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The "SW_VLAN-6-VTP_MODE_CHANGE: VLAN manager changing device mode from CLIENT to TRANSPARENT" message is displayed on Catalyst 2900/3500XL, 2940, 2950/2955, 2970, 3550, 3560 and 3750 series switches
There are more VLANs running the Spanning Tree Protocol (STP) than the switch can support.
Some switch devices must automatically change VLAN Trunking Protocol (VTP) device modes upon receipt of a VLAN configuration containing more than a set number of VLANs. The SW_VLAN-6-VTP_MODE_CHANGE: VLAN manager changing device mode from CLIENT to TRANSPARENT message indicates that such a spontaneous conversion has occurred.
Catalyst Layer 2 (L2) and Layer 3 (L3) fixed configuration switches support a different maximum number of instances of STP when using Per-VLAN Spanning-Tree (PVST)+. For example, the Catalyst 2940 supports four instances of STP in PVST+ mode, while the Catalyst 3750 supports 128 instances of STP in PVST+ mode. If more than the maximum number of VLANs is defined in the VTP, the remaining VLANs operate with STP disabled.
If more than the maximum number of instances of STP is already in use, you can disable STP on one of the VLANs and enable it on the VLAN where you want it to run. Issue the no spanning-tree vlanvlan-id global configuration command to disable STP on a specific VLAN, and issue the spanning-tree vlanvlan-id global configuration command to enable STP on the desired VLAN.
Switches that do not run STP still forward the Bridge Protocol Data Units (BPDUs) that they receive so that the other switches on the VLAN that have a running STP instance can break loops. Therefore, STP must run on enough switches to break all the loops in the network. For example, at least one switch on each loop in the VLAN must run STP. It is not necessary 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 could introduce a loop into the network and could result in a broadcast storm.
To resolve the issue, reduce the number of VLANs configured to a number supported by the switch.