BGP ( Boarder Gateway Protocol )

AS - Sharing similar routing policies and in single administrative domain.
AS number -  1 - 65535 ( 2 Byte) 
There is  4 byte AS number .
Reserved  64512 - 65535 for  private use.


Single Homed  - Only one exit point to outside.(only one ISP )


BGP message types


1.Open
2.Keeplive
3.Update
4.Notification


Path Attributes 


1. Well known mandatory 
   1.Origin
   2.AS_Path
   3.Next_Hop
2. Well known optional 
   1.Local_preference
   2. Atomic_aggregate
   3.Aggregator
3. Optional transitive
   1.Community
4. Optional non-transitive
   1.Muliti_exit_disc
   2.Originator_id (CISCO)
   3.Cluster list (CISCO)