unstable BGP sessions after debian 10 -> 11 upgrade

[Christoph]

Fran:
> Hey,
> 
> I ran the following combinations without problems:
> 
> Debian 10, kernel 5.8 backports, bird 2.0.7
> Debian 10, kernel 5.10 backports, bird 2.0.8 (from suse repo)
> Debian 11, kernel 5.10, bird 2.0.8 (from Debian Bookworm repo).
> 
> sysctl values (unchanged since ~ 2 y):
> 
> net.core.rmem_max=67108864
> net.core.wmem_max=67108864
> net.core.wmem_default=8388608

thanks for your feedback.
Christoph