[FROG] /31s, scope link and FRR BGP

James Bensley jwbensley at gmail.com
Sat Oct 13 06:36:42 EDT 2018

On 13 October 2018 09:19:48 BST, "Chris Malton (Delta V)" <chris at deltav-tech.co.uk> wrote:
>Hi all,
>I have /31s from an upstream provider on v4. If I specify "scope link" 
>in /etc/network/interfaces for these, FRR seemingly doesn't allow the 
>address to be used for update-source.
>Minimal reproduction example (taken from one of my core routers):
>/e/n/i config:
>auto lo
>iface lo inet loopback
>	up ip addr add scope global dev lo
>auto ens4
>iface ens4 inet static
>	address
>	scope link
>FRR config:
>router bgp 65000
>  neighbor remote-as 65001
>  neighbor interface ens4
>  neighbor update-source
>What happens in this case, is that FRR sends updates from 
>instead of (despite being explicitly told otherwise!).
>Have I found a bug, or have I done something stupid?
>Thanks in advance.
>Chris Malton

Hi Chris,

Have you tried "nei X update-source ens4"? I always prefer that  in case you ever re-IP the  interface.

Also, when you say FRR sends the updates from, do you mean the TCP packets come from that IP, or that is the IP used as the router ID in the BGP UPDATE messages, or both?

What do you have for "bgp router-id",


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