Extreme Networks – EXOS BGP MED

I came across a scenario where a user had two data centers in different locations connecting back to the same ISP via BGP. These two data centers would be advertising a unique /24 at each site. However, the user also wanted to advertise the other DC’s /24, but not in an active state for failover. Being that the user was connecting back to the same provider AS, I decided to test using the BGP MED (Multi Exit Discriminator) attribute to determine which /24 would be the preferred route from the provider end. The MED with the lowest value takes a higher priority.

We’re using Extreme Networks summit series switches, so I tested the configuration on exos 22.6 using my EXOS virtual lab. I made sure to apply a lower MED value to the /24 I wanted to prioritize at each primary site and also applied a higher MED value to the backup /24 at the opposite site. 

In summit series switches you start with a policy file that matches the network address used in the BGP network statement. You can then apply a med value to that match. EXOS uses vi when creating these policy files. Here are the commands:

exos-vm#vi med_add.pol

entry med_add {

If   {

nlri 192.168.1.0/24;

}

Then   {

med set 100;

Permit;

}

}

You can finish the policy file with a :wq

Apply the policy using the BGP network command

exos-vm#Configure bgp add network 192.168.1.0/24 network-policy med_add

In my lab, I was using two different AS numbers representing each DC connected back to the same AS. Therefore I also had to use the “enable bgp always-compare-med” command on the simulated provider AS exos virtual switch as MED values are not compared between routes advertised from different autonomous systems by default.

Of course, your provider has to be willing to accept MED. If not, you could also try prepending your AS number to the AS_Path. This is another way to manipulate what route is less preferred. However, this method is not always supported as some providers ignore duplicate AS in the AS_path. The change for AS_Path is simple, just replace the med set 100; with As-path “2020“; in the policy file. This example is using AS path 2020 and should be applied to the BGP network statement that serves as the backup route at the opposite DC location.

@javi_isolis

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