About IGMP snooping and IGMP proxy

Original link http://blog.163.com/song_jk/blog/static/235693562007022112149449/

 

At present, in an IP network, the carrier's network does not want to provide users in the form of xDSL for the purpose of providing more services, but directly provides a uniform width interface, this form is particularly common in some broadband communities.

Currently, in addition to Internet services, most applications in broadband communities are provided with IPTV. In the promotion of IPTV, it is generally required to support VLAN and Multicast Technologies for access layer switches. For access layer switches, the following two types of multicast support are available: IGMP snooping and IGMP proxy.

IGMP snooping is relatively simple. It listens to IGMP messages between the client and the router to create multicast table entries for a specific multicast. It includes the ing between multicast addresses, physical ports, and VLANs. By using IGMP snooping, you can create a multicast table on a layer-2 switching device that does not support IGMP to save bandwidth and control the forwarding direction of packets.

Here we will focus on IGMP proxy, because there is almost no support for snooping for the original switching device, but it is not supported for proxy.

IGMP proxy sets up a multicast table by intercept IGMP messages between the user and the router. the uplink port of the proxy device executes the role of the host, and the downlink port executes the role of the router.

1. the downlink port executes the role of the router in full accordance with the mechanism specified in IGMP V2, including the queryers' election mechanism, regularly sending General query information, and sending specific queries when receiving the exit packet.

2. the uplink port executes the host role and responds to the query from the vro. When a new user group or the last user in a group exits, it actively sends the member report package or leaves the package.

IGMP proxy implements different functions on two ports, with a relatively large workload. The advantage is that when there is no router in the network, the IGMP proxy device can act as the queryer, in addition, if you want to extend the multicast routing function, proxy is more convenient than snooping.

It is not hard to see that IGMP snooping only listens to IGMP messages to create multicast tables, while IGMP messages are generated through the routers and clients at the upper end. That is to say, if no vro exists, you cannot create multicast tables. Instead, you can only transmit data in the form of broadcast. Proxy is more involved. It directly communicates with the client and the router at the upper end, instead of being a completely transparent role. Because it does more local processing, it reduces the dependency on the upper-end router, but also increases the local system burden. However, on the other hand, using IGMP proxy can reduce the load on the upper-end router. As mentioned above, the proxy will participate in the establishment of multicast groups. When a user request is submitted, the proxy will first check the local multicast group, if this multicast group already exists locally, it adds the user to the multicast group member without sending an application to the upstream router. Otherwise, if no corresponding multicast group is found locally, the proxy will send an application to the upstream router and create a multicast group locally. When a multicast member exits, the proxy also checks whether other multicast members exist in the multicast group. If yes, then, it only deletes the members applying to withdraw from the multicast group. Otherwise, it notifies the upper-end router to cancel the multicast group.

Therefore, the above two application solutions have their own advantages and disadvantages, and the application should be based on the overall environment.

It should be noted that the multicast function is introduced in the LAN environment, and similar principles and applications are also introduced in the dslam and xDSL environments, the dslam device acts as a switch that supports IGMP snooping or proxy in the LAN. The principle is the same.