My company has a Cabletron SSR 8600 Router/Switch connecting everything on the network. A new Cisco Catalyst 6509 switch has been purchased to take its place. We have quite a few subnets on the SSR and will be migrating those to the new Catalyst. My challenge is to keep the 8600 online while migrating to the 6509. What are my options?
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First, thoroughly document the Cabletron passwords you'll need to get into config mode to print out the current configuration, as well as delete the virutal LANs (VLAN) as you move them from one switch to the other. Your message didn't detail your network configuration, so I will assume you have the servers on one VLAN with the other subnets each in their own VLAN.
 
There are two ways that 6509 could have been ordered - hybrid or native mode. Depending on your background with Layer 3 switches, you'll find that native mode is easier to deal with. Other than creating the actual VLAN, everything is done from the command prompt. It has most of the same commands you would expect to find in a regular Cisco router. If your router was sent in hybrid mode, it can be converted to operating in native mode by flashing the firmware with a different IOS. There is documentation on the Cisco Web site on how to do this.
 
A good starting point is to create a VLAN on the Cisco that matches one on the SSR. You will have to use different IP addresses when setting it up and put routing statements in both switches so they know where to look for the subnets/VLANs that are not on the switch that is looking for it. Once you have verified that the routing is working, you can delete one VLAN/subnet from the SSR, create it on the Cisco, move the subnet, and you should be in business. Another option is to set up VLAN trunking, but this may be more trouble than it's worth depending on your level of expertise with handling VLAN trunking on Cabletron and Cisco platforms.