Stub Multicast Routing

Stub Multicast Routing

Networks that have remote sites connected in a hub and spoke arrangement over lower speed links can benefit by configuring the spoke routers as stub networks (see Figure 9-5). If PIM-Dense or Sparse-Dense mode is configured on the main campus network, then without additional configuration, multicast traffic would periodically be flooded to the stub network. PIM-Dense mode can also flood multicast traffic on links where a PIM neighbor has been discovered. To prevent this periodic flooding of traffic, the PIM neighbor relationship must be prevented and an IGMP proxy needs to be configured. If PIM-Sparse mode is being employed on the campus, a stub network would not need to know RP-group mappings.

Figure 9-5: A stub multicast network is configured with an IGMP proxy because the PIM neighbor relationship has been prevented from forming.

The configurations for the routers in Figure 9-5 that are needed to create a stub network are listed below:

Router A

ip multicast-routing

interface serial 0

ip address 172.16.1.1 255.255.255.0

ip pim dense-mode

ip pim neighbor-filter 5

access-list 5 deny host 172.16.1.2

Router stub

ip multicast-routing

interface e0

ip address 172.16.2.1 255.255.255.0

ip pim dense-mode

ip igmp helper-address 172.16.1.1

interface serial 0

ip address 172.16.1.2 255.255.255.0

ip pim dense-mode

The stub router forwards IGMP messages from hosts on the ethernet network to router A, which has an access list that blocks the PIM neighbor relationship from forming between the two routers. Only multicast traffic for a group that has been joined on the stub router is forwarded by router A, reducing the multicast traffic on the link.