MAC addresses are learned from the source addresses of frames, whether received
from a local attachment circuit or from a pseudowire. Frames are forwarded toward
known MAC destinations and are replicated by the ingress PE to all pseudowires and
local attachment circuits in cases where the destination address is broadcast, multicast,
or unknown. In order to avoid traffic loops a ???split horizon??? rule is applied to the
pseudowire ports. That is, if a packet is received from a pseudowire, it may never be
forwarded to another pseudowire. The split horizon rule may only be applied because
there is a full mesh of pseudowires between the PEs. If that mesh is broken, then the
VPLS instance will fail to operate correctly. As with point-to-point pseudowire services,
444 Chapter 14
the assumption is made that the MPLS network will ensure full connectivity and will
repair any network faults other than PE failure.
VPLS Forwarding The selection of the multipoint learning bridge as the basis for the
VPLS forwarding architecture constrains its implementation to MAC-learning-capable
devices??”except in cases where the MAC addresses are statically configured at the PE
(an undesirable overhead).
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