IS-IS on FRR - Is Anyone Running It?

Mark Tinka mark.tinka at
Mon Apr 6 09:43:04 UTC 2020

On 6/Apr/20 11:28, Saku Ytti wrote:

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

The services running on the server benefit from the high MTU. That's the
whole point of the server... to run the services, not to run a routing

So I don't want to lower the physical MTU because that will impact the
performance of the services on the server that depend on it.

For OSPF on Quagga, we just run "ip ospf mtu-ignore" and "ipv6 ospf6
mtu-ignore" to get around this, and this works fine.

> 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.

Hello Padding is disabled by default in our IS-IS core, so the other
side isn't doing it already.

The problem you have is IS-IS on FreeBSD won't initialize because it
sees the physical interface running at 9,000 bytes, and yet its code can
only go up to 8,192 bytes.

Fair point, that bug has been fixed and just needs to be committed to
upcoming code. But again, if I can manually tell IS-IS to run at a lower
MTU than what it is trying to infer from the interface, we shall be golden.

How do I do that, is the question?

> 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.

    "no hello padding" is default for our core.


More information about the NANOG mailing list