eBGP

BGP is a path-vector protocol. Consider the following simple BGP network. There are four AS numbered 1-4 with a single router in each. Router 1 has a local network 192.168.1.0/24 and a loopback address of 10.255.255.1.

Example Network

In AS2, Router 2 has a local network 192.168.2.0/24 and a loopback address of 10.255.255.2.

In AS3, Router 3 has a local network 192.168.3.0/24 and a loopback address of 10.255.255.3.

In AS4, Router 4 has a local network 192.168.4.0/24 and a loopback address of 10.255.255.4.

There are some class C networks used for linking the routers.

From AS1 to 2 we use 10.1.12.0/24.

From AS2 to 3 we use 10.1.23.0/24.

From AS3 to 4 we use 10.1.34.0/24.

Each router is directly connected, BGP has a TTL of one and needs to be specially configured using BGP Multi-hop if routers do not connect directly. Looking from Router 1;

Router1#sh ip bgp
BGP table version is 17, local router ID is 10.255.255.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

Network 			Next Hop 	Metric 	LocPrf 		Weight 		Path
*> 10.255.255.1/32 	0.0.0.0 		0 	0 		32768 		i
*> 10.255.255.2/32 	10.1.12.2 	0 	0 		0 		2 i
*> 10.255.255.3/32 	10.1.12.2 	0 	0 		0 		2 3 i
*> 10.255.255.4/32 	10.1.12.2 	0 	0 		0 		2 3 4 i
*> 192.168.1.0/24 		0.0.0.0 		0 	0 		32768 		i
*> 192.168.2.0/24 		10.1.12.2 	0 	0 		0 		2 i
*> 192.168.3.0/24 		10.1.12.2 	0 	0 		0 		2 3 i
*> 192.168.4.0/24 		10.1.12.2 	0 	0 		0 		2 3 4 i

Not only does Router 1 see all the advertised networks and loopback addresses, it also sees the path to every network. It can see its vector (which way to send the traffic, the next hop).

The routing table can also see the path to 192.168.4.0 goes via AS 2, 3 and 4; the letter i means internal and you may see the letter e represent external.