Msdp Message Types; Sa Messages - Cisco Catalyst 3850 series Configuration Manual

MSDP Message Types

1 The SA message identifies the source address, the group that the source is sending to, and the address or
the originator ID of the RP, if configured.
2 Each MSDP peer that receives the SA message floods the SA message to all of its peers downstream from
the originator. In some cases (such as the case with the RPs in PIM-SM domains B and C in the figure),
an RP may receive a copy of an SA message from more than one MSDP peer. To prevent looping, the RP
consults the BGP next-hop database to determine the next hop toward the originator of the SA message.
If both MBGP and unicast BGP are configured, MBGP is checked first, and then unicast BGP. That
next-hop neighbor is the RPF-peer for the originator. SA messages that are received from the originator
on any interface other than the interface to the RPF peer are dropped. The SA message flooding process,
therefore, is referred to as peer-RPF flooding. Because of the peer-RPF flooding mechanism, BGP or
MBGP must be running in conjunction with MSDP.
1 When an RP receives an SA message, it checks to see whether there are any members of the advertised
groups in its domain by checking to see whether there are interfaces on the group's (*, G) outgoing interface
list. If there are no group members, the RP does nothing. If there are group members, the RP sends an (S,
G) join toward the source. As a result, a branch of the interdomain source tree is constructed across
autonomous system boundaries to the RP. As multicast packets arrive at the RP, they are then forwarded
down its own shared tree to the group members in the RP's domain. The members' DRs then have the
option of joining the rendezvous point tree (RPT) to the source using standard PIM-SM procedures.
2 The originating RP continues to send periodic SA messages for the (S, G) state every 60 seconds for as
long as the source is sending packets to the group. When an RP receives an SA message, it caches the SA
message. Suppose, for example, that an RP receives an SA message for (172.16.5.4, 228.1.2.3) from
originating RP 10.5.4.3. The RP consults its mroute table and finds that there are no active members for
group 228.1.2.3, so it passes the SA message to its peers downstream of 10.5.4.3. If a host in the domain
then sends a join to the RP for group 228.1.2.3, the RP adds the interface toward the host to the outgoing
interface list of its (*, 224.1.2.3) entry. Because the RP caches SA messages, the device will have an entry
for (172.16.5.4, 228.1.2.3) and can join the source tree as soon as a host requests a join.
In all current and supported software releases, caching of MSDP SA messages is mandatory and cannot
Note
be manually enabled or disabled. By default, when an MSDP peer is configured, the ip multicast
cache-sa-state command will automatically be added to the running configuration.
MSDP Message Types
There are four basic MSDP message types, each encoded in their own Type, Length, and Value (TLV) data
format.

SA Messages

SA messages are used to advertise active sources in a domain. In addition, these SA messages may contain
the initial multicast data packet that was sent by the source.
SA messages contain the IP address of the originating RP and one or more (S, G) pairs being advertised. In
addition, the SA message may contain an encapsulated data packet.
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3.6E (Catalyst 3850 Switches)
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Configuring MSDP
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