BGP notes

Neighbor states:

Idle: all connections are refused
Connect: wait for TCP to establish
Active: initiates the TCP connections
OpenSent: Local waits for Open message from peer. After receiving open, if no errors BGP sends keepalive
OpenConfirm: BGP waits for keepalive or notification
Established: Can exchange update, notification and keepalive

Message types:

Open: sent when TCP 3way is complete. Initiates the BGP session and contains details about BGP neighbor and supported and negotiated potions
Update: Transports routing information between BGP peers
Keepalive: On BGP level. Contains the BGP header and has no data.
Notification: sent when something is wrong (eg. unsupported options in the open message, hold time expires)
Refresh: BGP does not readvertise sent routes by default. Route refresh supports soft clearing of BGP sessions by allowing routes already sent to be re-advertised

BGP attributes:

- contained in the Update message, describes the prefixes in the message
- used to influence route selection and select best path

Local preference:
- exchanged by IBGP peers only
- used to set the exit path from the local AS