topic BGP MED attributes don't work in ExtremeSwitching (EXOS/Switch Engine)

BGP MED attributes don't work

Karfax — Tue, 04 Dec 2018 06:59:32 GMT

Hello!

x480, EXOS 12.6.5.2 patch1-3
I received some prefix via BGP and over router-policy IN add MED 30 and MED 50 for them:

code:

# sh bgp routes network 5.45.121.0/24

Feasible Routes
---------------
    Destination        Peer            Next-Hop        LPref Weight MED   AS-Path
*>i 5.45.121.0/24      193.25.180.255  193.25.180.244  500   1000   50    3326 21219 3327 198068
  i 5.45.121.0/24      193.25.181.0    193.25.180.244  500   1000   50    3326 21219 3327 198068
  i 5.45.121.0/24      10.17.21.3      10.17.21.3      500   1000   30    ( 65003 ) 21219 3327 198068


Unfeasible Routes
-----------------
 Destination        Peer            Next-Hop        LPref Weight MED   AS-Path


Flags: (*) Preferred BGP route, (>) Active, (d) Suppressed, (h) History
       (s) Stale, (m) Multipath

Origin: (?) Incomplete, (e) EGP, (i) IGP

Why first route active? First and third routes have same weight, LPref, AS-PATH (4), origin code.
But third route have lower MED - it should be active route.

Re: BGP MED attributes don't work

BrandonC — Wed, 05 Dec 2018 03:29:49 GMT

By default, EXOS doesn't consider the MED for routes with differing AS-paths. Try the following command and see if it changes it:

code:

enable bgp always-compare-med

If not, can you get the output of:

code:

sh bgp routes detail network 5.45.121.0/24

Re: BGP MED attributes don't work

BrandonC — Wed, 05 Dec 2018 03:57:40 GMT

Also, why not set the LPref instead of MED via policy in this case?

Re: BGP MED attributes don't work

Stephane_Grosj1 — Wed, 12 Dec 2018 21:31:37 GMT

To add to Brandon's comment, by default EXOS compares MED only for paths in the same AS. In my experience, the use of MED is quite rare. It can be used to try to influence traffic entering your AS, but with little luck to succeed. If you want to influence how traffic can exit your AS, prefer Local Preference.