IS-IS Error (FRR)

adamv0025 at adamv0025 at
Mon Apr 20 14:26:40 UTC 2020

Interesting, so it is an MTU problem after all, I take it there’s no way to adjust the BPF (Berkeley Packet Filter) limit to let the 6020B sized PSNPs through?




From: Mark Tinka <mark.tinka at> 
Sent: Sunday, April 19, 2020 1:47 AM
To: Tony Li <tony1athome at>; adamv0025 at
Cc: nanog at
Subject: Re: IS-IS Error (FRR)


So we've been discussing this on Slack.

It seems the packet handlers for IS-IS in FRR and how they work on FreeBSD is not completely optimized (not an issue on Linux).

FRR-based IS-IS on FreeBSD is sending PSNP's that are 6,020 bytes large. However, FreeBSD's BPF (Berkeley Packet Filter) has a packet data limit of 4,096 bytes. This is what is causing the error.

While it is possible to set the IIH MTU in FRR with the "lsp-mtu" command, there is not way to set the PSNP MTU.

In short, IS-IS on FRR does not understand that there is an MTU limit for PSNP's on FreeBSD.

I'm working with the IS-IS FRR team, but as of now, no idea on if/when there will be a fix for this.

While routes are being successfully exchanged between FRR and Cisco IOS XE for IS-IS, this is a show-stopper! So for the moment, I cannot recommend anyone runs IS-IS on FRR in production.

As it stands, it appears that this is the only thing standing in the way of a successful deployment, as far as simple Anycast services go.

Will keep you all posted as this develops.


On 18/Apr/20 16:34, Tony Li wrote:


No, IIH’s are padded, hot PSNP’s.



On Apr 18, 2020, at 3:08 AM, adamv0025 at <mailto:adamv0025 at>  wrote:


Just shooting in the dark, Isn’t the PSNP padded to some ISIS defined max MTU and that somehow fails due to mismatch on em0 L2 MTU?



From: NANOG  <mailto:nanog-bounces at> <nanog-bounces at> On Behalf Of Mark Tinka
Sent: Friday, April 17, 2020 11:05 AM
To: nanog at <mailto:nanog at> 
Subject: IS-IS Error (FRR)


Hi all.

I'm almost there getting IS-IS to work without issue. I'm now faced with the following error log:

2020/04/17 10:02:01 ISIS: IS-IS bpf: could not transmit packet on em0: Input/output error
2020/04/17 10:02:01 ISIS: [EC 67108865] ISIS-Snp (1): Send L2 PSNP on em0 failed

This repeats every second.

Anyone know what this could be?

Systems is FreeBSD-12.1-RELEASE-p3.

Despite the error, IS-IS is running and routing is good, talking to a Cisco IOS XE implementation on the other side. So the error is throwing me off.

No feedback from the Slack feed, Google doesn't say much, and waiting to hear back from the FRR list.



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