Wake on LAN and VTP Auto Prunning

diggy · ‎07-13-2009

I'm helping our Windows Admins configure WOL. To make it so I didn'have to worry about IP directed broadcast, I am simply trunking all the needed VLANS (like 4 or 5) to the windows box, and they will bring up a local interface on each VLAN to send the broadcast packet on.

We are using VLAN auto pruning, and am wondering if this will prevent WOL broadcast packets from possibly getting passed down trunks to other switches.

For example WOL Server (on vlan5) is on a 2950 edgeswitch, which goes to a 6500. There are say 1 or 2 PC's (also on VLAN5) on another 2950 edgeswitch hanging off the same 6500. The links to the edgeswitches are trunks.

If the 2 PCs are the only thing on VLAN5, and they are turned off, I'm assuming that that trunk will be pruned of VLAN5 to the switch they are on, and hence not get any broadcast packets for vlan5 until something on that switch comes back up on vlan5. So basically this would break WOL if you use VTP auto pruning? Am I right in this assumption, and is there any way around this except to go back to manually pruning VLANS?

Giuseppe Larosa · ‎07-14-2009

Hello David,

I agree that VTP pruning could block wake on lan broadcasts unless modified to exclude these frames from its action.

Be aware that VTP pruning stops unnecessary traffic on unused vlans but doesn't prevent STP instances to be started for them

Using swithport trunk allowed vlan command provides this level of control and avoid scalability issues of STP protocol on low end switches.

Hope to help

Giuseppe