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BGP Basics
- EGP Autonomous System:
- Set of routing devices under single technical administration.
- 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.
- 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
- iBGP
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