Information Technology Reference
In-Depth Information
In Figure 11-23, there is an MSDP session between R6 and R4; however, there is no BGP
session between these routers. An RPF check that requires the MSDP peer to match with a
BGP peer would fail. However, the topology can be checked to prevent loops by ensuring
that the next AS in the path to the RP is the AS in which the MSDP peer that sent the SA
resides.
The MSDP debugs for the RPF check are shown in Example 11-15, which specifies
e(m)BGP as the RPF mechanism. There are no e(m)BGP sessions on R6; the RPF check,
however, passes.
Example 11-15
MSDP Debug for a Successful SA RPF Check
*Mar 3 10:22:35.739: MSDP(0): 10.1.0.4: Received 20-byte msg 13 from peer
*Mar 3 10:22:35.739: MSDP(0): 10.1.0.4: SA TLV, len: 20, ec: 1, RP: 10.1.0.1
*Mar 3 10:22:35.739: MSDP(0): 10.1.0.4: Peer RPF check passed for 10.1.0.1,
used EMBGP peer
The condition to pass an RPF check without a direct relationship between an MSDP and
BGP peering is that the first AS in the path to the originating RP must be the same as the
last AS in the path to the MSDP peer that sent the SA. Example 11-16 shows the BGP path
information. The first path in the AS_PATH for the originating RP is AS 4. The debug
shown in Example 11-15 shows that the MSDP peer that the SA was received from is
10.1.0.4. The prefix for 10.1.0.4 originates in AS 4. Hence, the RPF check passes.
Example 11-16
mBGP Best Path for Originating RP and MSDP Peers
R6#show ip mbgp 10.1.0.3
BGP routing table entry for 10.1.0.3/32, version 4
Paths: (1 available, best #1, table Default-MBGP-Routing-Table)
Not advertised to any peer
5
10.3.1.5 (metric 128) from 10.1.0.5 (10.1.0.5)
Origin IGP, metric 0, localpref 100, valid, internal, best
R6#show ip mbgp 10.1.0.4
BGP routing table entry for 10.1.0.4/32, version 5
Paths: (1 available, best #1, table Default-MBGP-Routing-Table)
Not advertised to any peer
4
10.3.1.10 (metric 128) from 10.1.0.5 (10.1.0.5)
Origin IGP, metric 0, localpref 100, valid, internal, best
R6#show ip mbgp 10.1.0.1
BGP routing table entry for 10.1.0.1/32, version 2
Paths: (1 available, best #1, table Default-MBGP-Routing-Table)
Not advertised to any peer
4 2 3
10.3.1.10 (metric 128) from 10.1.0.5 (10.1.0.5)
Origin IGP, localpref 100, valid, internal, best
