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As shown in Figure 115 “MSDP working principles (II)”, Switch A, Switch B, Switch 
C, Switch D, Switch E and Switch F belong to domain 1, domain 2 and domain 3 
respectively. MSDP peer relationship is established between them, indicated with 
bi-directional arrows in the figure. Among them, Mesh Group is created among 
Switch B, Switch C and Switch D.

Figure 115   MSDP working principles (II)

The SA message forwarding and RPF check among these MSDP peers are 
illustrated as follows:

If the SA message is from a MSDP peer that is the RP of the multicast source as 
from Switch A to Switch B, it is received and forwarded to other peers.

If the SA message is from a MSDP peer that has only one peer as from Switch B to 
Switch A, it is received.

If the SA message is from a static RPF peer as from Switch D to Switch E, it is 
received and forwarded to other peers.

If the SA message is from a MSDP peer in Mesh Group as from Switch B to Switch 
D, it is received and forwarded to the peers outside the Mesh Group.

If the SA message is sent from a MSDP peer in a same domain, and the peer is the 
next hop along the optimal path to the RP in the domain of source, as in the case 
when the message is from Switch E to Switch F, it is received and forwarded to 
other peers.

AS 1

AS  2

AS 3

SwitchD

SwitchF

SwitchE

SwitchC

SwitchB

SwitchA

Source

mesh group

Static peer