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
ibgp unicast,
ebgp unicast,
labeled unicast,
layer3 vpn,
vpls ldp,
vpls bgp,
evpn pbb,
evpn vxlan,
evpn mpls,
sr and
bier.
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 have
compressed updates.
then we got two colorful labelled afis,
ctp and
car.
and now e can turn these
on-off one by one without breaking the session...
