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 (172.123.2) and R3 (172.123. Let's see which of the two paths R1 is using. R1#show ip bgp 4.4 BGP routing table entry for 4.4/32, version 8 Paths: (2 available, best #2, table Default-IP-Routing-Table) Flag: 0x208 Advertised to non peer-group peers: 172.
123.3 234 172.123.3 from 172.123.3 (3.3) Origin IGP, localpref 100, valid, external 234 172.123.2 from 172.123.2 (2.2) Origin IGP, localpref 100, valid, external, best R1 is using 172.123.2 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.
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