??? This may be used to speed
up VPLS convergence when there is an attachment circuit failure that results in MAC
addresses moving to another attachment circuit (generally because spanning tree or
multi-chassis 802.3ad is operating in a Layer 2 access network between the MAC addresses
and the PE devices) or when a dual-homed MTU-s switches over from one serving
N-PE to another (see ???H-VPLS??? section below).
BGP Autodiscovery One advantage that the BGP VPLS approach had over the LDP
approach was the ability to autodiscover VPLS peers through BGP??”in other words, for
a PE device to learn the IP addresses of the other PE devices in the VPLS using BGP
rather than the administrator having to statically configure those addresses. Various
approaches to autodiscovery were investigated for LDP VPLS, including LDP itself
(operating in hop by hop mode), OSPF-TE, and RADIUS, but none of these were ultimately
adopted. More recently, consensus has formed around the use of BGP for autodiscovery
while still using LDP for signalling. The BGP attributes used for autodiscovery are
modelled closely on those used for IP VPNs and are similar to those used in BGP VPLS.
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