Babel stops operating on interface after link flap

[MEschenbacher]

Describe the bug

A link flap causes babel to stop operating on the interface.

[x] Did you check if this is a duplicate issue? Yes -- could not find
[x] Did you test it on the latest FRRouting/frr master branch? Yes

To Reproduce

/etc/frr/daemons

...
babeld=yes
...

/etc/frr/frr.conf

frr version 8.2.2
frr defaults traditional
hostname debian11-template
log syslog informational
no ip forwarding
no ipv6 forwarding
service integrated-vtysh-config
!
debug babel all
!
router babel
 network enp1s0
exit
!

Watch babel packets leaving the interface

22:44:21.919096 enp1s0 Out IP6 fe80::5054:ff:fea7:a55b.6696 > ff02::1:6.6696: babel 2 (8) hello

Now cause a link flap

ip l set down enp1s0; ip l set up enp1s0

No more packets are sent now (verify via tcpdupm) although the interface shows as up:

2: enp1s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000

show babel interface 
enp1s0 is up
  ifindex 2, MTU 1500 bytes <UP,BROADCAST,RUNNING,MULTICAST>
  Babel protocol is running on this interface
  Operating mode is "wireless"
  Split horizon mode is Off
  Hello interval is 4000 ms
  Update interval is 16000 ms
  Rxcost multiplier is 256

I found two ways to cause babel to start again operating the flapped interface:

• via vtysh

conf
router babel
no network enp1s0
network enp1s0

• via deamon restart systemctl restart frr

Expected behavior

I verified that other protocols e.g. ospf continue running on an interface even if the link flapped.

Versions

• OS Version:
  • Debian 11

• Kernel: Linux debian11-template 5.10.0-10-amd64 SMP Debian 5.10.84-1 (2021-12-08) x86_64 GNU/Linux

• FRR Version:
  • 7.5.1 (Debian 11)
  • 8.2.2 (from the FRR deb repo)
  • master 5bc17d7

[MEschenbacher]

Thank you for the swift fix! I'm not familiar with the backport mechanisms but is there any chance the commit 93d9b1a may be backported to appear in the current debian 11 stable package?