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:
If the source is on an SMLT link that is not the receiver SMLT, the switch that directly received the data on its side of the source SMLT link forwards it down to the receiver on the receiver SMLT regardless of which switch is the DR for the receiver VLAN. The forwarding switch sends a copy of the data over the vIST link to the peer switch, which drops the data because it knows that the remote SMLT is up and therefore the remote peer has already forwarded the data. If the forwarding switch goes down, the other switch receives the data directly over its source SMLT link and takes over forwarding to the receivers. After the original switch comes back up, the original switch again receives the data directly over its source SMLT. The original switch may not be ready to forward the data because of the protocol reconvergence, so the original switch loses traffic until reconvergence is complete.
If the source is not learned on another SMLT link or the vIST link on each aggregate switch; they have a route to the source which is not on an SMLT or across the vIST. The switches must choose which one forwards the data down the receiver SMLT link; which one is the designated forwarder, so that duplicate data does not occur. The highest IP address is the designated forwarder. If the designated forwarder becomes disabled, the other takes over. When it is reenabled, the other switch sees that it is no longer the highest IP address and it sees that the remote SMLT link comes up. The other switch then assumes that the vIST peer is capable of being the designated forwarder and it stops forwarding down to the receivers. If the original switch is not ready to forward the data due to reconvergence, traffic loss occurs.
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.