Zte ZXR10 5900 Product Description page 28

ZXR10 5900 5200 Product Description
sink can be switched over to SPT (shortest path tree). PIM-SM is independent from
unicast routing protocol. It uses unicast routing table instead of a certain unicast routing
protocol to implement RPF checking, PIM-SM is more suitable for the multicast network
with potential group member at the end of WAN connection. In addition, PIM-SM allows
SPT, so it reduces the latency caused by share tree which in other words enhances the
efficiency greatly.
PIM-SM uses share tree to deliver multicast data packet. One share tree has one center
point responsible for sending data packets for all source delivery port of one multicast
group. The message from each source delivery port will be routed along SPT to the
center point which is considered as the root node, and the packets will be dispatched to
each receiving port along SPT. The center of PIM-SM group is called "RP" (Rendezvous
Point). Every network may have multiple RP, but only one RP exists in one multicast
group.
There are three ways for router to get the location of RP. First of all, it can configure RP
statically on every router of the running PIM-SM. The other two ways are dynamic
depending on the PIM-SM version used in the network. PIM-SM V1 uses "Auto-RP", and
PIM-SM V2 adopts candidate-RP.
PIM-SM V2 manually configures some routers running PIM-SM as candidate BSR
(Bootstrap Router), and selects the candidate BSR with the highest priority as official
BSR. BSR is in charge of collecting the candidate RP message of each multicast router,
finding out which candidate RP are in multicast domain, and informing all PIM routers
in PIM domain uniformly, therefore, all PIM routers will follow the unique RP selection
rule to pick up the best RP for each group in RP set. The candidate RP is configured
manually
The router running PIM-SM discovers each other and maintains the neighbor relations
by exchanging hello message. In multi-access network, hello message also consists of
the priority information of router, so that, DR can be chosen accordingly.
The multicast source and the first-hop router (DR connects with the source directly)
encapsulates data packet in a register message, and then this packet will be sent to RP
via unicast route. When RP receives this register message, it will capsulate the packet
and dispatches the message to the receiver of the group all the way along SPT.
Every host used for receiving will add in the multicast group via IGMP member report.
The last-hop router (or the DR in the multi-access network) will deliver the received add-
in message to RP for registration tier-by-tier. The media-router will check if the route of
this group has existed. If it has been there, the downstream query router will be added in
the share tree as a branch. If not, the message will be added in and sent to RP.
When RP or multicast router directly connects receiver, the share tree can be switched
over to source-based SPT. When RP receives a registration message from a new
multicast source, RP will return an add-in message to DR which directly connects with
the multicast source. So a STP from source to RP should be built.
When one DR or the router directly connecting with multicast members receive the first
multicast data packet from the multicast group, or when their received packets reach a
certain threshold , it can switch the share tree to source-based STP. As soon as the
switchover occurs, the router will send a graft message to upstream neighbor asking it to
leave the share tree.
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