As a further optimisation, selective trees may be used for IP multicast
traffic being carried in a VPLS. Each selective tree only sends traffic to a set of PE devices,
each of which has interested receivers for the IP multicast traffic being sent. Again,
one selective tree may be shared between multiple VPLS instances.
When mapping a single VPLS onto one point-to-multipoint LSP, a single label is
used to identify the VPLS instance. This is in contrast to unicast VPLS traffic where a
two-level stack of tunnel label and PW label is used. However, when using one point-tomultipoint
or multipoint LSP to carry traffic for multiple VPLS instances, it is necessary
to use a two-level label stack so the inner label can identify the VPLS instance.
In this case, all egress PEs will see the same inner label since it is not swapped in the
network. Thus, upstream label allocation is required for these inner labels??”in contrast
to the downstream label allocation normally used in MPLS.
Economic Assessment
It is hard to provide generic costs for the EoMPLS solution because network CAPEX
(capital expense) and OPEX (operational expense) are functions of many variables.
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