IPv6 multihoming without BGP, quo vadis?#

David 'equinox' Lamparter (NetDEF / FRRouting)

• Why: A deep technical exploration of IPv6 multihoming using source-specific routing (SADR) instead of BGP — relevant to the PM's routing protocol internals and Linux networking interests. Written by a core FRRouting maintainer.
• What: A systematic walkthrough of how to achieve multihomed IPv6 connectivity without PA assignment and BGP, using: DHCPv6-PD for prefix distribution (RFC 8415 §6.3), RFC 6724 rule 5.5 for host source address selection, RFC 8028 / dst-src-routing for source-specific routes on hosts, multiple link-local addresses on routers (draft-link-6man-gulla) to differentiate which uplink advertised which prefix, and MP-QUIC (draft-ietf-quic-multipath) for connection recovery. The talk is brutally honest that "the good shit isn't quite ready yet."
• How: Hosts do a route lookup with :: as source, prefer addresses advertised by the chosen gateway (identified by router link-local address), then look up the route again with the chosen source. Linux ip route add ::/0 from <prefix> syntax installs source-specific routes in a single table (not policy routing). Multiple LL addresses per router (one per uplink prefix) make all topologies look the same to the host. Remaining gaps: PD snooping relays, multi-IA_PD handling, and preference/de-prefer mechanisms for backup uplinks.
• Where applicable: End-sites too small for PA+BGP but needing multihoming. Linux hosts have the best (and worst) support. FRRouting/OpenWRT have partial implementations. Commercial CPE largely doesn't ship these features. The IETF homenet WG attempted this 2015–2018 and "went really poorly" — this talk represents the current best-thinking on a genuinely hard problem.