IPv6 multihoming without BGP, quo vadis?#

David 'equinox' Lamparter (NetDEF / FRRouting)

• Why: A deep technical exploration of source-specific routing (SADR/dst-src) as the "nice" IPv6 multihoming solution for sites too small for PA+BGP. Written by a core FRRouting maintainer with deep IETF involvement — relevant to the PM's routing internals and Linux networking interests.
• What: A systematic walkthrough of the deployable IPv6 multihoming path without BGP: DHCPv6-PD prefix distribution (RFC 8415 §6.3), host source address selection (RFC 6724 "rule 5.5", being upgraded to MUST in RFC 6724bis), source-specific routes on hosts (RFC 8028, draft-ietf-rtgwg-dst-src-routing), multiple link-local addresses per router to disambiguate which uplink advertised which prefix, and MP-QUIC (draft-ietf-quic-multipath) for connection recovery after failover. Honest assessment: "the good shit isn't quite ready yet."
• How: Hosts do a route lookup with :: as source (step 1), prefer addresses advertised by the chosen gateway identified by link-local address (step 2), then re-lookup with the chosen source (step 3). Linux ip route add ::/0 from <prefix> installs source-specific routes in a single table (not policy routing with multiple tables). Multiple LL addresses per router (draft-link-6man-gulla) make all topologies look the same to the end host. Remaining gaps: hierarchical DHCPv6-PD across multiple routers (no open-source PD snooping relay), multi-IA_PD handling, and preference/de-prefer mechanisms for backup uplinks.
• Where applicable: End-sites too small for PA+BGP but needing genuine multihoming. Linux has both the best router support and the worst end-host behavior. OpenWRT has most features but they don't ship by default; commercial CPEs largely lack them. The IETF homenet WG attempted this 2015–2018 and failed (BABEL vs IS-IS routing protocol fight) — this talk represents current best thinking on a genuinely hard problem.