OSPF and VLAN Question

certmister · October 2011

Hopefully this is a quick question. Routing is not my strong point so bear with me here. I set up OSPF on a 3 Router, 3 Switch Network and the Guide I was using said you should be able to ping the Vlan IP of the Switches. I was under the impression that was what ROAS and L3 Switches were for, not Routing Protocols. I mean I can ping all the interfaces on the Routers but if I try to ping the address assigned to Vlan1 it dies. Am I right to assume that without a Trunk and Subinterfaces setup between the Router and the Switch, i.e. ROAS, that OSPF is not supposed to be able to reach the switches.

pham0329 · October 2011

ROAS provides inter-vlan routing, but without routing protocols, every network would have to be directly connected to the router for routing to occur. Consider the following

Sw1 is connected to R1, and contains vlan 10 and 20
Sw2 is connected to R2, and contains vlan 100 and 200
R1 is connected to R2

Without a routing protocol, how would a client in Vlan 10 reach a client in vlan 100? It couldn't because R1 wouldn't have a route to get to vlan 100 and vice versa.

I mean I can ping all the interfaces on the Routers but if I try to ping the address assigned to Vlan1 it dies. Am I right to assume that without a Trunk and Subinterfaces setup between the Router and the Switch, i.e. ROAS, that OSPF is not supposed to be able to reach the switches.

No. As long as vlan interface and the router is on the same subnet, and the switchport connecting the router to the switch is assigned to the correct vlan, there should be connectivity between the two.

certmister · October 2011

Forgot about the switchport. I have had some success with hardcoding the switchport on the switch to an access port, but one of switches is still not reachable even after changing the switchport. The Routers are in the same subnet and Vlan 1 is the only vlan.

advanex1 · October 2011

certmister wrote: »

Forgot about the switchport. I have had some success with hardcoding the switchport on the switch to an access port, but one of switches is still not reachable even after changing the switchport. The Routers are in the same subnet and Vlan 1 is the only vlan.

Well, I'm assuming you have your switches all connected to one router and then the other 2 routers as "out sites". This may sound basic, but are you sure that the router interface and the switches management IP's are all within the same subnet?

certmister · October 2011

I figured out the Problem, it's an IP Addressing problem. I'm using the subnet192 guide for this lab. Each Switch connects to a different Router. Here's a basic idea of the Topology:

Here is the IP Info:

I think S2's Vlan 1 IP address is wrong, and should be 172.16.0.126 not 172.16.5.126 and I used the proper address on S2 but I never changed the address on R2 to put it in the same subnet. So problem solved I guess but the Guide really needs to be redone because this is not the first error I've seen.

pham0329 · October 2011

Good to hear it's working, but looking at the diagram, it says

R2's F0/0 ip = 172.16.5.1/25
S2's VLAN ip = 172.16.5.126/25

those are on the same subnet...i'm not sure how changing S2's ip to 172.16.0.126 fixed your problem

certmister · October 2011

Part of the lab was to calcuate the subnets and their ranges and when I did S2's Vlan subnet, it was 172.16.0.126/25 but R2 was 172.16.5.1/25

I configured S2's vlan as 172.16.0.126 but forgot to change R2 so it was on the same subnet, I have since put them both on the same subnet, i.e. R2 is now 172.16.0.1

I guess looking back if I had left them as they were in the Table, it would have worked but technically the subnet range was incorrect.

OSPF and VLAN Question

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