3Com Switch 7750 Configuration Guide Guide

Overview 443
Assume that three autonomous systems (ASs) exist. They are AS1, AS2, and AS3.
Each AS has a PIM-SM domain associated with it. Each PIM-SM domain contains
at least one RP. See
Figure 108.
Figure 108 Forwarding SA messages between MSDP peers
As shown above, RP1 belongs to AS1. RP2, RP3 and RP4 belong to AS2. RP5 and
RP6 belong to AS3. An MSDP peering relationship exists among these RPs. RP2,
RP3, and RP4 form a mesh group. These MSDP peers perform RPF check and
process SA messages forwarded to one another according to the following rules:
1 If an RP has only one MSDP peer (for example, when RP2 sends an SA message to
RP1), the receiver accepts the SA message from the peer.
2 If an SA message comes from a peer that belongs to the same MSDP mesh group
with the receiver, the receiver accepts the SA message and forwards it to peers out
of the mesh group. For example, when RP2 sends an SA message to RP4, RP4
accepts the message and forwards it to RP5 and RP6.
3 If the MSDP peer sending the SA message is the RP of the PIM-SM domain of the
multicast source, only this SA message is received and forwarded to other peers.
Such a message includes a SA message from RP1 to RP2.
4 If the SA message, such as SA message from RP4 to RP5, is from a static RPF peer,
only this message will be received and forwarded the other peers.
5 If an MSDP peer is the next hop of the EBGP route of the RP where the multicast
source resides, and this peer will is an RPF peer, only SA messages from this peer
will be received and forwarded to the other peers.
6 If an MSDP peer is the next hop of the IGP route of the RP where the multicast
source resides, and this peer is an RPF peer, only messages from this peer will be
received and forwarded to the other peers.
7 If an MSDP peer is in the nearest AS on the optimal path in the RP where the
multicast source resides, this peer is an RPF peer. If more than one peer matches
with the rule above, the peer with the highest IP address is the RPF peer. Only the
messages from this peer will be received and forwarded to the other peers.
8 The other SA messages are neither received nor forwarded.
n
The RPF check for MSDP peers is performed according to their priority. If an MSDP
peer matches with a rule, the following rules are unavailable to the peer.
RP1
Source
AS2
MSDP peers
AS1 AS3
RP2
RP3
RP4
RP6
RP5
mesh group
static peer
(1)
(2)
(3)
(4)
(5)
(6)
SA message