BGP, "Invalid NEXT_HOP attribute"

[Ondrej Zajicek]

On Sat, Mar 15, 2025 at 11:07:04AM +0000, Lexi Winter via Bird-users wrote:
> hello,
> 
> recently i noticed some entries in syslog like this:
> 
> Mar 15 11:03:22 uk-myb-2 bird[60388]: nl-myb-1: Invalid NEXT_HOP attribute - neighbor address fd5b:a83:b06b:500::1
> Mar 15 11:03:22 uk-myb-2 bird[60388]: nl-myb-1: Invalid route 172.20.212.0/26 withdrawn
> Mar 15 11:03:22 uk-myb-2 bird[60388]: nl-myb-1: Invalid NEXT_HOP attribute - neighbor address fd5b:a83:b06b:500::1
> Mar 15 11:03:22 uk-myb-2 bird[60388]: nl-myb-1: Invalid route 172.20.212.0/26 withdrawn
> Mar 15 11:03:22 uk-myb-2 bird[60388]: nl-myb-1: Invalid NEXT_HOP attribute - neighbor address fd5b:a83:b06b:500::1
> Mar 15 11:03:22 uk-myb-2 bird[60388]: nl-myb-1: Invalid route 172.20.212.0/26 withdrawn
> Mar 15 11:03:22 uk-myb-2 bird[60388]: nl-myb-1: Invalid NEXT_HOP attribute - neighbor address fd5b:a83:b06b:500::1
> Mar 15 11:03:22 uk-myb-2 bird[60388]: nl-myb-1: Invalid route 172.20.212.0/26 withdrawn
> 
> i am not sure how to understand this log message.
> 
> what is BIRD trying to indicate with this log message?

Hello

The message is generated during route export and means that the NEXT_HOP
attribute for the route that would be announced to the neighbor is the
same as the IP address of that neighbor. This is not valid, so BIRD sends
a route withdraw instead of an update.

Because it is an error during route export, the local situation is fine,
but the route is not announced to the neighbor.


> i still have the route (including via nl-myb-1):
> 
> root at uk-myb-2:~ # birdc show route 172.20.212.0/26
> BIRD 2.16.1 ready.
> Table master4:
> 172.20.212.0/26      unicast [uk-jmp-2 11:03:22.032 from fd5b:a83:b06b:400::1] * (100/61) [AS4242422622i]
>         via fe80::e0e3:b1bf:8d1d:abcd on wg.nl-myb-1
>                      unicast [nl-myb-1 11:03:20.704 from fd5b:a83:b06b:500::1] (100/61) [AS4242422622i]
>         via fe80::e0e3:b1bf:8d1d:abcd on wg.nl-myb-1
>                      unicast [ny-czy-1 11:03:20.589 from fd5b:a83:b06b:200::1] (100/694) [AS4242422622i]
>         via fe80::bc1b:dbed:690a:6840 on wg.uk-jmp-2
>                      unicast [mia-czy-1 11:03:20.680 from fd5b:a83:b06b:900::1] (100/694) [AS4242422622i]
>         via fe80::bc1b:dbed:690a:6840 on wg.uk-jmp-2
> 
> and it's in the FIB as well:
> 
> root at uk-myb-2:~ # route -n get -inet 172.20.212.0/26
>    route to: 172.20.212.0
> destination: 172.20.212.0
>        mask: 255.255.255.192
>     gateway: fe80::e0e3:b1bf:8d1d:abcd%wg.nl-myb-1
>         fib: 0
>   interface: wg.nl-myb-1
>       flags: <UP,GATEWAY,DONE>
>  recvpipe  sendpipe  ssthresh  rtt,msec    mtu        weight    expire
>        0         0         0         0      1420         1         0
> 
> so everything seems fine.

-- 
Elen sila lumenn' omentielvo

Ondrej 'Santiago' Zajicek (email: santiago at crfreenet.org)
"To err is human -- to blame it on a computer is even more so."