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unwanted vlan pruning..

I have two 3500 switches and a 3660 router.

Today the two switches are connected to each other via a dot1q trunk.

the router is then connected to one of the 3500 switches, also with a trunk. it is a "router-on-a-stick".

my problem is this. trunking does not seem fully functional. I cannot ping computers attached to the non-local switch.

If a device on VLAN 3 is connecetd on the 3548 (the first 3500 swtich) it cannot ping another device also on VLAN 3 on the 3512(the second 3500 switch).

Although if I connect a device to the 3512 and assign that port VLAN 3. the first device which couldn't ping befoore, now can!

so it's like the 3512 is pruning the VLANs when there are no devices on the 3512 that is connected to that partiular VLAN. Although there is at least one interface - the router interface, which is a dot1q trunk with the 3512..

anyone else got into the same problem?


thanks in advance



Re: unwanted vlan pruning..

You have device in VLAN3 on switch A but don't have

any devices on VlAN3 on switch B.

Switch B is not going to have VLAN 3 up if there are no ports up/active in VLAN 3. VLAN 3 will not be trunked until VLAN3 is active.

When you plug a device into Switch B and put it in VLAN 3 you can ping. VLAN3 is now up/active and trunked if trunk is configured to trunk VLAN3.

On the trunk to router, you can make the native VLAN 3 so VlAN3 is up/active thus it gets trunked.

Offhand, I don't think tagged VLANs on a trunk will be trunked unless that VLAN is active on a access port or native VLAN on a trunk port, on that switch.

Re: unwanted vlan pruning..


have you tried to disable VTP prunning?


for details. (Just use NO VTP prunning command.)



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