• EGP Autonomous System:
    • Set of routing devices under single technical administration.
      • All using an IGP
    • Uses inter-AS routing protocol to determine how to route packets to other ASes.
    • Autonomous System Numbers (ASNs) are allocated by IANA.
  • BGP ASN Values:
    • Originally 2 byte field
      • 0-65535
      • Public – 1-64511
      • Private – 64512-65535
    • New/Current 4 byte field
      • RFC 4893
      • IOS 12.4(24)T and after support this field
      • 0.0 Notation
        • 0.[0-65535] denote original 2 byte field.
        • Needs command ‘bgp asnotation dot’
        • Requires backwards compatibility with old code.
      • 4 byte ASN support negotiated during capability exchange.
        • old BGP speakers are sent ASdot numbers encoded as ASN ‘23456’
          • If ‘23456’ are seen in the AS path, means somewhere in path the 4 byte field was not supported.
        • Real AS-Path encoded with optional transitive attributes
          • AS4_Aggregator and AS4_Path
  • BGP Neighborship:
    • Does not have own transport.
      • TCP 179
    • Different types of neighbors.
    • No need to be directly connected.
    • Neighbors not discovered by default, need neighbor statements.
    • BGP Neighbor statement:
      • Tells process to listen on TCP 179
      • Initiate session to remote address via TCP 179
      • If collision (both devices try initiating session at same time) higher router-id becomes TCP Client, other server.
    • Session:
      • If server does not expect session from client, it will send TCP-Reset
      • ‘show control-plane host open-ports’
      • Client’s packet is sourced from outgoing interface.
        • Can be modified with ‘update-source’ under BGP process.
    • EBGP
      • Neighbors outside of AS
    • iBGP
      • Neighbors inside AS

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