Solution: Multicast Traffic Not Crossing Router¶

Triage¶

Check if multicast routing is enabled globally:
```
show running-config | include multicast-routing
```
Must see ip multicast-routing.
Check PIM configuration on interfaces:
```
show ip pim interface
```
Each VLAN interface that participates in multicast must have PIM enabled.
Check the multicast routing table:
```
show ip mroute
show ip mroute 239.1.1.1
```
If empty, the router is not processing multicast at all.
Check IGMP group memberships on each interface:
```
show ip igmp groups
show ip igmp interface GigabitEthernet0/0
```
Receivers' join requests should appear here.
Verify multicast traffic is reaching the router on the source side:
```
show ip mroute count
```
Or capture on the source-side interface.

Root Cause¶

PIM (Protocol Independent Multicast) is not configured on the router interfaces. Without PIM, the router: - Does not listen for IGMP membership reports from receivers - Does not build multicast routing table entries (S,G) or (*,G) - Does not forward multicast traffic between interfaces

Additionally, ip multicast-routing may not be enabled globally. Both the global command and per-interface PIM configuration are required.

Multicast works within a single VLAN because it is handled at Layer 2 (switches flood or use IGMP snooping to deliver within the VLAN). Crossing a router boundary requires Layer 3 multicast routing (PIM).

Fix¶

Enable multicast routing globally:

configure terminal
ip multicast-routing

Enable PIM on each participating interface:

interface GigabitEthernet0/0
  ip pim sparse-dense-mode
interface GigabitEthernet0/1
  ip pim sparse-dense-mode
interface GigabitEthernet0/2
  ip pim sparse-dense-mode

If using PIM sparse mode, configure a Rendezvous Point:
```
ip pim rp-address 10.1.1.1
```
Or use Auto-RP / BSR for dynamic RP discovery.

Verify multicast state is building:

show ip pim interface
show ip igmp groups
show ip mroute 239.1.1.1

Confirm receivers are now getting traffic:

# On receiver host
tcpdump -i eth0 -nn host 239.1.1.1

Rollback / Safety¶

Enabling PIM is non-disruptive to unicast traffic.
If PIM dense mode is used, be aware it floods multicast everywhere initially and then prunes -- this can cause temporary bandwidth spikes.
PIM sparse mode requires an RP; without it, no multicast tree is built.
Test with a single group and small set of receivers first.

Common Traps¶

Enabling ip multicast-routing but forgetting PIM on the interfaces (or vice versa) -- both are required.
Using PIM sparse mode without configuring an RP -- sparse mode requires an RP for (*,G) joins.
Forgetting that the source-side interface also needs PIM, not just receiver-side.
Multicast application sending with TTL=1 -- the router will not forward packets with TTL=1 (they expire at the router).
IGMP snooping on switches dropping IGMP reports before they reach the router -- ensure IGMP querier is properly configured.
Not checking switch-level IGMP snooping configuration, which can silently drop multicast in the wrong VLAN ports.