Default GW

Do you know that if a computer has a default gateway on different subnet than the IP address will still work?? If not read the below. If yes again read the below. It’s just 5 minutes and a great experience 😊  

 

As I am almost done from the review of the CWAP and almost ready for the exam (at least that’s what I believe), I will share my experience I had today. Due to today’s workload (around 10h troubleshooting/configuration/design) I am tired to focus on wireless and I am here to write this post.

One of my colleagues called me and told me:

I see one computer that the gateway is on different subnet with the ip address. He sent me a screenshot. See below.

 

To be fair I never faced this before.

The scenario was: My colleague removed the old router and added the new one (migrating to a new network that we don’t need to know the old router. Only the old data subnet and we are seeing them from the PCs most of the times)

I told him to connect via console on the old router but he didn’t know the credentials.

The place had around 10 users.

 

We checked for any static routes on windows PC

- NO any extra routes installed.

I asked him if they had internet before.

- The answer was yes. Then I really got confused.

I thought that they may had secondary IP address on the router interface (on data subnet range). 

In order to test it I added one random IP address within the data subnet as a secondary on the router interface. I advertised this subnet on the dynamic routing protocol and everything restored (internet services etc..)

After this I thought. What was the secondary IP address of the old router they had?? 

- Who knows with no router credentials?

My colleague connected the physical interface on his laptop and he saw the ARP.

Then we found the old router interface secondary IP address and I added it as a secondary on routers interface  😊

 

With this we temporarily resolved the issue as we couldn’t see the old router config and we couldn’t rollback as the provider deleted the old VN. Now we will inform the customer to correct the gateway of the PCs and inform us to delete the secondary.