sparse-mode中share-tree和spt的收敛过程

PIM SM Shared Tree Join

1.In the example, an active receiver has joined multicast group G by multicasting an IGMP Membership Report that is heard by the Designated Router on this LAN segment—in this case, the leaf router at the bottom of the example.

2.The Designated Router knows the IP address of the Rendezvous Point (RP) router for group G and sends a (*, G) Join for this group towards the RP.

3.This (*, G) Join travels hop-by-hop towards the RP, building a branch of the Shared Tree that extends from the RP to the last-hop router directly connected to the receiver.

4.At this point, group G traffic can flow down the Shared Tree to the receiver.

PIM SM Sender Registration

5.As soon as an active source for group G starts sending multicast packets, its first-hop, Designated Router registers the source with the RP. In order to register a source, the Designated Router encapsulates the multicast packets in a PIM Register message and unicasts it to the RP.

6.When the RP receives the Register message, it de-encapsulates the multicast packets and starts sending them down the Shared Tree towards the receivers. At the same time, the RP initiates the building of a Shortest Path Tree from the source to the RP by sending (S, G) joins towards the source. This results in an (S, G) state being created in all routers along the SPT, including the RP.

7.After the SPT is built from the first-hop, Designated Router to the RP, the multicast traffic starts to flow from source S to the RP without being encapsulated in Register messages.

8.Once the RP begins receiving multicast data down the SPT from source S, it sends a PIM Register Stop message to the source’s first-hop, Designated Router, to inform it that it can stop sending the unicast Register messages.

9.At this point, the multicast traffic from the source is flowing down the SPT to the RP and, from there, down the Shared Tree to the receivers.

PIM SM SPT Switchover

10.PIM SM has the capability for last-hop, Designated Routers (that is routers with directly connected members), to switch to the Shortest Path Tree and bypass the RP if the traffic rate is above a set threshold called the SPT-Threshold.

11.The default value of the SPT-Threshold in Cisco routers is zero. This means that the default behavior for PIM SM Designated Routers attached to active receivers is to immediately join the SPT (to the source) as soon as the first packet arrives via the (*, G) Shared Tree.

12.In the example, the last-hop, Designated Router (at the bottom of the drawing) sends a (S, G) Join message toward the source to join the SPT and bypass the RP.

13.This (S, G) Join message travels hop-by-hop to the first-hop, Designated Router (the router connected directly to the source), hence creating another branch of the SPT. This also creates (S, G) state in all routers along this branch of the SPT.

14.(S, G) traffic is now flowing directly from the source to the receiver via the SPT.

15.Next, a special (S, G) RP-bit Prune message is sent up the Shared Tree to prune off the (S, G) traffic from the Shared Tree. If this were not done, duplicate (S, G) traffic would continue flowing down the Shared Tree to the receiver.

16.After the (S, G) RP-bit Prune has reached the RP, the branch of the SPT from the source to the RP still exists. However, once the RP has received an (S, G) RP-bit Prune via all branches of the Shared Tree (such as in the example), the RP no longer needs this (S, G) traffic—because all the receivers in the network are receiving the traffic via an SPT bypassing the RP.

17.In this example the RP no longer needs the (S, G) traffic, therefore, it will send (S, G) Prune messages back toward the source to shut off the flow of now unnecessary (S, G) traffic.

18.When the (S, G) Prune reaches the first-hop, Designated Router, it prunes off the branch of the (S, G) SPT built between the source and the RP. The traffic is now only flowing down the remaining branch of the (S, G) SPT built between the source and the last-hop router that initiated this switchover.