RSTP MAXIMUM INSTANCES

[Madhan Kumar]

Hi All,

We are using Cisco 4500 series switches in our environment. Core and access switches are 4500 only. We are using RSTP. I came to know that RSTP in cisco 4500 will support 3000 instances only. Currently my core switches are showing beyond 3000 instances. Can some one help me on this?. How to reduce the no of instances, etc..

Thanks & Regs

R.MADHANKUMAR

[Giuseppe Larosa]

Hello Madhan,

one way to reduce STP instances is to use manually configured lists of permitted vlans on trunk ports, but this method works well only on access layer switches.

If really in your network there are so many different Vlans you should consider moving to 802.1s MST that allows to map multiple Vlans to a single MST instance and is the most scalable STP protocol. Each MST instance is an 802.1W Rapid STP so there is no penalty in convergence.

However, MST requires a change of mind and approach because an MST region is defined by:

the region name

the revision number

the Vlans to MST instances mapping

the MST BPDU contains the region name, the revision number and an hash of the Vlans to MST instances mappings.

Simply put, MST requires provisioning: the best thing you can do is to divide the whole VLAN space of 4094 possible vlans among the MST instances so that you don't need to change the MST configuration.

The reason for this is that if you change the Vlans to MST instances mapping on one switch you are actually partitioning the MST regions in two parts with severe impact until the change is not propagated to all switches.

see

http://www.cisco.com/en/US/docs/switches/lan/catalyst4500/12.2/54sg/configuration/guide/spantree.html#wp1083005

Hope to help

Giuseppe

[Madhan Kumar]

Hi,

Thanks for your valuable reply. Though Moving to MST is the right solution it is not pssible right now. Can you please help me manual steps where I can reduce atleast some instances.

Thanks & Regards

R.MADHANKUMAR

[Giuseppe Larosa]

Hello Madhan,

this can work only on access layer switches on uplink ports connecting to core/distribution you specify the list of allowed Vlans to reflect the effective port vlan mappings on access ports.

interface type x/y

switchport trunk allowed vlan x1,x2-x3,x4,x5-x6

you can specify ranges of vlans using '-' you separate vlans with a comma. Note do not use spaces the list of vlans does not contain a space char.

configure the same list of vlans also on the other side of the uplink for consistency

If later you need to add a vlan you need to use

interface type x/y

switchport trunk allowed vlan add x7

Note: missing the add statement causes the vlan list to be reduced to x7 vlan only !!!!!!!! caution here!

No benefits on core switches with this and the right tool is MST in the long term.

Hope to help

Giuseppe