Troubleshooting Linux Bridge Networking Issue on KVM Host

Have you ever restarted your Linux Host machine or Host networking which is running KVM Guest machines ? If so, have to encountered connectivity issue between host and guest machines ?
If answer to questions above is resounding “YES”, then you are in luck. This post is all about how to resolve the issue.

Background on how I encountered this problem or if you are adventurous type, would like to replicate the issue. I have production linux server which is running critical applications on Guest VMs. Host and Guest networking is pretty straightforward. Host Machine has 2 interface to which bridges (br0 and br1) are associated. And Each Guest Machine has been associated to both the bridges and assigned static IPs. Pretty straightforward. Problem arose when i had to update the default Gateway for both the network associated (cisco asr, gateway failover, etc. etc. I don’t want to bore you) and very naively i ran “systemctl restart network” and all hell broke loose. Host connectivity was fine but I was not able to connect to Guest Machines and services running on Guest Machines were unavailable.

So this is what happened which broke the guest machine connectivity.When the default gateway was updated on host, vnetxx association to bridge was lost as you can see below:
VM Interface association for my first Guest machine:
VM Interface association for my 2nd Guest machine:
To resolve the issue, add the respective vnet interface to corresponding bridge:

Voila, interfaces associated, Guest Machine connectivity restored and like is good as you can see below.