I'm actually not sure if this is going to be a possibility.

Unfortunately the Nokia SR-Linux platform does not allow us to set a BGP policy which matches routes based on the MED value on them. This is what we do on the Juniper switches to add the needed BGP community strings to flag the routes as prefered/backup etc.

set policy-options policy-statement LVS_import term add_avoid_community from protocol bgp
set policy-options policy-statement LVS_import term add_avoid_community from metric 200
set policy-options policy-statement LVS_import term add_avoid_community then community add AVOIDED_PATH

set policy-options policy-statement LVS_import term add_backup_community from protocol bgp
set policy-options policy-statement LVS_import term add_backup_community from metric 100
set policy-options policy-statement LVS_import term add_backup_community then community add BACKUP_PATH

Without this the MED set by PyBal will influence preference between different LVS hosts connected to the same IBGP cluster (in this case the Nokia switches in eqiad), but won't be visible to BGP speakers outside that (i.e. the core routers or Juniper switches in eqiad).

I can't really think of any way around this for now, we probably need to leave the LVS peering with the core routers, so that the community will be set. The downside there is all traffic from internal hosts needs to go up through the spine to core router and back down to the given LVS. Those links should have plenty of bandwidth though so could be worse.

Just to update this we are not planning to move the IP gateways for vlans to the Nokia switches in the short term (see T409286), and we won't be tackling this until that is done. So we can review the situation at that point and see where we are with Liberica.

@cmooney: We plan to move to Liberica in Q1 or Q2 of FY2026. Do you think we should still consider working on this?

In T405630#11948942, @ssingh wrote:

@cmooney: We plan to move to Liberica in Q1 or Q2 of FY2026. Do you think we should still consider working on this?

Yes it absolutely has to be done. But we hit some blockers with moving the IP gateway to the Nokias (T420706) which means it doesn't make a huge difference for right now. There are also some advantages to keeping things as they are while we are still running PyBal as it's complicated to support the backup routes with it. So the sooner we move to Liberica the better.

Once we are looking at the gateway move post switch upgrade I'll be focusing on this again anyway whether we're upgraded or not (we can do some annoying manual policy stuff to support PyBal even on Nokia if it comes to it).

Thanks for the update and the explanation, @cmooney!