Configuring Point-to-Multipoint LSP with IGMP Snooping

By default, IGMP snooping in VPLS uses multiple parallel streams when forwarding multicast traffic to PE routers participating in the VPLS. However, you can enable point-to-multipoint LSP for IGMP snooping to have multicast data traffic in the core take the point-to-multipoint path rather than using a pseudowire path. The effect is a reduction in the amount of traffic generated on the PE router when sending multicast packets for multiple VPLS sessions.

Figure 1 shows the effect on multicast traffic generated on the PE1 router (the device where the setting is enabled). When pseudowire LSP is used, the PE1 router sends multiple packets whereas with point-to-multipoint LSP enabled, only a single copy of the packets on the PE1 router is sent.

The options configured for IGMP snooping are applied on a per routing-instance, so all IGMP snooping routes in the same instance will use the same mode, point-to-multipoint or pseudowire.

Figure 1: Point-to-multipoint LSP generates less traffic on the PE router than pseudowire.

In a VPLS instance with IGMP-snooping that uses a point-to-multipoint LSP, mcsnoopd (the multicast snooping process that allows Layer 3 inspection from Layer 2 device) will start listening for point-to-multipoint next-hop notifications and then manage the IGMP snooping routes accordingly. Enabling the use-p2mp-lsp command in Junos allows the IGMP snooping routes to start using this next-hop. In short, if point-to-multipoint is configured for a VPLS instance, multicast data traffic in the core can avoid ingress replication by taking the point-to-multipoint path. If the point-to-multipoint next-hop is unavailable, packets are handled in the VPLS instance in the same way as broadcast packets or unknown unicast frames. Note that IGMP snooping is not supported on the core-facing pseudowire interfaces. PE routers participating in VPLS will continue to receive multicast data traffic regardless of how Point-to-Multipoint is set.