Today we’ll go over another important BGP topic: MED. From Cisco:

MED is an optional nontransitive attribute. MED is a hint to external neighbors about the preferred path into an autonomous system (AS) that has multiple entry points. The MED is also known as the external metric of a route. A lower MED value is preferred over a higher value.

So, to summarize, MED is used to influence incoming traffic from a multi-homed neighbor AS.

Here’s our diagram:

In this case we’ll say that the link between PE1 and CE1 is only a T1, while the link between PE2 and CE1 is a DS3. Obviously we will want traffic to use the faster link, the PE2-CE1 DS3.

First we’ll configure our interfaces and basic BGP:

PE1:
hostname PE1
!
interface Loopback0
 ip address 1.1.1.1 255.255.255.255
!
interface Serial0/0
 description To PE2
 ip address 10.1.1.2 255.255.255.254
!
interface Serial0/1
 description To CE1
 bandwidth 1500
 ip address 172.16.1.2 255.255.255.254
!
router bgp 5300
 no synchronization
 bgp log-neighbor-changes
 neighbor 2.2.2.2 remote-as 5300
 neighbor 2.2.2.2 update-source Loopback0
 neighbor 2.2.2.2 next-hop-self
 neighbor 172.16.1.3 remote-as 1200
 no auto-summary
!
ip route 2.2.2.2 255.255.255.255 10.1.1.3
 
 
PE2:
hostname PE2
!
interface Loopback0
 ip address 2.2.2.2 255.255.255.255
!
interface Serial0/0
 description To PE1
 ip address 10.1.1.3 255.255.255.254
!
interface Serial0/2
 description To CE1
 bandwidth 45000
 ip address 172.16.1.4 255.255.255.254
!
router bgp 5300
 no synchronization
 bgp log-neighbor-changes
 neighbor 1.1.1.1 remote-as 5300
 neighbor 1.1.1.1 update-source Loopback0
 neighbor 1.1.1.1 next-hop-self
 neighbor 172.16.1.5 remote-as 1200
 no auto-summary
!
ip route 1.1.1.1 255.255.255.255 10.1.1.2
 
 
CE1:
hostname CE1
!
interface Loopback0
 ip address 3.3.3.3 255.255.255.255
!
interface Loopback1
 ip address 192.168.1.1 255.255.255.0
!
interface Loopback2
 ip address 192.168.2.1 255.255.255.0
!
interface Loopback3
 ip address 192.168.3.1 255.255.255.0
!
interface Loopback4
 ip address 192.168.4.1 255.255.255.0
!
interface Serial0/1
 description To PE1
 bandwidth 1500
 ip address 172.16.1.3 255.255.255.254
!
interface Serial0/2
 description To PE2
 bandwidth 45000
 ip address 172.16.1.5 255.255.255.254
!
router bgp 1200
 no synchronization
 bgp log-neighbor-changes
 network 192.168.1.0
 network 192.168.2.0
 network 192.168.3.0
 network 192.168.4.0
 neighbor 172.16.1.2 remote-as 5300
 neighbor 172.16.1.4 remote-as 5300
 no auto-summary