Information Technology Reference
In-Depth Information
The address family style of configuration is used when advertising multicast NLRI. This
topic is covered in Chapter 10, “Multiprotocol BGP and MPLS VPN,” for MPLS-VPN
deployment. Configuration examples are provided later in this chapter, in the case study.
mBGP/MSDP Interaction
MSDP and BGP operate hand in hand. The acronym (m)BGP is used here to refer to a BGP
session that is carrying either unicast, multicast, or unicast/multicast NLRI. This is because
both multicast and unicast NLRI can be used for peer-RPF checking, with multicast NLRI
taking precedence over unicast NLRI. The MSDP peering topology should mirror the BGP
peering topology to ensure that proper peer-RPF checks are made.
There are six major peer-RPF rules for incoming SA messages based on the BGP and
MSDP peering congruency. These rules are discussed in detail in the next sections. These
six rules are broken down into three cases where a peer-RPF check is not required and three
specific peer-RPF scenarios.
The peer-RPF check is not required when any of the following conditions are met:
•
The sending MSDP peer is the originating RP for the SA.
•
The sending MSDP peer is a mesh group peer.
•
The sending MSDP peer is the only MSDP peer.
The peer-RPF check rules depend on the MSDP and (m)BGP peering congruency. The
(m)BGP session with the same address as the MSDP peering session is identified. The type
of BGP session, internal or external, is used to determine the criteria for performing the
peer-RPF check. The following rules are used for the peer-RPF check:
•
Rule 1: The sending MSDP peer is also an i(m)BGP peer.
•
Rule 2: The sending MSDP peer is also an e(m)BGP peer.
•
Rule 3: The sending MSDP peer is not an (m)BGP peer.
It should be noted that because the third rule indicates that there is no (m)BGP peer, the
MSDP peering address does not match any of the (m)BGP peer addresses. There must still
be an (m)BGP NLRI on the router for the SA messages to pass the peer-RPF check. This
third rule just deals with the lack of a congruent MSDP and (m)BGP topology.
The next three sections examine these peer-RPF rules in more detail.
Peer-RPF Checking Rule 1: i(m)BGP Session
The router looks for the best path to the originating RP for the MSDP SA message. The
mRIB is checked first, followed by the uRIB. If the path is not found, the RPF check fails.
