Border Gateway Protocol (BGP) Basics

 

Routing involves two basic activities: determination of optimal routing paths and the transport of information groups (typically called packets) through an internetwork. The transport of packets through an internetwork is relatively straightforward. Path determination, on the other hand, can be very complex. One protocol that addresses the task of path determination in today’s networks is the Border Gateway Protocol (BGP).

• It is a Exterior gateway protocol (EGP) used to connect b/w AS defined in RFC 1711
• BGP is a
  • Advanced distance vector routing protocol.
  • Path vector routing protocol.
  • Policy based routing protocol.
  • Inter domain routing protocol. (IDRP)
• It is a Classless routing protocol, hence supports VLSM, CIDR.
• Latest version of BGP is BGP 4.0
• BGP exchanges routing information by forming a unicast neighbor relationship with other devices running BGP. This connection is formed using the TCP protocol on port 179.
• BGP describes path by using Attributes which is similar to Metrics.
• BGP routes exchange network reach ability info called Path Vectors, made up of path attributes.
• BGP peers initially exchange their entire routing table and then only periodic updates as changes in the routing table.
• Timers
  • Route exchange Internal Peer: 5 seconds
  • Route exchange External Peer: 30 seconds
  • Keepalive: 60 seconds
  • Holdtime: 180 seconds
• BGP keeps a version of the routing table and it should be identical for all its peers. The version number changes whenever BGP updates its routing table due to some routing information changes. So if the version increases often there is a flapping issue.
• Two types of BGP
  • EBGP: Communication b/w two Autonomous system
  • IBGP: Communication b/w the same Autonomous system (Gotcha: BGP neighbors in the same autonomous system SHOULD be fully meshed)
• AD for BGP:
  • External BGP :20
  • Internal BGP: 200
• When to use BGP
  • Dual or Multi-homed
  • Providing partial or full internet routing to a downstream router
  • Anytime the AS path information is required