BGP – Neighbour and Sessions

Session based protocol – TCP running on port 179. Reliability comes from TCP.

Two types of neighbours – iBGP and eBGP (internal and external). The AS number after the neighbour statement defines whether the neighbour is internal or external. They are referred to as peers and the relationship forming process can be a slow affair.

There are no hellos. Neighbours are established manually. There is an element of “trust” between organisations running BGP.

Neighbours do not have to be directly connected. Neighbours can form over an underlay network being connected across multiple hops by an IGP or static routes.

Numerous rules around BGP – syncronisation, next hop etc.

BGP tyically used by service providers and enterprise networks where redundant egress points exist.

Session communications:

  • OPEN – starts a BGP session
  • KEEPALIVE – maintains the session i.e. it stays up
  • UPDATE – Routing updates
  • NOTIFICATION – BGP error – used when the session will be terminated.

Forming neighbour relationships pass through six states

  • IDLE – Neighbours configured but not connected.
  • CONNECT – Attempts to form TCP/179 connection to other peer
  • ACTIVE – Attempting to form a BGP relationship
  • OPENSENT – All of the configuration of peer is sent to the other side and tested.
  • OPENCONFIRM – All is good – ready to establish.
  • ESTABLISHED – BGP relationship is up.

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