions-of-bgp-address-family-nerd-knobs-ipspace-net-blog-hc5al2eku","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/three-dimensions-of-bgp-address-family-nerd-knobs-ipspace-net-blog-hc5al2eku"},"datePublished":"2026-08-11T08:52:14.775Z","dateModified":"2026-08-11T09:00:33.769Z","description":"BGP has three dimensions of address family configurability: transport sessions (TCP over IPv4/IPv6, with QUIC drafts emerging), address families enabled per...","image":"https://media.daily.dev/image/upload/s--HRgLpUt6--/f_auto/v1722860399/public/Placeholder%2003","thumbnailUrl":"https://media.daily.dev/image/upload/s--HRgLpUt6--/f_auto/v1722860399/public/Placeholder%2003","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/three-dimensions-of-bgp-address-family-nerd-knobs-ipspace-net-blog-hc5al2eku","interactionStatistic":[{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":0},{"@type":"InteractionCounter","interactionType":{"@type":"CommentAction"},"userInteractionCount":0}],"keywords":"networking,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":"Three Dimensions of BGP Address Family Nerd Knobs « ipSpace.net blog"}]} {"@context":"https://schema.org","@type":"FAQPage","@id":"https://daily.dev/posts/three-dimensions-of-bgp-address-family-nerd-knobs-ipspace-net-blog-hc5al2eku#faq","mainEntity":[{"@type":"Question","name":"What AFI/SAFI numbers are used for EVPN in BGP?","acceptedAnswer":{"@type":"Answer","text":"EVPN uses AFI 25 (L2VPN) and SAFI 70, making the combination 25/70. 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