Guest loses Network connection after Host Network restart (KVM)

This document (7018994) is provided subject to the disclaimer at the end of this document.

Environment

SUSE Linux Enterprise Server 11
SUSE Linux Enterprise Server 12
SUSE Linux Enterprise Server 15

Situation

In a setup where a KVM Guest System is running and has a working Network Configuration and the Network on the KVM Host is restarted, the Guest system loses all Network Connectivity.

From a Guest side this cannot be debugged as the Guest only sees that nothing happens on the Network Devices anymore. Seemingly all Network hangs.

On the Host side, after a restart of the Network the ip command, for example

ip a s

shows the virtual network in state down

32: vnet0: <BROADCAST,MULTICAST> mtu 1500 qdisc pfifo_fast state DOWN qlen 500

no restart of Host Network or Guest Network can fix this issue.

Resolution

To connect the virtual Network with the Network again, it is necessary to reconnect on the Host system, this can be done with the command

/usr/share/qemu-kvm/qemu-ifup <Virtual Network> <Desired Device>

which for example, in a setup where the DOWN Virtual Network would be

vnet0

and the Device that provides Network Connectivity would be

br0

would be

/usr/share/qemu-kvm/qemu-ifup vnet0 br0

After this, issuing the command ip a s will show

37: vnet0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master br0 state UNKNOWN qlen 500

and the Guest System will have Network Connectivity again.

Cause

This is a result of removing an underlying element of a stack. In this case the Virtual Network builds on the existing Network on the Host. If this Network is removed, as is done during a restart, then the Virtual Network goes DOWN and does not recover by itself.

In this case Administrative Intervention is necessary to connect the Virtual Network to the Network again.

Disclaimer

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