BGP preferred path

In this scenario R3 is advertising network 154.1.5.0/24 to bgp peers R1 and R2. The lab requirement is for AS 300 to be configured so that the link R1 to R3 is the preferred path to reach this network.

In such a scenario there are 2 usual candidates to meet this requirement: as-path prepending and MED. In this scenario as-path pre-pending is not allowed.

So the configuration for MED on R3 is as follows

ip prefix-list VLAN10 permit 154.1.5.0/24
route-map R1 permit 10
match ip address prefix-list VLAN10
set metric 100
route-map R1 permit 20


route-map R2 permit 10
match ip address prefix-list VLAN10
set metric 200
route-map R2 permit 20


router bgp 300
neighbor 154.1.13.1 route-map R1 out
neighbor 154.1.23.2 route-map R2 out

The above was my first configured solution. I examined the bgp routing table on R2 to verify my results:-

R2#s ip bgp
Network Next Hop Metric LocPrf Weight Path
*>i154.1.5.0/24 154.1.13.3 100 100 0 300 400 i
* 154.1.23.3 200 0 300 400 i

As expected R2 had 2 paths to network 154.1.5.0/24. However R2s next hop for both learned routes was R3! I had missed one vital configuration element in terms of meeting the lab requirement. R1s advertisement of 154.1.5.0/24 to R2 was NOT adjusting the next hop. This is the correct beahviour since R1 has an EBGP peer relationship with R3.

To ensure traffic from R2 destined to R3 goes via R1 it is necessary for R1 to adjust the next hop of EBGP learned routes to itself.

R1
router bgp 200
neighbor 192.10.1.2 next-hop-self

Once applied i examined the bgp table on R2 again..

Rack1R2#s ip bgp
Network Next Hop Metric LocPrf Weight Path
*>i154.1.5.0/24 192.10.1.1 100 100 0 300 400 i
* 154.1.23.3 200 0 300 400 i


Now R2s preferred route to reach R3 is via R1. Job done!