Hello. I am migrating from a 3750 to a 4510 in my core. I have started the migration, but I am stumped on a basic configuration issue.

My plan has been as follows:

internet <--> FW <--> 3750 <--> 4510

I added the 4510 to the core and attached it via a 802.1q trunk to the 3750. I have then been configuring the 4510 with corresponding VLANs and moving hosts over to the 4510. The 3750 still has the VLAN interface configurations w/IP addresses, and the 3750 still serves as the core distribution (i.e., routing) to the rest of the network. So far, so good. All hosts can reach all internal and external destinations without any problem.

However, I am running into trouble when I try to remove the VLAN interface config from the 3750 and add it to the 4510. I lose connectivity to internal and external destinations.

So, I tried setting up a test VLAN on the 3750 to mimmic my live environment. I built a VLAN on the 3750, gave it an IP address in an unused subnet range, no shutdown, added it to EIGRP. I then configured the same VLAN on the 4510, added the network to EIGRP. I then attached a w/s to that switchport on the 4510, but I get different results than I got with my "live" hosts. I can access all internal hosts, but nothing external. I have checked the trunk link, and the VLAN is active on the trunk link. It should work.....

So, two questions.

First, what am I missing as far as my test VLAN is concerned?

Secondly, how should I proceed from here on my "live" transition?

Note: my goal is to move my FW link and my internal connection to my distribution layer as the last step in the removal of the 3750 from operation.

Thanks in advance, Patrick