ExtremeSwitching (VSP)

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 Preserve OSPF Tags across Switching Fabric

Tim Schnepper's profile image
Tim Schnepper posted 05-20-2022 02:55

Hello Community,

we have a switching fabric on vsp hardware since several years with some ospf links at the edges mostly to route external networks.
I now have the task to filter routes on site B based on ospf tags which are set on site A.
My Problem, from what i see, is that these ospf tags get lost during the redistribution from ospf into is-is entering our fabric.
(And maybe would't preserved during the redistribution from is-is back to ospf at site B, i guess.)

So my question is how do i preserve ospf tags across an (isis) switching fabric?
Is this possible?

Kind regards,

Tim

Sam Pirok's profile image
Sam Pirok
Hi Tim, thanks for reaching out. No OSPF (or other routing protocol) tagging info is passed when OSPF or any other protocol route is redistributed into ISIS.

You can choose to deny routes of interest on any ISIS node (that is learning those route via ISIS after those routes are being redistributed into ISIS) via route-map (with action "no permit")/isis accept policies. This article has more details.

Hope this helps!

Ludovico Stevens's profile image
Ludovico Stevens
Hi; you can't today, as there is no tag support on ISIS routes. The only field you can use in the ISIS routes is the metric field. The attached (use case 2) shows a way to use those metrics as tags.
Another thing to consider is to let OSPF run over (or in parallel) with ISIS, that way your tags are preserved.
Tim Schnepper's profile image
Tim Schnepper

Hi Ludovico,

many thanks for that attched document. Use case 2 corresponds almost exactly to our topology. I will give it a try to (ab)use the ISIS metric field as an translator for the OSPF tags. (As described on page 61.)

Kind regards,

Tim

Ludovico Stevens's profile image
Ludovico Stevens
Glad it helped!