3Com Switch 7750 Configuration Guide Guide

440 CHAPTER 45: MSDP CONFIGURATION

between RPs in the same PIM-SM domain, between an RP and a common router,

or between common routers.

Figure 105 shows the MSDP peering relationship

between RPs. Unless otherwise specified, examples in the following descriptions

are based on MSDP peering relationship between RPs.

An active multicast source S exists in the PIM-SM1 domain. RP1 in this domain

learns the specific location of the multicast source S through multicast source

MSDP peers (RP nodes) in other PIM-SM domains. An SA message contains the IP

address of the multicast source S, the multicast group address G, the address of

the RP that has generated the SA message, and the first multicast data received by

the RP in the PIM-SM1 domain. The SA message is forwarded by peers. Finally, the

SA message reaches all the MSDP peers. In this way, the information of multicast

source S in the PIM-SM domain is delivered to all PIM-SM domains.

By performing reverse path forwarding (RPF) check, MSDP peers accept SA

messages only from the correct paths and forward the SA messages, thus avoiding

SA message loop. In addition, you can configure a mesh group among MSDP

peers to avoid SA flooding among MSDP peers.

Assume that RP4 in the PIM-SM4 domain receives the SA message. RP4 checks

whether receivers exist in the corresponding multicast group. If yes, RP4 sends a

(S, G) Join message hop by hop to the multicast source S, thus creating a shortest

path tree (SPT) based on the multicast source S. However, a rendezvous point tree

(RPT) exists between RP4 and receivers in the PIM-SM4 domain.

Through MSDP, a PIM-SM domain receiving information from the multicast source

S does not rely on RPs in other PIM-SM domains; that is, receivers can directly join

the SPT based on the multicast source without passing RPs in other PIM-SM

domains.

MSDP application

You can also implement Anycast RP through MSDP. Anycast RP refers to such an

application that an MSDP peering relationship can be established between two

RPs with the same IP address in the same PIM-SM domain, to enable load

balancing and redundancy backup between the two RPs in the same domain. The

candidate RP (C-RP) function is enabled on an interface (typically the loopback

interface) of each of multiple routers in the same PIM-SM domain, and these

interfaces have the same IP address. An MSDP peering relationship is formed

among these interfaces, as shown in

Figure 106.