[bullseye] Output of iproute2 v5 parsed incorrectly by swss for VLAN removal

[alexrallen]

THIS IS A BUG TARGETING the Debian 11 / Kernel 5.10 UPGRADE

Description

The new major version of iproute2 (5.10) has a different output than the version currently used in SONiC (4.20)

Directly after removing a port from a vlan bridge in 4.20 we see the following results

root@r-lionfish-16:/# bridge vlan show dev Ethernet0
port    vlan ids
Ethernet0       None

SWSS looks at this result and greps for "None" in order to determine that it needs to remove the interface from the bridge device as the master.

https://github.com/Azure/sonic-swss/blob/master/cfgmgr/vlanmgr.cpp#L234

However in 5.10 we see the following

root@r-lionfish-16:/# bridge vlan show dev Ethernet0
port    vlan ids

The lack of entry in this table means that swss does not remove the interface from the bridge as its master. In effect, this means the interface is not fully removed from the vlan.

Steps to reproduce the issue:

config vlan add 50
config vlan member add 50 Ethernet0
config vlan member remove 50 Ethernet0
ip link show Ethernet0

Describe the results you received:

The bridge device is still the master of the interface.

Describe the results you expected:

The interface should have no master set.

[alexrallen]

FYI @saiarcot895

We thought we had fixed this, but it seems that in our current configuration the Ethernet0 entry will be removed entirely from bridge vlan show instead of showing None like we expected.

[saiarcot895]

This should actually get fixed by sonic-net/sonic-swss#1970. The code there updates the detection code in swss to correctly remove the interface from the bridge.

[alexrallen]

Verified this is fixed.