How to enable BGP and make the neighbors most stable?
 

According to the figure above, configure R1-R4 with BGP routing, so that:

1. R3 and R4 are in AS 11, R1 and R2 are in AS 111.

2. Establish peering relationships as shown: blue arrow means iBGP, red arrow means for eBGP.

3. Each BGP router should start announcing routes to neighbor immediately without waiting for IGP protocol to learn of the same routes

4. Configure BGP in a way that its peering relationship is most stable

Knowledge Points
 

Knowledge Points:

• The source IP of BGP peering needs to be reachable from neighbor router. Use logical interface such as loopback as the source help increase the stability of BGP peering:

(config-router)#neighbor X.X.X.X update-sourceLoopback 0

• When source IP of eBGP neighbors are not directly connected , e.g for R2 & R3, the following configuration is required:

(config-router)#neighbor X.X.X.X ebgp-multihop-hop (1~255)

• BGP synchronization is enabled by default, which states that a BGP router will not advertise to eBGP neighbors for the routes learned from iBGP neighbors, unless those routes are also known via an IGP. To disable it, configure the following Cisco BGP subcommand:

(config-router)#no synchronization