Friday, 5 February 2010

Cisco Gotchas - Max VLANs and STP Instances

Cisco switches have separate limitations on:

  • 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:

  1. Standard 802.1D Spanning-Tree

  2. PVST, Per-VLAN Spanning-Tree (ISL trunking only)

  3. PVST+, Per-VLAN Spanning-Tree plus (compatible with 802.1q trunks)

  4. Rapid PVST - RSTP version of PVST+

  5. 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.

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