Information Technology Reference
In-Depth Information
Peer-RPF Checking Rule 3: No (m)BGP Session
If there is no matching (m)BGP peer when the check is performed on the IP address of the
sending MSDP peer, a different set of criteria is used. The router checks for a best path to
the originating RP. The mRIB is checked first, followed by the uRIB. If no path is found,
the RPF check fails.
If a path to the originating RP is found, the BGP table is checked for a path to the MSDP
peer. The mRIB is checked first, followed by the uRIB. If no path is found, the RPF check
fails.
The originating (last) AS from the path to the MSDP peer is compared to the first AS in
the path to the originating RP. If they are the same, the RPF check succeeds. If they are
different, the RPF check fails. This procedure ensures that the MDT built to the domain
originating the (S,G) passes through the AS from which the MSDP SA was received.
Figure 11-23 shows an RPF check that succeeds.
Figure 11-23
MSDP SA RPF Succeeds Without Matching the BGP Session
Source
ISP 3
ISP 2
R1
R2
BGP Peer
ISP 5
MSDP Peer
ISP 4
SA Message
R3
R4
R5
ISP 1
R6 RP
