If PIM-SM or other multicast protocols are used in an SMLT environment, enable the protocol on the vIST. Routing protocols in general are not run over an vIST but multicast routing protocols are an exception. When using PIM-SM and a unicast routing protocol, ensure the unicast route to the BSR and RP has PIM-SM active and enabled. If multiple OSPF paths exist and PIM-SM is not active on each pair, the BSR is learned on a path that does not have PIM-SM active. The following figure demonstrates this issue.
The network configuration in the preceding figure is as follows:
5510A is on VLAN 101.
5510B is on VLAN 102.
Switch B is the BSR.
Switch A and Switch B have OSPF enabled.
PIM is enabled and active on VLAN 101.
PIM is either disabled or passive on VLAN 102.
In this example, the unicast route table on Switch A learns the BSR on Switch B through VLAN 102 using OSPF. The BSR is either not learned or does not provide the RP to Switch A.