Some CatalystLayer 2 and Layer 3 fixed configuration switches change the VTP modeautomatically from client to transparent with this error message:
%SW_VLAN-6-VTP_MODE_CHANGE: VLAN managerchanging device mode from
CLIENT to TRANSPARENT.
Either of thesetwo reasons can cause the automatic VTP mode change in these switches:
· More VLANsrun on the Spanning Tree Protocol (STP) than the switch can support.
CatalystLayer 2 and Layer 3 fixed configuration switches support a different maximumnumber of instances of STP with the use of per-VLAN spanning tree+ (PVST+). Forexample, the Catalyst 2940 supports four instances of STP in PVST+ mode, whilethe Catalyst 3750 supports 128 instances of STP in PVST+ mode. If more than themaximum number of VLANs is defined in the VTP, the VLANs that remain operatewith STP disabled.
If the number of instances of STP that is already in use is greater thanthe maximum number, you can disable STP on one of the VLANs and enable it onthe VLAN where you want STP to run. Issue the no spanning-tree vlan vlan-id globalconfiguration command in order to disable STP on a specific VLAN. Then, issuethe spanning-tree vlan vlan-id global configuration command in order to enableSTP on the desired VLAN.
Note: Switches that do not run STP stillforward the bridge protocol data units (BPDUs) that they receive. In this way,the other switches on the VLAN that have a running STP instance can breakloops. Therefore, STP must run on enough switches in order to break all theloops in the network. For example, at least one switch on each loop in the VLANmust run STP. You do not need to run STP on all switches in the VLAN. However,if you run STP only on a minimal set of switches, a change to the network canintroduce a loop into the network and can result in a broadcast storm.
Workarounds:
· Reduce the number of VLANs that areconfigured to a number that the switch supports.
· Configure the IEEE 802.1s MultipleSTP (MSTP) on the switch in order to map multiple VLANs to a single STPinstance.
· Use switches and/or images (EnhancedImage [EI]) which support a greater number of VLANs.
· The switch receives more VLANs from a connectedswitch than the switch can support.
An automaticVTP mode change also can occur if the switch receives a VLAN configurationdatabase message that contains more than a set number of VLANs. This normallyhappens in Catalyst Layer 2 and Layer 3 fixed configuration switches when they areconnected to a VTP domain that has more VLANs than are supported locally.
Workarounds:
· Configure the allowed VLAN list onthe trunk port of the connected switch in order to restrict the number of VLANsthat are passed to the client switch.
· Enable pruning on the VTP serverswitch.
· Use switches and/or images (EI) whichsupport a greater number of VLANs.