IGMP LÀ GÌ

PIM source-specific multicast (SSM) uses a subset ofPIM sparse mode và IGMPhường version 3 (IGMPv3) khổng lồ allow a clientlớn receive multicast traffic directly from the source. PIM SSM usesthe PIM sparse-mode functionality khổng lồ create an SPT between the receiverand the source, but builds the SPT without the help of an RP..

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Any Source Multicast (ASM) was the Original Multicast

RFC 1112, the original multicast RFC, supported both many-to-many& one-to-many models. These came to be known collectively as any-sourcemulticast (ASM) because ASM allowed one or many sources for a multicastgroup"s traffic. However, an ASM network must be able to determinethe locations of all sources for a particular multicast group wheneverthere are interested listeners, no matter where the sources mightbe located in the network. In ASM, the key function of is a required function of the network itself.


Source Discovery in Sparse Mode vs Dense Mode

Multicast source discovery appears khổng lồ be an easy process, butin sparse mode it is not. In dense mode, it is simple enough lớn floodtraffic to lớn every router in the whole network so that every routerlearns the source address of the nội dung for that multicast group.However, the flooding presents scalability and network resource useissues & is not a viable option in sparse mode.

PIM sparse mode (like any sparse mode protocol) achieves therequired source discovery functionality without flooding at the costof a considerable amount of complexity. RP routers must be added andmust know all multicast sources, and complicated shared distributiontrees must be built to lớn the RPs.


PIM SSM is a Subphối of PIM Sparse Mode

PIM SSM is simpler than PIM sparse mode because only the one-to-manyMã Sản Phẩm is supported. Initial commercial multicast Internet applicationsare likely to lớn be available to lớn (thatis, receivers that issue join messages) from only a single source(a special case of SSM covers the need for a backup source). PIM SSMtherefore forms a subset of PIM sparse mode. PIM SSM builds shortest-pathtrees (SPTs) rooted at the source immediately because in SSM, therouter closest to lớn the interested receiver host is informed of theunicast IPhường. address of the source for the multicast traffic. That is,PIM SSM bypasses the RP connection stage through shared distributiontrees, as in PIM sparse mode, and goes directly to lớn the source-baseddistribution tree.


Why Use PIM SSM

In an environment where many sources come & go, such as fora videoconferencing service, ASM is appropriate. However, by ignoringthe many-to-many model & focusing attention on the one-to-many source-specificmulticast (SSM) model, several commercially promising multicast applications,such as television channel distribution over the Internet, might bebrought khổng lồ the Internet much more quickly and efficiently than iffull ASM functionality were required of the network.

An SSM-configured network has distinct advantages over a traditionallyconfigured PIM sparse-mode network. There is no need for shared treesor RP. mapping (no RPhường is required), or for RP-to-RPhường source discoverythrough MSDP..

PIM SSM is simpler than PIM sparse mode because only the one-to-manymã sản phẩm is supported. Initial commercial multicast Internet applicationsare likely to be available to lớn (thatis, receivers that issue join messages) from only a single source(a special case of SSM covers the need for a backup source). PIM SSMtherefore forms a subphối of PIM sparse mode. PIM SSM builds shortest-pathtrees (SPTs) rooted at the source immediately because in SSM, therouter closest khổng lồ the interested receiver host is informed of theunicast IP address of the source for the multicast traffic. That is,PIM SSM bypasses the RP connection stage through shared distributiontrees, as in PIM sparse mode, & goes directly to the source-baseddistribution tree.


PIM Terminology

PIM SSM introduces new terms for many of the concepts in PIMsparse mode. PIM SSM can technically be used in the entire 224/4 multicastaddress range, although PIM SSM operation is guaranteed only in the232/8 range (232.0.0/24 is reserved). The new SSM terms are appropriatefor Internet đoạn phim applications & are summarized in Table 1.

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Table 1: ASM andSSM Terminology

Term

Any-Source Multicast

Source-Specific Multicast

Address identifier

G

S,G

Address designation

group

channel

Receiver operations

join, leave

subscribe, unsubscribe

Group address range

224/4 excluding 232/8

224/4 (guaranteed only for 232/8)


Although PIM SSM describes receiver operations as & , the samePIM sparse mode join & leave sầu messages are used by both forms ofthe protocol. The terminology change distinguishes ASM from SSM eventhough the receiver messages are identical.


How PIM SSM Works

PIM source-specific multicast (SSM) uses a submix of PIM sparsemode và IGMP.. version 3 (IGMPv3) to allow a client to lớn receivemulticast traffic directly from the source. PIM SSM uses the PIM sparse-modefunctionality to create an SPT between the receiver and the source,but builds the SPT without the help of an RPhường.

By mặc định, the SSM group multicast address is limited to lớn theIPhường address range from 232.0.0.0 through 232.255.255.255. However,you can extend SSM operations into another Class D range by includingthe ssm-groups statement at the hierarchy màn chơi. The default SSM address range from232.0.0.0 through 232.255.255.255 cannot be used in the ssm-groups statement. This statement is for adding other multicast addresseskhổng lồ the mặc định SSM group addresses. This statement does not overridethe default SSM group address range.

In a PIM SSM-configured network, a host subscribes khổng lồ an SSMchannel (by means of IGMPv3), announcing a desire lớn join group G& source S (see Figure 1). The directlyconnected PIM sparse-mode router, the receiver"s DR, sends an (S,G)join message lớn its RPF neighbor for the source. Notice in Figure 1 that the RPhường. is not contacted in this processby the receiver, as would be the case in normal PIM sparse-mode operations.


Figure 1: Receiver Announces Desire lớn JoinGroup G & Source S
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The (S,G) join message initiates the source tree và then buildsit out hop by hop until it reaches the source. In Figure 2, the source tree is built across the networklớn Router 3, the last-hop router connected khổng lồ the source.


Figure 2: Router 3 (Last-Hop Router) Joinsthe Source Tree
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Using the source tree, multicast traffic is delivered khổng lồ thesubscribing host (see Figure 3).


Figure 3: (S,G) State Is Built Between theSource and the Receiver
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Using PIM SSM

You can configure Junos OS to accept any-source multicast (ASM)join messages (*,G) for group addresses that are within the defaultor configured range of source-specific multicast (SSM) groups. Thisallows you lớn support a mix of any-source and source-specific multicastgroups simultaneously.

Deploying SSM is easy. You need to lớn configure PIM sparse modeon all router interfaces và issue the necessary SSM commands, includingspecifying IGMPv3 on the receiver"s LAN. If PIM sparse mode is notexplicitly configured on both the source and group member interfaces,multicast packets are not forwarded. Source lists, supported in IGMPv3,are used in PIM SSM. As sources become active sầu & start sending multicastpackets, interested receivers in the SSM group receive the multicastpackets.

To configure additional SSM groups, include the ssm-groups statement at the hierarchymàn chơi.