NOTE With H-VPLS, there is no requirement for nodes to have fixed roles. Some VPLS
instances in a provider??™s network may be provisioned as ???flat??? VPLS instances with no
hierarchy, whereas others are provisioned as H-VPLS instances where some nodes act
as N-PEs and others as U-PEs.
IP Multicast in VPLS Many service providers have a requirement to carry IP multicast
traffic over VPLS, either for corporate services or when backhauling IP TV traffic in
residential broadband networks. The standard VPLS design replicates multicast traffic
at ingress and floods it to all attachment circuits in the VPLS, making it unsuitable for
this application.
Attempts to improve multicast handling in VPLS have tended to focus either on replacing
ingress replication with another technique (for example, point-to-multipoint LSPs) or
on applying IGMP or PIM snooping to the VPLS to prune IP multicast traffic from PEs
and attachment circuits that have no active listeners for a specific multicast group.
When PE devices are deployed in rings, it is possible to improve VPLS multicast
efficiency??”this is described in ???Typical Deployment Scenarios,??? later in the chapter.
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