Migrating a Large Telco Networks IGP from OSPF/OSPFv3 to ISIS#

Sam Faunt - Aussie Broadband

• Why: IGP migrations at telco scale are rare war stories with real failure detail — core routing/WAN interest, and a candid postmortem-style account of what broke.
• What: Aussie Broadband moved its backbone IGP from OSPF/OSPFv3 (two processes for two address families) to IS-IS, motivated by one protocol for v4+v6, unifying the IGP across AS4764 and AS9268, segment routing readiness, and vendor support. Method: lab first (convergence testing, choosing FRR/SRLG-disjoint/node-protection/microloop-avoidance features, prefix policy — loopbacks only vs loopbacks + links, OSPF/IS-IS coexistence on the same box, cross-platform behavior); deploy IS-IS alongside running OSPF; cut over by lowering OSPF/OSPFv3 admin distance below IS-IS so IS-IS takes over. AS4764 cut over cleanly. During AS9268, traffic blackholed — the RIB still preferred OSPF for some loopbacks; they assumed stale routes, rebooted the core router, and it did not come back. The network was eventually migrated fully to IS-IS.
• How: Admin-distance-based cutover with both IGPs running simultaneously; phased AS-by-AS; lab validation before deployment.
• Where applicable: SP/telco backbones running dual OSPFv2/v3. The specific blackhole and failed reboot are org- and platform-specific, but the lab-first, coexistence-then-cutover method is broadly transferable. The slides are thin on detail — the talk is likely richer than the deck.