unstable BGP sessions after debian 10 -> 11 upgrade

[Christoph]

Hello,

we upgraded our debian BGP routers from debian buster to bullseye.

On debian 10 they used the repo https://bird.network.cz/debian/
now they use the bird2 package directly from the official debian repos,
both repos contain BIRD version 2.0.7.


After upgrading and rebooting we noticed that iBGP sessions are 
constantly dying every few minutes.


The logs also show multiple of these log entries:

<WARN> Kernel dropped some netlink messages, will resync on next scan.
<WARN> Event ... took 31415 ms
<WARN> I/O loop cycle took 31529 ms for 9 events
<WARN> Netlink: File exists


Especially the warning "Kernel dropped some netlink messages" was not 
new but we previously solved them with these sysctl.conf settings:


# solves the "Kernel dropped some netlink messages, will resync on next 
scan." BIRD Warnings
# https://bird.network.cz/pipermail/bird-users/2017-September/011542.html
# (we did not change the wmem_default since this is apparently not needed)
net.core.rmem_max=4194304
net.core.rmem_default=4194304
net.core.wmem_max=4194304


The most obvious change is the linux kernel version:
  4.19.208 (buster)
  5.10.70 (bullseye)

To avoid constantly dying iBGP sessions we increased the keepalive and 
hold timers.
Are you observing similar issues on your debian 11 systems?
Does the newer kernel need higher rmem values?

best regards,
Christoph