border gateway protocol is the swiss army knife of an mpls core. it can distribute
unicast, multicast or vpn reachability information between the pe
routers (probably through the route reflector, which is basically a gossipmonger).
this is called internal bgp operation. this kind of session is usually built between
loopback addresses advertised by isis or ospf.
the reachability information heard from this ibgp session is used to forward
packets from outside world toward other pe routers, and information
advertised into this ibgp session is used to get traffic from other pe
and forward them to the outside world.
external bgp operation is the bgp session spoken to an other pe router in
an other autonomous system (probably an other mpls core too) in order to excange traffic between the ases.
there are other modes, for example route reflector client, route server and confederation.
these modes differ in the remote as number, the flooding scope of reachability information,
and the handling of different path attributes of the reachability info.
operation of bgp session starts with the bidirectional open followed an inital flood of
information using updates then the asyncroniously sent updates when something changed from
the previously sent state. an update may contain a reachability for prefixes with an identical
set of path attributes, or a withdrawal.
here are the captures for
these address families carried within the multiprotocol reachable attribute (except ipv4 unicast),
but there are a lot of other extensions (for example addpath that is negotiated
for each afi within the open) to the bgp. probably this is the most actively evolving protocol in the industry.
the unicast afi is used for global internet connectivity, the labeled unicast is used in
case of ias mpls vpn to send reachability of
remote as's loopbacks with a label.
alternatively it could be used instead of ldp or rsvp.
vpn afis are used to provide vpn services on the pe routers.
please note that those captures looks fine now but we may have bgp-compress soon....