PIM Traffic Delay and SMLT Peer Reboot

PIM uses a designated router (DR) to forward data to receivers on the DR VLAN. The DR is the router with the highest IP address on a LAN. If this router is down, the router with the next highest IP address becomes the DR. However, if the VLAN is an SMLT VLAN, the DR is not a factor in determining which switch forwards the data down to the receiver. Either aggregate switch can forward data to the receiver, because the switches act as one. The switch that forwards depends on where the source is located (on another SMLT/vIST link or on a non-SMLT/non-vIST link) and whether either side of the receiver SMLT link is up or down. If the forwarder switch is rebooted, traffic loss occurs until protocol convergence is completed.

Consider the following cases:

In either case, configuring a static RP helps the situation. To avoid this traffic delay, a workaround is to configure a static RP on the peer SMLT switches. This configuration avoids the process of selecting an active RP router from the list of candidate RPs, and also of dynamically learning about RPs through the BSR mechanism. Then, when the DR comes back, traffic resumes as soon as OSPF converges. This workaround reduces the traffic delay.