Lab 17 — Multicast PIM Sparse Mode — Auto-RP and MSDP

Active Nodes

Active routers: CORE-1, CORE-2, CORE-3, CORE-4, EDGE-1, EDGE-4, BORDER-1, BORDER-2

Key Concepts

PIM Sparse Mode and RP

PIM-SM requires a Rendezvous Point (RP) that receivers use to join multicast groups and sources register with. All initial multicast traffic flows through the RP via a shared tree ((*,G) state). Once traffic flows, the last-hop router can optionally switch to the shortest-path tree (SPT) by sending a PIM join directly toward the source. SPT switchover is triggered by the ip pim spt-threshold command — default is 0 kbps (immediate switchover). Setting it to infinity keeps traffic on the shared tree permanently.

Auto-RP vs BSR

Auto-RP uses Cisco-proprietary RP announcement and mapping agent mechanisms. RP candidates send announcements to 224.0.1.39; the Mapping Agent listens, selects the best RP, and announces the RP-to-group mappings to 224.0.1.40. BSR (Bootstrap Router) is the RFC standard alternative: BSR candidates flood bootstrap messages with RP candidate information throughout the PIM domain, and each router independently runs the hash algorithm to select the RP. BSR is preferred for multi-vendor environments.

MSDP — Multicast Source Discovery Protocol

MSDP connects multiple PIM-SM domains (or multiple RPs in the same domain). When a source registers with its local RP, the RP sends an MSDP Source Active (SA) message to its MSDP peers. Receiving RPs cache the SA and can join the source tree if they have active receivers. In this lab, BORDER-1 and BORDER-2 each run an RP and peer via MSDP — allowing multicast sources in one region to reach receivers in the other without merging the PIM domains.

Multicast Configuration

CORE-2RP Candidate + Mapping Agent

ip multicast-routing
ip pim rp-address 10.255.1.2

interface Loopback0
 ip pim sparse-mode

interface GigabitEthernet0/0
 ip pim sparse-mode

ip pim send-rp-announce Loopback0 scope 16 group-list MULTICAST-GROUPS
ip pim send-rp-discovery Loopback0 scope 16

ip access-list standard MULTICAST-GROUPS
 permit 239.0.0.0 0.255.255.255

BORDER-1MSDP peer for inter-domain multicast

ip multicast-routing
ip msdp peer 10.255.3.2 connect-source Loopback0
ip msdp sa-filter out 10.255.3.2 list MSDP-EXPORT
ip msdp originator-id Loopback0

ip access-list standard MSDP-EXPORT
 permit 239.0.0.0 0.255.255.255

Verification

show ip pim rp mapping

Expected: RP 10.255.1.2 mapped to group ranges. Source "PIM Auto-RP" or "BSR" shown. All routers should agree on RP mapping.

show ip mroute

Expected: (*,G) entries for active groups. (S,G) entries appear after source registers. OIF list shows interfaces with active receivers.

show ip msdp peer

Expected: MSDP peer 10.255.3.2 in Up state. SA messages received/sent counters incrementing.

show ip msdp sa-cache

Expected: Source Active cache shows (source, group) pairs learned from MSDP peers. Expiry timer indicates freshness.

Research and direction by Ioannis (John) Konstas; compiled and finalized with AI assistance.