Research On Layer2 Management Technique For Multicast Traffic In Layer3 Switch

With the popularization of wideband network, many multicast applications such as online video conference and distance education are becoming more and more popular. On the other hand, because of the ability to forward IP datagram at high speed, more and more layer3 switches have been participating in the construction of networks. But, when a layer3 switch wants to forward multicast data through a VLAN, which can be seen as a layer3 IP interface, it replicates the multicast datagram onto all of the layer2 ports who belong to the VLAN just as broadcast does. This causes the multicast traffic reach the downstream network devices who don't want to receive it, and thus wastes the resource of those devices.In response to this problem, this paper designs a layer2 forwarding method to handle the multicast traffic. After comparing PIM protocol with the other multicast routing protocols, the author found PIM protocol much more popular than the others because of its simplicity, so the author decides to do the research based on PIM protocol. A method called PIM-Snooping is proposed in this paper as a solution for handling layer2 multicast traffic. By snooping and parsing the PIM messages, PIM-Snooping can build a forwarding state for each layer2 port. And a forwarding table for multicast traffic should be built according to the changes of the forwarding state. When multicast traffic arrives at the layer3 switch, the switch will efficiently replicate multicast datagram onto layer2 ports where receivers for the specific multicast group exists according to the forwarding table.In this paper, the author also realizes the layer2 forwarding method for multicast traffic, and builds a testing environment under FreeBSD. The test results show that, the PIM-Snooping method can build an accurate forwarding table to forward the multicast traffic, and thus the bandwidth and the resource of the downstream network devices will be greatly saved.