CDP, VTP and Spanning-tree

Just a few things I have found over the last couple of days:
If you are getting duplex mismatches reported by CDP when there really isn't one you can use no cdp advertise-v2 on the link to turn that detection off. You may get that reported if you have the following layout:
cisco device (full duplex) -> non-cisco switch -> cisco device (half duplex)

Most commonly either the half-duplex device will be an ATA that only supports half-duplex OR you may have a non-cisco wireless device at one end that only supports half-duplex on the ethernet on one side and another non-cisco wireless device at the other end that does support full-duplex.

All that you lose with that CDP command is duplex mismatch detection, VTP domain advertisement and native vlan detection.

The other thing is a bit about spanning-tree and VTP. I'd always noticed that in companies with large numbers of VLANs most devices have been in VTP transparent mode. I assumed it was so that adding a switch with a higher VTP revision number would not nuke the VLAN database. There may be another good reason. Each vlan in the vlan database will have an STP instance. Not an issue for MST, but for PVST it is a big issue. You have higher memory requirements and you may exhaust the number of STP instances your switch can run.

VTP pruning does not help - it only reduces the number of VLANs that run over a trunk (verified by using show int trunk to see which vlans are allowed on the trunks and not pruned and then using show spanning-tree summary to verify which vlans have a spanning-tree instance).

Manually reducing the vlans running on a link using switchport trunk vlan allow does reduce the spanning-tree instances (you need to limit it on all trunk links on the switch you want to reduce spanning-tree instances on). The command must be used on the switch that you want to reduce spanning-tree instances for, using it on the switch at the other end of the link does not help (it only stops the VLAN traffic at that switches port, it doesn't stop the other one from sending traffic before it gets discarded).

The only other ways to reduce the STP instances reliably is to either turn spanning-tree off for some of the VLANs (not advisable), run MST or use VTP transparent and manually add the VLANs required on each of your switches as needed.

Updated: verified in Dynamips using a 3745 with a 16 port switch module and running 12.3-8.T5