151
Figure 52 MSDP peer-RPF forwarding
The process of peer-RPF forwarding is as follows:
1. RP 1 creates an SA message and forwards it to its peer RP 2.
2. RP 2 determines that RP 1 is the RP that creates the SA message because the RP address in
the SA message is the same as that of RP 1. Then, RP 2 accepts and forwards the SA
message.
3. RP 3 accepts and forwards the SA message, because RP 2 and RP 3 reside in the same AS
and RP 2 is the next hop of RP 3 to RP 1.
4. RP 4 and RP 5 accept the SA message, because RP 3 is in the same mesh group with them.
Then, RP 4 and RP 5 forward the SA message to their peer RP 6 rather than other members of
the mesh group.
5. RP 4 and RP 5 reside in the closest AS in the route to RP 1. However, RP 6 accepts and
forwards only the SA message from RP 5, because the IP address of RP 5 is higher than that of
RP 4.
6. RP 7 accepts and forwards the SA message, because RP 6 is its static RPF peer.
7. RP 8 accepts and forwards the SA message, because RP 7 is the EBGP or MBGP next hop of
the peer-RPF route to RP 1.
8. RP 9 accepts the SA message, because RP 8 is the only RP of RP 9.
MSDP support for VPNs
Interfaces on the multicast routers in a VPN can set up MSDP peering relationships with each other.
With the SA messages exchanged between MSDP peers, the multi-instance VPN implements the
forwarding of multicast data across different PIM-SM domains.
To support MSDP for VPNs, a multicast router that runs MSDP maintains an independent set of
MSDP mechanism for each VPN that it supports. These mechanisms include SA message cache,
peering connection, timers, sending cache, and cache for exchanging PIM messages.
One VPN is isolated from another, and MSDP and PIM-SM messages can be exchanged only within
the same VPN.
Protocols and standards
RFC 3618, Multicast Source Discovery Protocol (MSDP)
SA message
MSDP peers
AS 1
AS 2
AS 3
AS 4
AS 5
RP 1
RP 2 RP 3
RP 4
RP 5
RP 6 RP 7
RP 8RP 9
Mesh group
Source
(1)
(2)
(3)
(3)
(4)
(7)
(6)
(5)
(4)
Static RPF peers
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