Foaa

This article is about a strange network problem occuring when used bridging in a XEN environment.

Symptoms

• Some DomU is not reachable from the network anymore.
• The problem persists after restarting the DomU (all: reboot, shutdown+create, destroy+create).
• Possible: The DomU still has a virtual NIC, if pings are send from DomU via console only arp-requests are seen on the virtual NIC in Dom0, no answers.
• Possible: There could be log-entries as: xenbr1: port 4(domu_eth0) entering disabled state.

Cure

There are two possibilities. First: the DomU has lost it’s NIC on the Dom0 side, then re-add with

#> brctl addif xenbr0 domu_eth0

(xenbr0 = your bridge, domu_eth0 = the nic of the DomU, maybe looks like vif99.1)

If the NIC was not deleted from the bridge or the solution above doesnt work, first check:

brctl showstp xenbr0

This should output something like this:

domu_eth0 (4)
 port id		8004			state		         learning
 designated root	8000.0030487e5b0d	path cost		 100
 designated bridge	8000.0030487e5b0d	message age timer	 0.00
 designated port	8004			forward delay timer	 12582588.14
 designated cost	   0			hold timer		 0.00
 flags

The important information is learning as value for state. If this is the case, the following solution works probably. You have to reset the forward delay timer by:

brctl delif xenbr0 domu_eth0
brctl setfd xenbr0 0
brctl addif xenbr0 domu_eth0

This removes the NIC from the bridge, then sets the timer to zero and then re-adds the NIC to the bridge. Afterwards the showstp-command should return somthing like this:

domu_eth0 (4)
 port id              8004                 state                   forwarding
 designated root      8000.0030487e5b0d    path cost               100
 designated bridge    8000.0030487e5b0d    message age timer       0.00
 designated port      8004                 forward delay timer     0.00
 designated cost      0                    hold timer              0.28
 flags

The important part: forwarding (again).

I dont have any clue whe the NICs detach in the first place, but this works for me to repair it.