BGP Autonomous Systems
- An AS is a collection of networks under a single technical administration
- IGP operates within an AS
- BGP is used between ASs
- Exchange of loop-free routing info is guaranteed
BGP features
- Open standard
- Exterior – designed for Inter-AS Domain Routing
- Designed to scale huge inter-network like Internet
- classless (supports VLSM, CIDR, auto and manual summary)
- updates are incremental and trigger
- Path vector – bgp is exchanging the routes which carries also the path information attributes.
- it sends updates to manually defined neighbor as a unicast
- BGP is application layer protocol and uses TCP (port 179) for reliability
- Metric – is complicated and is formed from many attributes
- AD – 20 for external updates (EBGP), 200 internal updates (IBGP)
BGP loop prevention mechanism
- AS path loop detection – Whenever a router sees its own AS number in route update it will ignore that.
When to use BGP
- AS working as transit AS – for example ISP
- AS connected to multiple AS (multi-homed) – Data traffic entering or leaving A.S needs to be manipulated
When not to use BGP
- if it is single-homed AS
- lack of resources in your network equipment
- limited understanding about BGP