For routers to route to a system not within its autonomous system (AS), an inter-AS routing protocol is needed.

• Need to combine ASes together by providing reachability information of internet addresses beyond the scope of the current AS
• Need to advertise IP addresses beyond AS and broadcast best route

BGP (Border Gateway Protocol) is the de facto inter-domain routing protocol.

• Provides each autonomous system (AS) a means to:
  • eBGP - obtain subnet reachability from neighboring ASes
  • iBGP - propgate reachability information to all AS-internal routers
  • Determine “good” routes to other networks based on reachability information and policy
• BGP does not advertise IPs internal to system (part of intra-routing protocol)

Observe theere are 2 kinds of BGP connectivities/sessions - eBGP (between ASes), logical iBGP (within AS).

In a BGP session, 2 BGP routers (”peers”) exchange BGP messages over a semi-permanent TCP connection

• Paths for different destination network prefixes are advertised
  • BGP - “path vector” protocol
• e.g. Consider the following set of ASes
  • WHen AS3 gateway 3a advertises path AS3,X to AS2 gateway 2c:
    • AS3 promises to AS2 that it will forward datagrams toward X

• BGP route advertisement contains:
  • Prefix
    • Corresponds to destination being advertised
  • Attributes
    • AS-PATH - list of ASes through which prefix advertisement has passed
    • NEXT-HOP - indicates specific internal-AS router to next-hop AS
• Policy-based routing
  • Gateway receiving route advertisement uses import policy to accept/decline a path (e.g. never to route through AS Y)
  • AS policy determines whether to advertise path to other neighboring ASes
• e.g. BGP path advertisement

  • AS2 router 2c receives path advertisement AS3,X via eBGP from AS3 router a3

  • Based on AS2 policy:

    • AS2 router 2c accepts path AS3,X and propagates this message via iBGP to all AS2 routers
    • AS2 router 2a advertises via eBGP path AS2,AS3,X to AS1 router 1c

    

  • A gateway router can learn about multiple paths to a destination

    • AS1 gateway router 1clearns about the paths to X:
      • AS2,AS3,X from 2a
      • AS3,X from 3a
    • Based on policy, AS1 will choose AS3,X and advertise path within AS1 via iBGP
• BGP message types include:
  • OPEN - open TCP connection to remoge BGP peer, authenticate sending bGP peer
  • UPDATE - advertise new path or withdraw old
  • KEEPALIVE - keep connection alive in absence of UPDATES, ACK open request
  • NOTIFICATION - report errors in previous message + close connection
• BGP is an application-level protocol to maintain routing tables
• e.g. IBGP - inform routers in AS1 to use a particular interface to get to gateway