?– Broadcast TV may be supported using a modified H-VPLS topology where each
U-PE has pseudowires only to the two adjacent PEs on the ring (see Figure 14.10).
Each of the two N-PEs at the head-end of the ring has a single pseudowire to its
adjacent layer U-PE and acts as a multicast router for the VPLS instance with one
N-PE being elected as the IGMP querier for the ring. IP multicast traffic is forwarded
hop by hop around the ring (generally with IGMP snooping enabled toward
DSLAMs, if not on the pseudowires). Thus, instead of N copies of each multicast
packet being sent on a ring of N devices, just one copy is sent. In this topology, it is
possible to use MPLS fast reroute to protect against link failure, though IP multicast
techniques are used to protect against node failure (if a U-PE fails, then each
of the two N-PEs becomes the IGMP querier for the part of the ring between itself
and the failed node).
Figure 14.9 Multiple edge architecture for triple play
VoD
server
MPLS core
BRAS
IPTV feed
U-PE N-PE
DSLAM
462 Chapter 14
Scaling Metro Ethernet
Many service providers initially built Metro Ethernet networks in major cities using
enterprise-class Layer 2 switches in customer buildings and in PoP locations.
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