IP Multicast in a Campus Network
There is often some confusion about how HSRP active and standby devices route IP Unicast and IP multicast traffic. The issue appears when an HSRP active device is successfully routing IP Unicast traffic and the HSRP standby device is forwarding the IP multicast traffic. IP multicast forwarding is not influenced by which box is active or in standby.
The path that multicast traffic takes is based on the shortest path to the source or, in a shared-tree model, the shortest path to the RP. The shortest path to the source or RP is based on the route entries in the unicast routing table. In most Campus designs, the links between layers are equal cost paths. Therefore, the multicast traffic will follow the path through the DR. The DR is determined by which PIM router has the highest IP address on the shared subnet and also which has an RPF interface toward the source.
If multiple paths exist and they are not equal, it is possible for the DR to decide that the shortest path to the source or RP is actually back out the same VLAN that the host is on and through the non-DR router.
Figure 2-3 illustrates the possible issue with HSRP and IP multicast. In this example, it is assumed that the routes are equal-cost. Switch 1 is configured with an HSRP priority of 120 and Switch 2 is configured with a priority of 110. Therefore, HSRP will use Switch 1 as the active router. However, Switch 2 has a higher IP address than Switch 1. Therefore, PIM will use Switch 2 as the DR. The result is that Unicast traffic is sent through Switch 1 while Multicast traffic is sent through Switch 2.
HSRP and IP Multicast
HSRP active non-DR
Designated router-highest IP address WINS
CallManager with MoH
To avoid this situation, either adjust the HSRP priority to make Switch 2 the active router or change the IP addresses so that Switch 1 has the higher address. Either of these actions will make the HSRP active router and the PIM DR the same.
In IOS Release 12.2, the ip pim dr-priority command forces a router to be the DR (regardless of IP address). Not all platforms support the ip pim dr-priority command.
Figure 2-4 shows how changing the HSRP priority value on both switches will cause both unicast and multicast traffic will flow the same way.
Cisco AVVID Network Infrastructure IP Multicast Design