Routing within a subnet?

[Ben F]

I recently came across a setup that I haven't seen before. Basically, the L3 switch is the gateway for the user LAN. We'll say that address is 10.1.1.1/24. From there traffic is sent to a router. Here is the odd part (from my perspective).....the router's interface has an IP in the same subnet. We'll call it 10.1.1.2/24. Now, I've previously seen a setup similar to this, but instead of the router having an IP in the same subnet there is a /30 or something between the switch and router. Am I missing something or is this setup not the best way to go? Or, I am overthinking it and this is a common practice?

[shaps]

Is the /30 the linking subnet between the router and the L3 switch.  This is generally used for point to point links.

[Ben F]

In the cases that I've seen, the /30 was just for p2p between devices. In this instance the gateway is on the switch and the next top (on the router) are both in the same /24 subnet.

[Julio E. Moisa]

Hi Ben

Please correct me if I understanding wrong.

You have the IP 10.1.1.1 on the L3 switch but on the router connected to the switch you have the ip 10.1.1.2/30 for example?

[Ben F]

Right IP addresses, but they are on the same /24 as the LAN. PCs use the switch (.1) as their gateway and then the switch points to (.2) on the router.

[Joseph W. Doherty]

I would expect it's not all that common.

I've done similar, but generally only when the router is connected to another L2 switch and you're also using it to provide a virtual GW backup on the LAN facing subnets.  In such a situation, I want the L3 switch to be the active LAN GW, for faster routing between VLANs.

[cofee]

I agree with Joseph this is not a common practice. If you were using a /30 address on the same major network like you mentioned then I don't think there is anything wrong with that.

Based on your explanation this is how I think your network is set up, so please correct me if I am wrong. Subnet 10.1.1.0/24 is using L3 switch SVI address 10.1.1.1/24 as the gateway and L3 switch has an upstream connection to an edge router as a next hop which is also using address in the same subnet 10.1.1.2/24. Port on the switch that connects to router is probably an access port  and assigned to the vlan for 10.1.1.0/24.

If this is correct, I know I am assuming a lot here. I think the potential issue with this kind of set up will be that router will have to arp for each destination host (inbound traffic) that falls in this network 10.1.1.0/24

[Ben F]

Your understanding/assumptions are correct. It seems so odd to route traffic through 2 interfaces on the same subnet so I'm glad I'm not the only one that thought this isn't "normal".