Which two statements about RPF checks in Multicast Source Discovery Protocol (MSDP) are
true? (Choose two)
A.
RPF check should be done against the route to the source of the corresponding PIM-SM
domain.
B.
Checking session advertisement (SA) messages causes messages looping.
C.
The RPF check ensures that there is a working redundancy for anycast RPs.
D.
RPF check should be done against the route to the RP of the PIM-SM domain that originated
the SA.
E.
It prevents message looping, and session advertisement (SA) messages must be RPF
checked.
Explanation:
SA Message Receipt
SA messages are only accepted from the MSDP RPF peer that is in the best path back toward the
originator.
The same SA message arriving from other MSDP peers must be ignored or SA loops can occur.
Deterministically selecting the MSDP RPF peer for an arriving SA message requires knowledge of
the MSDP topology. However, MSDP does not distribute topology information in the form of
routing updates. MSDP infers this information by using (M)BGP routing data as the best
approximation of the MSDP topology for the SA RPF check mechanism. An MSDP topology,
therefore, must follow the same general topology as the BGP peer topology. Besides a few
exceptions (such as default MSDP peers and MSDP peers in MSDP mesh groups), MSDP peers,
in general should also be (M)BGP peers.
Rule 1 of RPF checking in MSDP is applied when the sending MSDP peer is also an i(M)BGP
peer. When Rule 1 is applied, the RPF check proceeds as follows:
The peer searches the BGP Multicast Routing Information Base (MRIB) for the best path to the RP
that originated the SA message. If a path is not found in the MRIB, the peer then searches the
Unicast Routing Information Base (URIB). If a path is still not found, the RPF check fails.
If the previous search succeeds (that is, the best path is found), the peer then determines the
address of the BGP neighbor for this best path, which will be the address of the BGP neighbor that
sent the peer the path in BGP update messages.