Maximum spanning tree instances - 128

[W S H FERNANDO]

HI

I read that maximum spanning tree instance number is 128, is there any switches that can go more than128 instances ? or can we do this from IOS updates ?

Regds

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[Bilal Nawaz]

Hello, there is an article on this here
http://sabotage-networks.blogspot.co.uk/2010/02/cisco-gotchas-max-vlans-and-stp.html?m=1

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.
Cisco 6500 doesn't directly limit the number of spanning-tree instances. It has limits on the number of virtual ports per line card.


A virtual port is a VLAN being forwarded on a trunk. So if you have 10 VLANs and 5 active trunks with no pruning then you have 50 virtual ports.

A 6500 is limited to 1800 virtual ports per line-card (with caveats, see below!).

If you have a full 48-port linecard and use PVST then you can only create 37 VLANs before you hit the limit. If you intend to go over this then either manually prune some VLANs or upgrade.

Some of the SX IOS versions have removed the virtual port limit on the newer linecards (6500 and 6700 series), however the per-chassis limits still apply.

The per-chassis limits are shown here.

They are 10000 for RPVST+ and 13000 for PVST+, updated in IOS 12.2(33)SXI to be 12000/15000 respectively.


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

Hope this helps

Sent from Cisco Technical Support iPhone App

[Peter Paluch]

Hello Bilal,

Awesome answer indeed!

To add a small remark, there is really no sensible need to have dozens of STP instances just because we have dozens (and more) of VLANs. As the number of possible spanning trees in a network is finite and usually sharply smaller than the number of VLANs running over it, many STP instances end up establishing identical spanning trees. That is one of the reasons to run MSTP as it allows you to run a reasonably small number of STP instances without any regards to the number of existing VLANs. In other words, you may have thousands of VLANs and still be fine with two or three STP instances. Running one unique STP instance for a VLAN is, in a certain way, extremal.

Best regards,

Peter

[Bilal Nawaz]

Hi Peter, I'm in total agreement with you.

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