Internet is collection of autonomous system that is interconnected to allow to communication among them. BGP provide routing between these autonomous systems. So in a brief we can say BGP is basically used to connect very large networks or autonomous systems.
Definition of BGP ( Border Gateway Protocol )?
Border Gateway Protocol (BGP) is a standardized exterior gateway protocol designed to exchange routing and reachability information among autonomous systems (AS) on the Internet. The protocol is often classified as a path vector protocol but is sometimes also classed as a distance-vector routing protocol.
How BGP works ?
Before going forward to understand how Border Gateway Protocol works, we should first understand below terms.
1. Autonomous System – An Autonomous System is any set of routers that share similar routing policies and operate within a single administrative domain. These Autonomous Systems should be registered autonomous system number (ASN), which they get from any of the Regional Internet Registry: AFRINIC, ARIN, APNIC, LACNIC or RIPE NCC.
2. Autonomous System Number – (AS Number) is allocated to each AS for use in BGP routing. AS numbers are important because the ASN uniquely identifies each network on the Internet
3. Peering – Two routers that have established connection for exchanging BGP information, are referred to as BGP peers. These BGP peers exchange routing information with each other via BGP sessions which is a reliable, connection oriented & error free protocol
Once BGP is enabled, your router will pull a list of Internet routes from your BGP neighbours, who in this case will be your two ISPS. It will then scrutinize them to find the routes with the shortest AS paths. These will be put into the router’s routing table.
BASIC ABOUT BGP ( Border Gateway Protocol)
- BGP is designed to connect autonomous systems, not connect subnets within an autonomous system.
- It is a path vector routing protocol.
- BGP supports variable-length subnet mask (VLSM) & CIDR
- Full routing updates are sent at the start of the session; Incremental triggered updates are sent subsequently.
- BGP uses a TCP as its transport protocol, which provide connection oriented reliable delivery; therefore it doesn’t have to implement retransmission or error recovery mechanism.
- It creates and maintains connections between peers, using TCP port 179.
- The connection is maintained by periodic keep lives.
- The metric, referred to as ‘Path Vector’ or also called as ‘Attributes’ BGP is connection and maintained.
- BGP is connection oriented. When a neighbour is seen, a TCP peering session is established and maintained.
- BGP have admin distance 20 for EBGP and 200 for IBGP.