Configuring the Content of SA Messages

Context

Some multicast sources send multicast data at intervals longer than the aging time of (S, G) entries. In this case, the source DR can only encapsulate multicast data into Register messages and send the messages to the source RP. The source RP transmits (S, G) information in SA messages to the remote RP. The remote RP then sends an (S, G) Join message to the source DR and creates a shortest path tree (SPT). By this time, the (S, G) entry has been aged out. As a result, hosts can never receive multicast data from these multicast sources.

To solve this problem, enable a source RP to encapsulate multicast data packets in SA messages. The source RP then sends multicast data packets in SA messages. After receiving the SA messages, the remote RP decapsulates them and transmits the multicast data packets to users in the local domain along the rendezvous point tree (RPT).

You can also set the minimum time to live (TTL) to control which multicast data packets will be encapsulated in SA messages and forwarded to the MSDP peer.

Before the RP creates an SA message to encapsulate a multicast data packet, it checks the TTL value in the IP header of the multicast data packet. If the TTL value of the packet is smaller than the minimum TTL value, the RP does not create an SA message. If the TTL value of the packet is greater than or equal to the minimum TTL value, the RP encapsulates the packet in an SA message and sends the message to its MSDP peer.
When the RP receives an SA message carrying a multicast data packet, it reduces the TTL value in the IP header by 1 and checks the TTL value. If the TTL value is smaller than the minimum TTL value, the RP does not forward the SA message. If the value is greater than or equal to the minimum TTL value, the RP forwards the SA message to its MSDP peer.

Procedure

Run system-view
The system view is displayed.
Run msdp [ vpn-instance vpn-instance-name ]
The MSDP view is displayed.
Run encap-data-enable
The RP is enabled to encapsulate multicast data packets in SA messages.
(Optional) Run peer peer-address minimum-ttl ttl
The minimum TTL value of multicast data packets is set.

By default, the minimum TTL value of multicast data packets is 0.