Posted on September 14, 2015 1:39 pm
 |  Asked by Chintan Shah
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I could not find any option that could allow me to disable/enable BGP deterministic MED support in EOS.. Is it enabled by default and there is no way to disable it ?




Posted by Mark Berly
Answered on September 14, 2015 5:05 pm

You can enable this with the command:

sw1(config-router-bgp)#bgp always-compare-med

It is disabled by default...
Posted by Chintan Shah
Answered on September 14, 2015 5:07 pm

Sure, but I was looking for ”deterministic MED” support, is it supported ?

Posted by Alexis Dacquay
Answered on September 14, 2015 9:41 pm

Hi Chintan,

EOS only supports deterministic MED. Could you explain the use case for wanting un-deterministic MED, if you wanted it ?

Note the following BGP MED features are also supported, just for information:

  • bgp default-metric
  • neighbor <addr> metric-out <metric>
  • bgp always-compare-med
  • bgp bestpath med missing-as-worst
  • bgp bestpath med confed
  • bgp bestpath med confed missing-as-worst




Posted by Chintan Shah
Answered on September 18, 2015 1:43 pm

Hi Alexis, I haven’t come across use case that would require to disable deterministic MED , in this case I was just checking if there is any knob to enable/disable it like Cisco devices, nothing more than that. It’s good to know its enabled by default. Thanks for sharing more features on BGP MED.

I was in particular interested¬†to evaluate MED in leaf-spine architecture with BGP where we can use MED from Spine to Leaf or to upper layer of Spine to advertise all prefixes with higher MED to drain out traffic from Spine before we take it for reload/upgrade etc. I guess with default MED setting, it works and won’t require any change.




Posted by George Bonser
Answered on November 18, 2019 1:46 am

As far as I can tell, ACOS does not support deterministic-med, it only supports "always-compare-med" which is different. With "deterministic-med" the med is only compared between peers in the same AS. With "always-compare-med" it compares all peers in all AS.

So for example, if I have two connections to AS9999 and one connection announces a MED of 10 and the other of 20 but I also have a connection to AS6666 that announces a MED to the same network of 1. With "always-compare-med" I would choose AS6666 to the destination but with "deterministic-med" I would choose the connection with a MED of 10 to AS9999 if AS9999 wins over AS6666 generally.

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