IP Multicast and IPv6 Deployment
Multicast questions test the PIM variants (dense, sparse, SSM) and the role of IGMP (v1/v2/v3) and MLD for IPv6. Know the shared tree (RPT) vs. shortest-path tree (SPT) switchover, the Rendezvous Point (static, anycast, or BSR-elected), and how RPF checks prevent loops - a multicast packet must arrive on the interface that routes back to the source. Traps include assuming PIM-DM uses an RP (it does not), confusing IGMP snooping with PIM, and forgetting that SSM requires IGMPv3. Also know MSDP for inter-domain source discovery.
For IPv6, candidates must handle address types (global unicast, link-local fe80::/10, unique local fc00::/7), NDP (neighbor discovery replacing ARP), SLAAC, and DHCPv6. Traps: link-local addresses are required for routing protocol adjacencies; EUI-64 formation; and the difference between ipv6 enable and a configured address. Dual-stack and tunneling (6to4, ISATAP) appear but are lower yield.
Worked example: a receiver joins a group but gets no stream. Check display igmp group on the last-hop router, confirm the RP is reachable with display pim rp-info, and verify RPF with display multicast routing-table. The most common failure is an RPF mismatch caused by asymmetric routing - the fix is a static multicast route (ip rpf-route-static) or correcting unicast routing.
Rule of thumb: multicast forwarding always obeys RPF; if unicast and multicast topologies disagree, multicast breaks.