Comment 2 for bug 1938191

I've checked it on the install you requested and got the same issue.
No VMs, just use the steps I described in the description.

I think the issue is in the create_rtr_2_fip_link method [1].
It checks if the rtr_2_fip_dev device exists. It exists, but this device is from the current external network.
And then it does fip_2_rtr_dev.link.set_mtu(mtu), where fip_2_rtr_dev is the device of the new external network that does not exist yet.

I have a patch to fix this issue by checking the correct netnsid of the rtr_2_fip_dev.
But my patch has a crutch to w/a the pyroute2 issue:
https://github.com/svinota/pyroute2/issues/826

[1] https://opendev.org/openstack/neutron/src/commit/5dee9bbe75cfa79961681677718c509549a584ab/neutron/agent/l3/dvr_fip_ns.py#L410