IS-IS on FRR - Is Anyone Running It?

Saku Ytti saku at
Mon Apr 6 09:25:43 UTC 2020

On Mon, 6 Apr 2020 at 12:13, Mark Tinka <mark.tinka at> wrote:

> Ummh, not really. As mentioned, we run 9,178 bytes on IOS and IOS XE,
> and 9,192 bytes on Junos and IOS XR, in our network.

Quite and then you specified but for ISIS you run 8000. I'm only
talking about your ISIS here, not rest. And that 8k doesn't do
anything useful, so you can run 1500B to your FRR and not lose

> The 8,000 bytes we run is done only on the IOS, IOS XE and CSR1000v
> boxes. It doesn't mean that we are trying to transport IS-IS PDU's at
> 8,000 bytes. It just means we can, and are capping it to just about what
> VMware ESXi can support.

As I explained this is not true, by standard ISIS cannot do this for
>1500B MTU. As ISIS is 802.3 and 802.3 cannot send larger frames.
Cisco has proprietary hack to do EthernetII for hellos, but even that
doesn't guarantee anything, because ISIS will still come up, even if
it receives subMTU hello as 802.3.


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