, the IP address of the next SPE
toward the destination T-PE, or of the destination T-PE itself if the pseudowire only
has one segment). The source T-PE then sets up a pseudowire segment to that next hop
using targeted LDP. The next-hop S-PE repeats this process until the final S-PE sets
up a pseudowire segment to the target T-PE.
NOTE Because FEC 129 implements ???single-sided signalling,??? only one T-PE needs to
initiate the pseudowire setup. To prevent both T-PEs starting pseudowire setup, the
T-PEs compare AIIs, and the one with the higher SAII initiates setup.
Of course, if the Layer 2 AII PW routing table at each T-PE and S-PE was configured
manually, the solution would fail to address the two challenges. The solution, therefore,
also includes mechanisms for using multiprotocol BGP to advertise NLRIs for the Type
2 AIIs. These NLRIs are advertised as 96-bit prefixes with a mask (in a similar manner
to the way in which IPv4 addresses are advertised by routers as 32-bit prefixes with a
mask). S-PEs may choose to advertise aggregated reachability??”for example, if a Global
ID is assigned to a service provider, then that service provider may configure its border
routers to advertise that Global ID with /32 mask to neighbouring border routers supporting
MS-PWs??”indicating that all PW endpoints matching that Global ID can be
reached through those border routers.
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