3com S7906E 安装指导

 

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A multicast routing protocol runs on Layer 3 multicast devices to establish and maintain multicast routes 

and forward multicast packets correctly and efficiently. Multicast routes constitute a loop-free data 

transmission path from a data source to multiple receivers, namely, a multicast distribution tree. 

In the ASM model, multicast routes come in intra-domain routes and inter-domain routes. 

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An intra-domain multicast routing protocol is used to discover multicast sources and build multicast 

distribution trees within an AS so as to deliver multicast data to receivers. Among a variety of 

mature intra-domain multicast routing protocols, protocol independent multicast (PIM) is a popular 

one. Based on the forwarding mechanism, PIM comes in two modes – dense mode (often referred 

to as PIM-DM) and sparse mode (often referred to as PIM-SM). 

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An inter-domain multicast routing protocol is used for delivery of multicast information between two 

ASs. So far, mature solutions include multicast source discovery protocol (MSDP) and multicast 

border gateway protocol (MBGP). MSDP is used to propagate multicast source information among 

different ASs, while MBGP, an extension of the Multi-protocol Border Gateway Protocol (MP-BGP), 

is used for exchanging multicast routing information among different ASs.  

For the SSM model, multicast routes are not divided into inter-domain routes and intra-domain routes. 

Since receivers know the position of the multicast source, channels established through PIM-SM are 

sufficient for multicast information transport.  

Layer 2 multicast protocols include IGMP Snooping/MLD Snooping and multicast VLAN/IPv6 multicast 

VLAN. 

 shows where these protocols are in the network.  

Figure 1-9 Position of Layer 2 multicast protocols 

 IPv4/IPv6 multicast packets

 

 

1)  IGMP Snooping/MLD Snooping 

Running on Layer 2 devices, Internet Group Management Protocol Snooping (IGMP Snooping) and 

Multicast Listener Discovery Snooping (MLD Snooping) are multicast constraining mechanisms that 

manage and control multicast groups by listening to and analyzing IGMP or MLD messages exchanged 

between the hosts and Layer 3 multicast devices, thus effectively controlling the flooding of multicast 

data in a Layer 2 network.  

2) Multicast 

VLAN/IPv6 multicast VLAN 

In the traditional multicast-on-demand mode, when users in different VLANs on a Layer 2 device need 

multicast information, the upstream Layer 3 device needs to forward a separate copy of the multicast