This is addressed by the ability to configure
dual-homed U-PEs that have pseudowires to two different N-PEs. Only one pseudowire
is active at a time, and if it fails, the U-PE switches over to the other pseudowire.
If the pseudowire has failed because the N-PE has failed, then traffic from other N-PEs
will immediately start flowing to the backup N-PE (since those N-PEs will remove FIB
448 Chapter 14
entries corresponding to the failed N-PE). If, however, the pseudowire has failed because
the LSP from the U-PE to the primary N-PE has failed, then other N-PE devices may
still attempt to send traffic through the primary N-PE as they are unaware of the LSP
failure. (It may be desirable to let the failure happen if the physical path from the U-PE
to the primary N-PE fails, to avoid ???tromboning??? traffic through the core of the network
to the primary N-PE.) To prevent traffic from being blackholed until MAC timers age
out, the U-PE may send an LDP address withdraw message, containing an empty MAC
list TLV, to the (now active) backup N-PE, which will then send a similar message to all
other N-PEs in the VPLS, resulting in them removing all FIB entries for the VPLS except
those destined for the backup N-PE.
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