Path preference: Layer 2 vs Layer 3

[network_geek]

Hello All,

I am looking for a simple logical reasoning behind a small thing that has been troubling me.

Please refer to attached diagram to better understand this situation. I am going to ping PC-2 from PC-1 in different subnets and VLANs, with a proper routing protocol running in between switches and having complete routing information. Gateways to the SVIs are the nearest attached switches, for example PC-1 has a a gateway on SW-1 but in shut state and having identical configurations on SW-2 with the SVI is in up state and vice versa.

If I ping from PC-1 to PC-2, will it work or will it fail? The gateway is shut on SW-1 but unshut on SW-2 having same IP configured and having complete layer-2 path. In this scenario you can assume that the configurations of both SVIs are up with the only difference being shut/unshut.

Awaiting your responses!

[Sergiu.Daniluk]

Hi @network_geek 

In the diagram you showed, if it is configured just as described there, connectivity between PC1 to PC2 will not work.

The flow is like this:

1. PC1 resolves ARP for it's gateway

2. PC1 sends ICMP request to SW1 (SVI-10)

3. SW1 checks the DMAC -> knows it's destined to itself so it forwards the packet to routing engine.

4. SW1 checks the routing table. There is no route to DIP so it drops the packet.

For this to work, you need SW-1 to have a route to VLAN20. You can have that either connected, by configuring SVI20, either have an interconnect between SW1 and SW2 (SVI30 for example) over which you run a routing protocol (can be even static).

Stay safe,

Sergiu