Following is the establishment of the peer relationship, the discovery process begins as soon as a new VPLS service instance is provisioned on the PE.
Two VPLS identifiers are used to indicate the VPLS membership and the individual VPLS instance:
VPLS-ID
Membership information, unique network wide identifier; same value assigned for all VPLS switch instances (VSIs) belonging to the same VPLS; encodable and carried as a BGP extended community in one of the following formats:
a two-octet AS specific extended community
an IPv4 address specific extended community
VSI-ID
The unique identifier for each individual VSI, built by concatenating a route distinguisher (RD) with a 4 bytes identifier (usually the system IP of the VPLS PE); encoded and carried in the corresponding BGP NLRI.
To advertise this information, BGP AD employs a simplified version of the BGP VPLS NLRI where just the RD and the next 4 bytes are used to identify the VPLS instance. There is no need for Label Block and Label Size fields as T-LDP takes care of signaling the service labels later on.
The format of the BGP AD NLRI is very similar with the one used for IP VPN, as shown in the following figure. The system IP may be used for the last 4 bytes of the VSI-ID further simplifying the addressing and the provisioning process.
Network Layer Reachability Information (NLRI) is exchanged between BGP peers indicating how to reach prefixes. The NLRI is used in the Layer 2 VPN case to tell PE peers how to reach the VSI instead of specific prefixes. The advertisement includes the BGP next hop and a route target (RT). The BGP next hop indicates the VSI location and is used in the next step to determine which signaling session is used for pseudowire signaling. The RT, also coded as an extended community, can be used to build a VPLS full mesh or a H-VPLS hierarchy through the use of BGP import or export policies.
BGP is only used to discover VPN endpoints and the corresponding far end PEs. It is not used to signal the pseudowire labels. This task remains the responsibility of targeted-LDP (T-LDP).