Cisco CCNP / BSCI Exam Tutorial: The BGP Attribute MED

Your BSCI exam and CCNP certification success depend on mastering BGP, and a big part of that is knowing how and when to use the many BGP attributes. And for those of you with an eye on the CCIE, believe me - you've got to know BGP attributes like the back of your hand. One such BGP attribute is the Multi-Exit Discriminator, or MED.

The MED attribute is sent from a router or routers in one AS to another AS to indicate what path the remote AS should use to send data to the local AS.

That sounds a little confusing on paper, so let's walk through an example. R1 is in AS 1, and R2, R3, and R4 are in AS 234. R4 is advertising a loopback into BGP, and R1 has two possible next-hops to get to that loopback - R2 (172.12.123.2) and R3 (172.12.123.3). Let's see which of the two paths R1 is using.

R1#show ip bgp 4.4.4.4

BGP routing table entry for 4.4.4.4/32, version 8

Paths: (2 available, best #2, table Default-IP-Routing-Table)

Flag: 0x208

Advertised to non peer-group peers:

172.12.123.3

234

172.12.123.3 from 172.12.123.3 (3.3.3.3)

Origin IGP, localpref 100, valid, external

234

172.12.123.2 from 172.12.123.2 (2.2.2.2)

Origin IGP, localpref 100, valid, external, best

R1 is using 172.12.123.2 as the next-hop to enter AS 234. If all values are left at their default, we could have 100 routes being advertised from AS 234 to AS 1 and the next-hop would remain the same.

We can configure R2 and R3 to send different MED values to R1, and the router sending the lowest MED would be the preferred next-hop. (The MED is a metric, and the lowest metric is always preferred.) We'll configure the MED attribute on both R2 and R3, sending a MED of 200 from R2 and 100 from R3.

R2(config)#route-map SET_MED_200 permit 10

R2(config-route-map)#set metric 200

R2(config-route-map)#router bgp 234

R2(config-router)#neighbor 172.12.123.1 route-map SET_MED_200 out

R3(config)#route-map SET_MED_100 permit 10

R3(config-route-map)#set metric 100

R3(config-route-map)#router bgp 234

R3(config-router)#neighbor 172.12.123.1 route-map SET_MED_100 out

After clearing the BGP table on R1, R1 will still see both next-hop addresses and will still consider both to be valid, but the path through R3 will be selected due to its lower metric.

Just keep in mind that the MED is actually a metric, and lower metrics are more desirable in path selection. That will put you one step closer to passing the BSCI and earning your CCNP Certification!

Cisco CCNP / BSCI Certification: The BGP Attribute “MED”

When you're preparing to pass the BSCI exam and earn your CCNP certification, one of the biggest challenges is learning BGP. BGP is totally different from any protocol you learned to earn your CCNA certification, and one of the differences is that BGP uses path attributes to favor one path over another when multiple paths to or from a destination exist.

Notice I said "to or from". In earlier free BGP tutorials, I discussed the BGP attributes "weight" and "local preference". These attributes are used to favor one path to a destination over another; for example, if BGP AS 100 has two paths to a destination in AS 200, these two attributes can be set in AS 100 to favor one path over another. But what if AS 100 wants to inform the routers in AS 200 as to which path it should use to reach a given destination in AS 100?

That's where the BGP attribute "Multi-Exit Discriminator", or MED, comes in. The MED value can be set in AS 100 to tell AS 200 which path it should use to reach a given network in AS 100.

As with many BGP attributes, the MED can be set with a route-map. What you need to watch is that there is no "set med" value in route maps. To change the MED of a path, you need to change the metric of that path. Let's say that there are two entry paths for AS 200 to use to reach destinations in AS 100. You want AS 200 to use the 100.1.1.0/24 path over the 100.2.2.0/24 path. First, identify the two paths with two separate ACLs.

R1(config)#access-list 22 permit 100.1.1.0 0.0.0.255

R1(config)#access-list 23 permit 100.2.2.0 0.0.0.255

Next, write a route-map that assigns a lower metric to the more-desirable path.

R1(config)#route-map PREFER_PATH permit 10

R1(config-route-map)#match ip address 22

R1(config-route-map)#set metric 100

R1(config-route-map)#route-map PREFER_PATH permit 20

R1(config-route-map)#match ip address 23

R1(config-route-map)#set metric 250

Finally, apply the route-map to the neighbor or neighbors.

R1(config-route-map)#router bgp 100

R1(config-router)#neighbor 22.2.2.2 route-map PREFER_PATH out

The key points to keep in mind is that while many BGP attributes prefer a higher value, the MED is basically an external metric - and a lower metric is preferred, just as with the protocols you've already studied to earn your CCNA certification.