Core Switch Migration Issue

[Mat Barry]

I've just migrated from a 3750 stack to a 3850 stack.  They were connected via trunk and all was well.   After the migration was complete, we disconnected the trunk and all devices lost connectivity on the new stack.    I cleared the arp table, but I'm guessing this may be due to something with STP.  

This is the first Migration I've done in this manner (without a straight replacement) and I'm sure it's something simple I need to do.   

Thanks in advance.

[Reza Sharifi]

Can you check to make sure VTP is turned off or is in transparent mode?

HTH

[Mat Barry]

There are no configurations regarding VTP on the switch. There isn't a domain or any others set.

If I set these to transparent mode, which one should I do it on first, and will it remove the vlans which are already running on it?

I've always set up VTP when I deployed the switch, but this is one that was set up by someone else, and I copied the config.   I've had mixed results when changing vtp info on switches that were already running, as far as losing connectivity.

[Mat Barry]

The issue was the IP addressing.  When I created the replacement, I configured it with a different address, which wasn't the gateway address.  When I removed the old switch, all devices lost their gateway.  

[paul driver]

Hello

Below was the way i did a recent core migration:
1 )attached new Core switch to existing core switch via trunk - provided it with a management ip adrress, made sure vtp/STP has synchronized

2 ) created the same SVi's on the new core as the existing core
Ie - Same IP address/subnet/default route/ip routing etc (MADE SURE THIS WERE IN SHUTDOWN MODE BEFORE ASSIGNING THE SAME ADDRESSING) - this also includeed the SVI and access port for the WAN router connections


3) Schedule some down time
1.1 relocated  the wan router connection over to the new core - attached it to the access port already created
1.2 shut down the old core svi interfaces and disabled ip routing and assign a new management ip addresss and D/|G
1.3 enable the new cores svi interfaces

l3 routing now went through the new CORE and I still had access to the old core

res
Paul