Implications Of Rule 2 Of Rpf Checking On Msdp; Rule 3 Of Rpf Checking Of Sa Messages In Msdp; Sa Message Processing - Cisco Catalyst 3850 series Configuration Manual
Ip multicast routing configuration guide
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SA Message Origination Receipt and Processing
Implications of Rule 2 of RPF Checking on MSDP
The MSDP topology must mirror the (M)BGP topology. In general, wherever there is an e(M)BGP peer
connection between two devices, an MSDP peer connection should be configured. As opposed to Rule 1, the
IP address of the far-end MSDP peer connection does not have to be the same as the far-end e(M)BGP peer
connection.The reason that the addresses do not have to be identical is that BGP topology between two
e(M)BGP peers is not described by the AS path.
Rule 3 of RPF Checking of SA Messages in MSDP
Rule 3 of RPF checking is applied when the sending MSDP peer is not a (M)BGP peer at all. When Rule 3
is applied, the RPF check proceeds as follows:
1 The peer searches the BGP 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 URIB. If a path is still not found, the RPF check fails.
2 If the previous search succeeds (that is, the best path to the RP that originated the SA message is found),
the peer then searches the BGP MRIB for the best path to the MSDP peer that sent the SA message. If a
path is not found in the MRIB, the peer then searches the URIB. If a path is still not found, the RPF check
fails.
The autonomous system of the MSDP peer that sent the SA is the origin autonomous system, which is
Note
the last autonomous system in the AS path to the MSDP peer.
1 If the first autonomous system in the best path to the RP is the same as the autonomous system of the
sending MSDP peer, then the RPF check succeeds; otherwise it fails.
SA Message Processing
The following steps are taken by an MSDP peer whenever it processes an SA message:
1 Using the group address G of the (S, G) pair in the SA message, the peer locates the associated (*, G)
entry in the mroute table. If the (*, G) entry is found and its outgoing interface list is not null, then there
are active receivers in the PIM-SM domain for the source advertised in the SA message.
2 The MSDP peer then creates an (S, G) entry for the advertised source.
3 If the (S, G) entry did not already exist, the MSDP peer immediately triggers an (S, G) join toward the
source in order to join the source tree.
4 The peer then floods the SA message to all other MSDP peers with the exception of:
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3.6E (Catalyst 3850 Switches)
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• The MSDP peer from which the SA message was received.
• Any MSDP peers that are in the same MSDP mesh group as this device (if the peer is a member of
a mesh group).
Configuring MSDP
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