Sa Request Messages; Sa Response Messages; Keepalive Messages; Sa Message Origination Receipt And Processing - Cisco Catalyst 3850 series Configuration Manual

Configuring MSDP
For more information about SA messages, see the
Note
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SA Request Messages

SA request messages are used to request a list of active sources for a specific group. These messages are sent
to an MSDP SA cache that maintains a list of active (S, G) pairs in its SA cache. Join latency can be reduced
by using SA request messages to request the list of active sources for a group instead of having to wait up to
60 seconds for all active sources in the group to be readvertised by originating RPs.

SA Response Messages

SA response messages are sent by the MSDP peer in response to an SA request message. SA response messages
contain the IP address of the originating RP and one or more (S, G) pairs of the active sources in the originating
RP's domain that are stored in the cache.

Keepalive Messages

Keepalive messages are sent every 60 seconds in order to keep the MSDP session active. If no keepalive
messages or SA messages are received for 75 seconds, the MSDP session is reset.

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.
A local source is denoted by the A flag being set in the (S, G) mroute entry on the RP (which can be
Note
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.
OL-32598-01
section.
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3.6E (Catalyst 3850 Switches)
SA Message Origination Receipt and Processing
SA Message Origination Receipt and Processing, on
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