Which link is used? L2 or L3 ... scratching my head

[NT_01]

Hi.

Our network has three core switches. We are running OSPF on each core switch for routing.

The design looks like this:

                   Firewall

                        |

SWA <----> SWB <-----> SWC

From A to B there is both a L2 Etherchannel and a L3 Etherchannel

From B to C there is a single L2 and single L3 link

SVIs exist on each core switch, and most, but not all associated subnets are being advertised via OSPF

The L2 trunk from B to C carries a handful of server related VLANs, vlan 10, 20, 30, 40, 50

Those same VLANS are also advertised via OSPF

If I am at either B or C location and trying to talk to a device on the above mentioned VLANs, which link will be used, L2 or L3?

Reason I am asking is because I would like to simplify this configuration, removing any VLANs that should not be on the trunk if they are being routed across the L3 link anyways.

The L2 link between B and C is 100 Gig

The L3 link between B and C is 20 gig

If anyone can help me unwind this I would greatly appreciate it!

Scratching my head.

[Joseph W. Doherty · Accepted solution]

Which link will be physically and/or logically used depends whether the two hosts, that wish to communicate, are in the same network or not and specifically how the L2 and L3 links, between network devices, are defined.

If the hosts are on the same network (e.g. same VLAN), they will use L2 connections.

If different network (e.g. 192.168.1.5/24 <> 192.168.2.5/24), they need to be routed, but supporting this might be done with L2 and/or L3 links.  What's used depends on the actual physical and logical topologies, and where the hosts are located within those topologies.

[NT_01]

Thank you for your help. Makes sense.