Configuring MSDP mesh groups

A PIM Sparse domain can have several RPs that are connected to each other to form an MSDP mesh group. To qualify as a mesh group, the RPs have to be fully meshed; that is, each RP must be connected to all peer RPs in a domain. (Refer to Figure 1.)

A mesh group reduces the forwarding of SA messages within a domain. Instead of having every RP in a domain forward SA messages to all the RPs within that domain, only one RP forwards the SA message. Since an MSDP mesh group is fully meshed, peers do not forward SA messages received in a domain from one member to any member of the group. The RP that originated the SA or the first RP in a domain that receives the SA message is the only one that forwards the message to the members of a mesh group. An RP can forward an SA message to any MSDP router as long as that peer is farther away from the originating RP than the current MSDP router.

The following figure shows an example of an MSDP mesh group. In a PIM-SM mesh group the RPs are configured to be peers of each other. They can also be peers of RPs in other domains.

Figure 1. Example of MSDP mesh group

PIM Sparse Domain 1 in Figure 1 contains a mesh group with four RPs. When the first RP, for example, RP originates or receives an SA message from a peer in another domain, it sends the SA message to its peers within the mesh group. However, the peers do not send the message back to the originator RP or to each other. The RPs then send the SA message farther away to their peers in other domains.The process continues until all RPs within the network receive the SA message.