Information Technology Reference
In-Depth Information
R4 receives the MSDP SA message for group 224.1.1.1. R4 then advertises this SA to both
R3 and R5, which both accept the SA, because it passes the RPF check. However, there is
also an MSDP peering between R3 and R5. R3 advertises the SA to R5, which you see in
the debugs shown in Example 11-11. In this example, the RPF check fails. Example 11-10
indicates that the best path to the originating RP, 10.1.0.1, was received from 10.1.0.4 and
not from 10.1.0.3, which is why the RPF check fails for the SA received from R3. The SA
check was looking for a peer of 10.1.0.4 (R4) to pass the RPF check.
Example 11-11
MSDP Debug for a Failed SA RPF Check
*Mar 3 09:21:18.499: MSDP(0): 10.1.0.3: Received 20-byte msg 29 from peer
*Mar 3 09:21:18.499: MSDP(0): 10.1.0.3: SA TLV, len: 20, ec: 1, RP: 10.1.0.1
*Mar 3 09:21:18.499: MSDP(0): 10.1.0.3: Peer RPF check failed for 10.1.0.1,
used IMBGP route's peer 10.1.0.4
Peer-RPF Checking Rule 2: e(m)BGP Session
The router looks in the BGP table for the best path to the originating RP. The mRIB is
checked first, followed by the uRIB. If a path to the originating RP is not found, the SA fails
the RPF check.
If the best path is found and the MSDP peer is the same IP address as an eBGP peer, the
first AS in the AS_PATH for the best path to the originating RP is compared to the ASN for
the eBGP peer that matches the MSDP peer. If they are the same ASN, the RPF check
succeeds. If they are a different ASN, the RPF check fails. Essentially, this ensures that the
upstream AS toward the originating RP is the same as the MSDP peer from which the SA
was received.
Figure 11-21 shows an example of an RPF check that succeeds.
R5 receives the MSDP SA for group 224.1.1.1. This SA is received from R4 in AS 4. The
MSDP SA RPF function is performed against an e(m)BGP peer. The MSDP debug shown
in Example 11-12 shows that the RPF check for the SA received from 10.3.1.10 (the link
address on R4) passes.
Example 11-12
MSDP Debug for a Successful SA RPF Check on R5
R5#
*Mar 3 09:58:18.291: MSDP(0): 10.3.1.10: Received 20-byte msg 101 from peer
*Mar 3 09:58:18.291: MSDP(0): 10.3.1.10: SA TLV, len: 20, ec: 1, RP: 10.1.0.1
*Mar 3 09:58:18.291: MSDP(0): 10.3.1.10: Peer RPF check passed for 10.1.0.1,
used EMBGP peer
