MSDP Packets
MSDP packets are encapsulated in TCP packets and are in the format of Type Length Value (TLV), as shown in Figure 6-2.
- Type: indicates the packet type. Table 6-1 lists types of MSDP packets.
- Length: indicates the packet length.
- Value: indicates the packet content that depends on the packet type.
Value |
MSDP Packet Type |
Function |
Major Information Contained |
|---|---|---|---|
1 |
Source-Active (SA) |
Carries multiple groups of (S, G) entries and is transmitted among several RPs. |
|
Encapsulates PIM-SM multicast data. |
|
||
2 |
Source-Active Request (SA-Req) |
Actively requests (S, G) list of a specified group G to reduce the delay for joining the group. |
Address of the group G. |
3 |
Source-Active Response (SA-Resp) |
Responds to SA request messages. |
|
4 |
KeepAlive |
Maintains MSDP peer connections. The Keepalive packet is sent only when no other protocol packet is exchanged between MSDP peers. |
- |
5 |
Reserved |
This packet type is reserved and used as Notification messages at present. |
- |
6 |
Traceroute in Progress |
Traces and detects the RPF path along which SA messages are transmitted. |
|
7 |
Traceroute Reply |
As described in Table 6-1, SA messages carry (S, G) information and encapsulate multicast packets. MSDP peers share (S, G) information by exchanging SA messages. If an SA message contains only (S, G) information, remote users may not receive multicast data because the (S, G) entry has already timed out when reaching the remote domain. If multicast data packets are encapsulated in an SA message, remote users can still receive multicast data when the (S, G) entry times out.
When a new user joins the group, the user must wait for the SA message sent by the MSDP peer in the next period because SA messages are sent periodically. To reduce the delay for the new user to join the source SPT, MSDP supports SA request and response messages of Type 2 and Type 3 to improve the update efficiency of active source information.