629
Protocol but make RPF Check using existing Unicast routing table.
1. PIM-SM Working Principle
The central working processes of PIM-SM are: Neighbor Discovery, Generation of
RP Shared Tree (RPT), Multicast source registration, SPT Switch, etc. We won’t describe
the mechanism of Neighbor Discovery here since it is same as that of PIM-DM.
(1) Generation of RP Shared Tree (RPT)
When a host joins a Multicast Group G, the leaf router that is connected to this host
directly finds out through IGMP message that there is a receiver of Multicast Group G,
then it works out the corresponding Rendezvous Point RP for Multicast Group G, and
send join message to upper lever nodes in RP direction. Every router on the way from
the leaf router to RP will generate a (*, G) table entry, where a message from any source
to Multicast group applies to this entry. When RP receives the message sent to Multicast
Group G, the message will get to the leaf router along the set up path and reach the host.
In this way the RPT with RP as root is generated.
(2) Multicast Source Registration
When a Multicast Source S sends a Multicast packet to Multicast Group G, the
PIM-SM Multicast router connected to it directly will take charge of encapsulating the
Multicast packet into registered message and unicast it to corresponding RP. If there are
more than one PIM-SM Multicast routers on a network segment, then DR (Designated
Router) takes charge of sending the Multicast packet.
(3) SPT Switch
When the Multicast router finds that the rate of the Multicast packet from RP with
destination address G exceeds threshold, the Multicast router will send Join message to
the next upper lever nodes in the source direction, which results in the switch from RPT
to SPT.
2. Preparation before PIM-SM configuration
(1) Configuration Candidate RP
More than one RPs (candidate RP) can exist in PIM-SM network and each C-RP
(Candidate RP) takes charge of transmitting Multicast packets with destination address in
a certain range. To configure more than one candidate RPs can implement RP load
share. No master or slave is differentiated among RPs. All Multicast routers work out the
RP corresponding to some Multicast group based on the same algorithm after receiving
the candidate RP message announced by BSR.
Note that one RP can serve more than one Multicast groups and all Multicast groups.
Each Multicast group can only correspond to one unique RP at any moment. It can’t
correspond to more than one RP at the same time.
(2) Configure BSR
BSR is the management center of PIMSM network. It is in charge of collecting
