Sa Message Origination Receipt And Processing; Sa Message Origination; Sa Message Receipt; How Rpf Check Rules Are Applied To Sa Messages - Cisco Catalyst 3650 Configuration Manual

SA Message Origination Receipt and Processing

SA Message Origination Receipt and Processing
The section describes SA message origination, receipt, and processing in detail.

SA Message Origination

SA messages are triggered by an RP (assuming MSDP is configured) when any new source goes active within
a local PIM-SM domain. A local source is a source that is directly connected to the RP or is the first-hop DR
that has registered with it. An RP originates SA messages only for local sources in its PIM-SM domain; that
is, for local sources that register with it.
Note A local source is denoted by the A flag being set in the (S, G) mroute entry on the RP (which can be
viewed in the output of the show ip mroute command). This flag indicates that the source is a candidate
for advertisement by the RP to other MSDP peers.
When a source is in the local PIM-SM domain, it causes the creation of (S, G) state in the RP. New sources
are detected by the RP either by the receipt of a register message or the arrival of the first (S, G) packet from
a directly connected source. The initial multicast packet sent by the source (either encapsulated in the register
message or received from a directly connected source) is encapsulated in the initial SA message.

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.

How RPF Check Rules Are Applied to SA Messages

The rules that apply to RPF checks for SA messages are dependent on the BGP peerings between the MSDP
peers:
• Rule 1: Applied when the sending MSDP peer is also an interior (M)BGP peer.
• Rule 2: Applied when the sending MSDP peer is also an exterior (M)BGP peer.
• Rule 3: Applied when the sending MSDP peer is not an (M)BGP peer.
RPF checks are not performed in the following cases:
• If the sending MSDP peer is the only MSDP peer, which would be the case if only a single MSDP peer
• If the sending MSDP peer is a member of a mesh group.
• If the sending MSDP peer address is the RP address contained in the SA message.
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3SE (Catalyst 3650 Switches)
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or a default MSDP peer is configured.
Configuring MSDP
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