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.The prune state has an associated timer, which onexpiration will turn into forward state, allowing data to go down the branchpreviously in prune state (see Figure 264 on page 479).478 TCP/IP Tutorial and Technical Overview Multicast Tree of Router ASourceAfterPruneAMessageBeforeB C3376C\3376F6P1VFigure 264.PIM-DM Flood and Prune Message ExampleThe prune state contains source and group address information.When a newmember appears in a pruned area, a router can graft toward the source for thegroup, turning the pruned branch into forward state.The forwarding branches forma tree rooted at the source leading to all members of the group.This tree is calleda source-rooted tree.9.3.3.3 PIM-DM and DVMRPIn order to understand PIM-DM, comparing it with DVMRP is helpful.PIM-DMdiffers from DVMRP in the following ways:PIM-DM forwards out all interfaces until explicit pruning occurs; DVMRP usesthe parent-child relationship, and split-horizon to recognize downstream routers.PIM-DM is independent of specific unicast routing technology; DVMRP usesdistance vectors such as RIP (see 3.2.2, Distance Vector Routing onpage 99).PIM-DM does not support tunnels; DVMRP does (see tunnels in 9.3.1.1, DVMRP Terminology on page 473).9.3.3.4 PIM-SM Protocol OverviewPIM-SM is designed for sparse multicast regions.The basic concept is thatmulticast data is blocked unless a downstream router explicitly asks for it in order tocontrol the amounts of the traffic that traverse the network.A router receives explicit join/prune messages from those neighboring routers thathave downstream group members.The router then forwards data packetsaddressed to a multicast group, G, only onto those interfaces on which explicit joinshave been received.The basic PIM-SM operation is described below.Note: All routers mentioned in this document are assumed to be PIM-SM capable,unless otherwise specified.1.A designated router (DR) sends periodic join/prune messages toward agroup-specific rendezvous point (RP) for each group for which it has activemembers.The DR is responsible for sending triggered join/prune and registermessages toward the RP.2.Each router along the path toward the RP builds and sends join/prunemessages on toward the RP.Routers with local members join thegroup-specific multicast tree rooted at the RP (see Figure 265 on page 480).Chapter 9.Multicast and Multimedia 479 PIM JoinRP-Rooted TreeRP3376C\3376F6P2Figure 265.PIM-SM Join and RP-Rooted Tree ExampleThe RP's route entry may include such fields as the source address, the groupaddress, the incoming interface from which packets are accepted, the list ofoutgoing interfaces to which packets are sent, timers, flag bits, etc.3.The sender's DR initially encapsulates multicast packets in the resistermessage and unicast it to the RP for the group (see Figure 266).PIM RegisterMulticast DataSRegister MessageRP3376C\3376F6P3Figure 266.PIM-SM Resister Message ExampleThe RP decapsulates each register message and forwards the data packetnatively to downstream routers on the shared RP tree.4.A router with directly connected members first joins the shared RP tree.Therouter can initiate a source's SPT based on a threshold (see Figure 267 onpage 481).480 TCP/IP Tutorial and Technical Overview PIM Join - SPTMulticast DataPIM Join TowardSource (S)SSPT DataRP3376C\3376F6P4Figure 267.PIM-SM SPT5.After the router receives multicast packets from both the source's SPT and RPtree, PIM prune messages are sent upstream towards the RP to prune the RPtree.Now multicast data from the source will only flow over the source's SPTtowards group members (see Figure 268).SPT MulticastDataSPIM PruneRP3376C\3376F6P5Figure 268.PIM-SM RP Tree to Source's SPT Switch ExampleThe recommended policy is to initiate the switch to the SP tree after receiving asignificant number of data packets during a specified time interval from aparticular source.To realize this policy the router can monitor data packetsfrom sources for which it has no source-specific multicast route entry andinitiate such an entry when the data rate exceeds the configured threshold.9.3.3.5 PIM-DM and PIM-SMIn relation to PIM-DM, PIM-SM differs in two essential ways:1.There are no periodic joins transmitted, only explicit triggered grafts/prunes inPIM-DM.2.There is no rendezvous point (RP) in PIM-DM.Chapter 9.Multicast and Multimedia 481 9.3.3 [ Pobierz całość w formacie PDF ]