The BGP MED Attribute
I know from experience that one of the hardest parts of earning your CCNP is mastering BGP and all the attributes, rules, aggregations, aggrevations, confederations, reflectors, and some other stuff I'm sure I left out! Today we're going to take a look at when and how to use the BGP MED attribute.
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 its loopback interface (4.4.4.4) via 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)
172.12.123.3 from 172.12.123.3 (3.3.3.3)
Origin IGP, localpref 100, valid, external
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!
If there's one thing I can tell you about BGP, it's that you can definitely learn it if you take it piece by piece. Looking at BGP as a whole can be very intimidating, but learning one attribute and rule at a time is the secret to mastering BGP.
To your Cisco certification success,
Chris Bryant
CCIE #12933
www.thebryantadvantage.com
I know from experience that one of the hardest parts of earning your CCNP is mastering BGP and all the attributes, rules, aggregations, aggrevations, confederations, reflectors, and some other stuff I'm sure I left out! Today we're going to take a look at when and how to use the BGP MED attribute.
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 its loopback interface (4.4.4.4) via 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)
172.12.123.3 from 172.12.123.3 (3.3.3.3)
Origin IGP, localpref 100, valid, external
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!
If there's one thing I can tell you about BGP, it's that you can definitely learn it if you take it piece by piece. Looking at BGP as a whole can be very intimidating, but learning one attribute and rule at a time is the secret to mastering BGP.
To your Cisco certification success,
Chris Bryant
CCIE #12933
www.thebryantadvantage.com
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