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Multicast-Aware VPLS
VPLS is a Layer 2 service, hence, multicast and broadcast frames are normally flooded in a VPLS.
Broadcast frames are targeted to all receivers. However, for IP multicast, normally for a multicast
group, only some receivers in the VPLS are interested. Flooding to all sites can cause wasted
network bandwidth and unnecessary replication on the ingress PE router.
In order to improve this condition, VPLS is IP multicast aware so it forwards IP multicast traffic
based on multicast states.
PIM Snooping for VPLS
PIM snooping for VPLS allows a VPLS PE router to build multicast states by snooping PIM
protocol packets that are sent over the VPLS. The VPLS PE then forwards multicast traffic based
on the multicast states. When all receivers in a VPLS are IP multicast routers running PIM,
multicast forwarding in the VPLS is efficient when PIM snooping for VPLS is enabled.
Because of PIM join/prune suppression, in order to make PIM snooping operate over VPLS
pseudowires, two options are available, plain PIM snooping and PIM proxy. PIM proxy is the
default behavior when PIM snooping is enabled for a VPLS.
Plain PIM Snooping
In plain PIM snooping configuration, VPLS PE routers only snoop, PIM messages generated on
their own. Join/prune suppression must be disabled on CE routers.
When plain PIM snooping is configured, a VPLS PE router detects a condition where join/prune
suppression is not disabled on one or multiple CE routers, the PE router should put PIM snooping
into PIM proxy state. A trap is generated which reports the condition to the operator and is logged
to syslog. If the condition changes, for example, join/prune suppression was disabled on CE
routers, the PE reverts to plain PIM snooping state. A trap is generated and is logged to syslog.
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