Connectivity between two L3 switches

[aok]

Hello

We are in the process of migrating off our old 3750 core switches and onto Nexus 3ks. Currently, all of our Layer 3 networks are on the 3750s, there is a trunk between the 3750s and N3Ks and our rack switches are connected to the N3Ks. So Layer 2 traffic flows through the Nexus to the 3750s as their gateway.

The next step is to start moving over the L3 to the N3Ks, I created a test L3 vlan on the N3ks but realized I have no routing to any of the other L3 networks from there and no route out to the Internet. I'm trying to wrap my ahead around what's needed in order to get everything talking to each other and L3 vlans working on the N3Ks with minimal service impact.

Thank you

[paul driver · Accepted solution]

Hello
You should add a additional physical connection between existing core and new core switches which will be a L3 connection (either routed port or a new svi with access-port assigned to the new svi) advertise this addressing into any routing process.

Create the same vlan SVi's on the new core but in a shutdown state and when you are ready migrate all the l2 interconnects over to the new core switch first and when that's complete, Shutdown the SVis one by one or all together on the old core and enable them on the new core, you may receive a slight drop for each L3 subnet whilst arp refreshes but all should be good.

Lastly relocate the wan uplink and any static routing and you then should be good to shutdown the old core switch.

[Giuseppe Larosa]

Hello @aok ,

as already noted you need a new IP subnet and VLAN and corresponding SVIs to be used as routing link between old core and new core.

The use of a routing protocol like OSPF will give you the capability to move each client VLAN SVI from old core to new core even one per time.

The routing protocol will be used to advertise existing connected SVIs and at the beginning the old core will advertise also a default route for internet connection.

When you shut an SVI on old core and enable the corresponding SVI on the new core the associated subnet will be seen via the L3 link as coming from new core.

You could even minimize ARP problems in migration if in the new SVI on new core  you use

int vlan XX

mac   <MAC-old-core>

that is you change the MAC address to be equal to that of the old core just shutted SVI.

the MAC of old core SVI can be seen with a simple show interface vlan XX on old core.

Hope to help

Giuseppe