Controlling the Single Forwarder Election
When a PE receives a customer join or prune message from a CE, the message identifies a particular multicast flow as belonging either to a source-specific tree (S,G) or to a shared tree (*,G). If the route to the multicast source or RP is across the VPN backbone, then the PE needs to identify the upstream multicast hop (UMH) for the (S,G) or (*,G) flow. Normally the UMH is determined by the unicast route to the multicast source or RP.
However, in some cases, the CEs may be distributing to the PEs a special set of routes that are to be used exclusively for the purpose of upstream multicast hop selection using the route-import community. More than one route might be eligible, and the PE needs to elect a single forwarder from the eligible UMHs.
The default metric for the single forwarder election is the IP address from the global administrator field in the route-import community. You can configure a router to use the unicast route preference to determine the single forwarder election.
To configure the router to use the unicast route preference, include the unicast-umh-election statement.
For a list of the hierarchy levels at which you can include this statement, see the statement summary section for this statement.
