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PIM Snooping for VPLS Use Cases


In a virtual private LAN service (VPLS), the provider edge (PE) devices provide a logical interconnect such that the customer edge (CE) devices belonging to a specific VPLS instance appear to be connected by a single LAN. Since a VPLS provides LAN emulation for layer 2 and layer 3 devices, the unicast and multicast traffic should use the same path for layer 2 protocols to work accurately. Hence, multicast traffic is treated like broadcast traffic and forwarded to all sites in the VPLS instance. This event of forwarding traffic out of every port other than the port it arrived on is called flooding and wastes network bandwidth in the VPLS.

Hence, in a VPLS, multicast traffic is replicated to non-member sites. This can be avoided by restricting traffic to only interested devices that are members of the specific multicast group. This process of directing traffic is called snooping, where a device snoops on Layer 3 multicast protocol packets to identify the devices that are going to receive the traffic.

Snooping depends on the type of multicast protocol used for multicasting traffic in the network. Internet Group Management Protocol (IGMP) snooping is used at Layer 2 when IGMP is configured as the multicast protocol. This document explains how Protocol Independent Multicast (PIM) snooping can be configured in a VPLS that uses PIM as the multicast protocol.

PIM shares many common characteristics with a routing protocol, such as neighbor discovery using hello messages, topology database using the multicast tree, and error detection and notification using the dead timer and designated router election. PIM does not exchange databases. It uses the unicast routing table to provide reverse path information for building multicast trees.

PIM snooping eliminates the service provider tasks of providing the multicast service (running PIM, managing group addresses and multicast tunnels) to customers, saves time, and reduces load on the network. PIM snooping thus optimizes the IP multicast bandwidth in the VPLS core and provides cost savings.