IS-IS on FRR - Is Anyone Running It?

Saku Ytti saku at
Mon Apr 6 09:28:56 UTC 2020

On Mon, 6 Apr 2020 at 12:16, Mark Tinka <mark.tinka at> wrote:

> Which is what this whole thread is about. How do I set that, manually,
> without changing the physical interface MTU?

You want to run your physical at high MTU and you also like ISIS to come up?

FRR doesn't seem to support Cisco proprietary EthernetII based ISIS,
so it'll only do 802.3 standard. Have you tried to just use what ever
interface MTU, and NOT pad on far-end cisco? If I read the code right,
this should be then standard max_size 802.3 and it should come up.

If you force Cisco to pad, it'll use the EthernetI hack, which is not
supported by FRR, and it won't work. I noticed in lab just now, if I
set 'pad sometimes' on IOS-XR, it won't pad at all for >1500B MTUs, so
seems like 'sometimes' stops Cisco from using the EthernetII hack.


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