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Using OSPF with Subinterfaces?


Hi all,

I am a first year networking student so please explain like I'm 5 when replying if you don't mind.

I'm currently working on a small-ish project in Packet Tracer.

I have three routers connected in a straight line I suppose you can say. Router A is connected to Router B which is connected to Router C.

Router A is then connected to a Switch, which is connected to two other switches to form a triangle. Any number of hosts is expected to be connected to these switches. It is being configured as router-on-a-stick, with three VLANs and three subnetworks accordingly. The interface attached the Switch has been divided into three subinterfaces with three networks for each VLAN.

Router B is just a central router, connecting Router A to Router C.

Router C is a stub network. It is using a default route to get outside of it's network, to the next hop on Router B. 

Router B and Router A are supposed to be configured with OSPF. 

I had everything working fine until I added the VLANs and three subnetworks to my network on Router A. Originally, it was just a single network and I had full connectivity. However, when I assigned the subinterfaces, I can no longer ping from the stub network to the Router A's network(s). The packets are dropped at Router B because it doesn't know where to go.

I tried removing the old single network attached to Router A from the OSPF on Router B and adding the three subnetworks in instead. This did not work however.

We were not taught how to deal with this scenario in our course so I'm quite lost. I tried googling but could only find information on configuring OSPF on the same router with the subinterfaces, involving Frame Relay? However my teacher says this is not the answer to the problem.

I was wondering if you guys could point me in the right direction.

To summarize again, how do I configure OSPF on Router B (my central router) to the subnetworks on the subinterfaces on Router A?



I have attached the topology and running configs for my three routers below.

"IT" is Router A

"Gateway" is Router B

"Accounting" is Router C