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...