BGP autodiscovery is used only in conjunction with the FEC 129. A VPLS instance is
identified by a BGP Layer 2 VPN Identifier extended community (known hereafter as
the VPLS ID). Additionally one or more BGP Route Target (RT) extended communities
are used to define connectivity within the VPLS. In the usual case where the VPLS has a
full mesh of pseudowires, only one RT is required, which will be advertised by all PEs.
446 Chapter 14
One or more 8-byte Route Distinguishers (RDs) are also assigned to each VPLS.
Each VSI is then identified by a combination of the RD and a globally unique 32-bit PE
identifier (which may be an IP address belonging to the PE where that VSI is located
or be locally configured). The PE identifier is then encoded into a BGP NLRI (Network
Layer Reachability Information, i.e., a route advertised by BGP). The RD is used solely
to disambiguate BGP NLRIs for different VPLS instances since, in general, all NLRIs
advertised by the same PE will have the same IP address portion??”usually the loopback
address of the PE. Each PE advertises the NLRI for each of its local VSIs using
BGP, including the BGP community identifying the VPLS and one or more RTs and
setting the BGP next hop for the NLRI to its local loopback IP address.
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