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DEMO: Configuring OSPF

A practical walkthrough of configuring OSPF on a router to establish full network connectivity across a multi-VLAN topology. Covers process ID assignment, wildcard mask usage, and network statement configuration in a single OSPF area.

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About this video

When a router has no routing protocol configured, it cannot forward traffic beyond its directly connected networks, leaving VLANs and remote segments unreachable. This scenario demonstrates that limitation in a live topology, using a continuous ping to a remote interface to illustrate the absence of connectivity before any OSPF configuration is applied. The show ip route command is used at the outset to confirm that only directly connected routes are present in the routing table. OSPF is then configured on the router by entering global configuration mode, launching the OSPF process with a locally significant process ID of 10, and issuing a network statement that covers the entire 10.0.0.0/8 address space using a wildcard mask of 0.255.255.255. The router is assigned to Area 0, which aligns with OSPF best practice for single-area deployments. Once the configuration is committed, OSPF begins advertising routes across all qualifying interfaces, neighbor relationships form with the already-configured adjacent switch, and the routing table populates with OSPF-learned routes spanning the full topology. Successful ping responses and an updated show ip route output confirm that end-to-end connectivity has been restored.

What you'll learn

What's covered

Configuring OSPF on Router

Aligned to

Cisco CCNA
3.4 Configure and verify single area OSPFv2
3.1 Interpret the components of routing table

Key terms

Router
A network device that forwards data packets between networks based on IP addresses.
Routing Protocol
A protocol used by routers to communicate and share routing information to determine the best path for data.
Virtual LAN
VLAN
A logical grouping of network devices that behave as if they are on the same network regardless of physical location.
Open Shortest Path First
OSPF
A link-state interior gateway routing protocol that maintains a complete map of the network topology and uses Dijkstra's Shortest Path First algorithm to calculate optimal, loop-free routes. OSPF divides networks into hierarchical areas to improve scalability and supports VLSM and rapid convergence.
Wildcard Mask
A 32-bit mask used in ACL statements and OSPF network commands where a 0 bit means the corresponding address bit must match and a 1 bit means it can be anything. Wildcard masks are the inverse of subnet masks and allow a single statement to match an entire range of IP addresses.
OSPF Area
A logical grouping of routers and networks within OSPF used to limit routing update traffic and scale large networks; Area 0 is the backbone area required in all OSPF deployments.
Subnet Mask
A 32-bit number that divides an IP address into network and host portions.

Transcript

The Topology and the Problem

We're using the same topology here. I am consoled into router one, which I don't have connectivity to right now, and I don't have connectivity because there's no routing protocol that's set up. What's happening is I'm on this demo laptop and I can ping out and it will get to this router one, but that can't come back because this router 1 is not aware of any of these VLANs here.

So what we're going to do is set up OSPF on here. I already have OSPF set up on this switch right here, so I don't need to set up OSPF there, and I'm using the same commands on this router one because my network statement encompasses all of my networks within here. So it's going to encompass all of the interfaces and all of the networks, and I'm good to go.

Right down here I'm pinging 10.240.2.2 2, which is this interface on router 1 right here, so as soon as I have connectivity I should be able to successfully ping. That's a continuous ping.

Configuring OSPF

Now what I'm going to do is get onto my router and get into the global configuration mode. I'm going to do the router ospf and then that process ID. We said the process ID is specific to these routers and it really doesn't matter what it is — we're not dealing with multiple processes, multiple OSPF processes, so we don't really need to worry about it. But I'm going to use 10 and be consistent across all of the different routers.

Then I'm into the router ospf. Now, still nothing is being sent out to other devices on here. Before I do this, I'm going to do a show ip route and let's just see what it looks like. You see the directly connected networks.

Now I'm going to type in my network statement. So network, and then it's the 10.0.0.0. We're doing a /8, and then for a subnet mask that would be a 255.0.0.0, but this is a wildcard mask, and as we mentioned it's the opposite, so I'm going to do 0.255.255.255. And then we do area. The area for this — we're just doing all one area for this topology, it's very simplistic and only needs one area, and the best practice is to be area zero. So I'm going to enter that in.

Verifying Connectivity

Now at this point in time it selects the interfaces that fall under that range, starts sending out information, and it's sending out the information from whatever networks fall into that range. We can see that our pings are successful now.

I can do the show ip route and I can see quite a few in there — all of the OSPF routes across the whole network. So now I have connectivity via OSPF.

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