- The number of VLANs that can exist in the database.
- The number of Spanning-Tree Instances that can run.
Cisco kit tends to use per-VLAN spanning-tree in which case the two values will be the same.
To understand the problem and solution requires knowledge of the different types of spanning-tree available, this subject is huge but very briefly the types are:
- Standard 802.1D Spanning-Tree
- PVST, Per-VLAN Spanning-Tree (ISL trunking only)
- PVST+, Per-VLAN Spanning-Tree plus (compatible with 802.1q trunks)
- Rapid PVST - RSTP version of PVST+
- MST - Multiple Spanning Tree
The limitations are as follows on switches running PVST, PVST+ or Rapid-PVST:
- 2950 SI: Maximum 64 STP instances, Maximum 128 VLANs.
- 2950 EI: Maximum 64 STP instances, Maximum 250 VLANS.
- 3550, 3560, 3750: Maximum 128 STP instances, Maximum 1005 VLANs.
- 6500: Based on logical ports. Article here.
If you exceed the number of VLANs then you'll get an error like this:
SPANTREE_VLAN_SW-2-MAX_INSTANCE: Platform limit of 64 STP instances exceeded. No instance created for VLANxxx
There are a couple of workarounds:
- Delete some VLANs!
- Manually prune VLANs off the trunk links and set the switch to VTP mode transparent so it doesn't know about the extra VLANs.
- Use MST instead of PVST.
MST is my preferred option. Most networks will only require 2 or 4 Spanning-Tree topologies anyway so it is wasteful to have STP running in every VLAN.