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..
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.
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