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