PIM-SM/MSDP Failure and SSM Migration
A content provider uses ASM group 239.100.1.1 for a market-data feed. Two data centers each have a PIM-SM RP: RP-A 10.1.1.1 and RP-B 10.2.2.2, configured as an Anycast RP pair with MSDP peering between the loopbacks. Sources are active only in DC-A. Branch receivers in a separate PIM domain use RP-C 10.3.3.3, which has an MSDP session to RP-A but not RP-B. After a maintenance window, branch receivers stop receiving the feed. RP-C's MSDP SA cache contains the source, but show ip mroute shows the (*,G) tree rooted at RP-C and no (S,G) join toward the source. The branch has a single PIM-SM path to RP-C and no SSM configuration. The provider wants to move this feed to 232.100.1.1 for SSM and keep legacy IGMPv2 receivers working during a six-week transition.
1. Why does RP-C have the MSDP SA but no (S,G) join, and what would you check?
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MSDP is a source-discovery mechanism, not a forwarding tree builder. The SA tells RP-C that a source is active, but to build the shortest-path tree RP-C must have a unicast route to the source and a PIM RPF neighbor toward it. If show ip rpf <source> fails or points to an interface without PIM, RP-C cannot send the (S,G) join, so only the shared tree remains. Check show ip rpf <source>, show ip mroute, show ip pim neighbor, and the MSDP SA cache. Common causes are a missing source prefix in the branch unicast table, an RPF route via a non-PIM link, or an MSDP mesh that is not full. In an Anycast RP design, the MSDP mesh should be full among the anycast RPs; a branch RP peering with only one member can learn SAs but still lack an RPF path for a source registered to another member. Fix the unicast/RPF path or the MSDP mesh, then verify that the (S,G) join is sent upstream.
2. How should the provider migrate the feed to 232.100.1.1 while legacy IGMPv2 receivers still need service?
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232.100.1.1 is in the SSM range 232.0.0.0/8. PIM-SSM does not use an RP or MSDP; the last-hop router needs an IGMPv3 source-specific join from the receiver, and it sends an (S,G) join directly toward the source. Legacy IGMPv2 receivers cannot express a source, so they cannot use true SSM. Run the new 232.100.1.1 feed in parallel with the existing 239.100.1.1 ASM feed during the transition. For legacy receivers that must receive the new group, configure IGMPv2-to-SSM mapping on the last-hop router with a static source mapping for 232.100.1.1; this is a transition aid, not a replacement for IGMPv3. Ensure all relevant routers have IGMPv3 and PIM-SSM enabled, and do not remove the RP/MSDP configuration for the ASM group until all receivers and applications are migrated. Verify with show ip igmp groups and show ip mroute that the (S,G) state is built directly.