Cisco CCNA Exam Tutorial: Five OSPF Hub-And-Spoke Details You Must Know!

CCNA exam success depends greatly on knowing the details, and if there's one protocol that has a lot of details, it's OSPF! This is true particularly of hub-and-spoke networks, so in this CCNA OSPF tutorial we'll take a look at some of the more important hub-and-spoke OSPF details. This will help you in working with real-world networks as well, since this OSPF network type is one of the more typical network topologies.

In OSPF, the hub must become the designated router (DR). The DR election's deciding value is the OSPF interface priority, and the default value is 1. It's not enough to set the hub's OSPF interface to 2, however, since the spoke routers must not become the DR or BDR. You must set the spoke interfaces to an OSPF priority of zero.

R2(config)#int s0

R2(config-if)#ip ospf priority 0

This ensures that the spokes will not become the DR or BDR if the hub goes down.

The hub does require a bit more configuration, though. The neighbor command must be used on the hub to indicate the IP address of the potential neighbors.

R1(config)#router ospf 1

R1(config-router)#neighbor 172.12.123.2

R1(config-router)#neighbor 172.12.123.3

It's common to have an ISDN link as a backup in an OSPF network, and when that ISDN link comes up the hello packets must be able to cross the link. What you don't want is to have the hellos keep the link up! By configuring the ISDN link as an OSPF demand circuit, the link will drop in the absence of interesting traffic, but the OSPF adjacency that formed across the ISDN link will be assumed by the router to still be up. (You usually see this command configured on both sides of the ISDN link, but it's only needed on one side. It doesn't hurt anything to put it on both sides, though.)

R2(config)#int bri0

R2(config-if)#ip ospf demand-circuit

A final detail of OSPF hub-and-spoke and demand circuits actually takes place at Layer 2. For the OSPF hello packets to successfully be transmitted across an ISDN link or a frame relay network, the broadcast option must be enabled in the appropriate frame and dialer map statements. Failure to enable this option can lead to a situation where pings will be successful, but OSPF adjacencies will not form.

R2(config-if)#dialer map ip 172.12.21.1 name R1 broadcast 5551111

R2(config-if)#frame map ip 172.12.123.1 221 broadcast

When you're troubleshooting OSPF in a production network or your CCNA / CCNP home lab, don't just look at Layer 3 - because everything's got to be right at the physical and data link layers in order for the network layer to function correctly!

Cisco CCNA Certification Exam Tutorial: OSPF Hub-And-Spoke

CCNA certification demands that you master the basics of OSPF, and for many studying for the CCNA exam, their first exposure to OSPF is a hub-and-spoke configuration. That's a tough way to get started, because a hub-and-spoke configuration built over an NBMA technology such as Frame Relay requires quite a bit of attention to detail. Let's take a quick look at several common OSPF configuration errors and how to avoid them on your CCNA test.

Make sure the hub is the designated router and that there are no backup designated routers. This is done by setting the OSPF interface priority to zero on the spoke routers. This not only ensures that the hub wins the DR election with its default OSPF interface priority of 1, but it prevents the spokes from ever having a chance to become the DR or BDR.

Configure neighbor statements on the hub. Since we're dealing with an NBMA network, the hub cannot dynamically discover its neighbors. Neighbor statements are not needed on the spokes. (They don't hurt anything, but they don't do anything, either.)

Finally, if your OSPF adjacencies do not form as expected, make sure to use your OSI model knowledge to approach the problem. The issue may actually be at Layer Two, with your Frame Relay configuration. If you don't use the "broadcast" option on your frame relay statements, OSPF hellos will not be transmitted successfully between potential neighbors. OSPF hellos are multicast, but the "broadcast" option for Frame Relay includes multicasts.

By paying special attention to these details, you're that much close to CCNA exam day success and earning your certification. I recommend that you get some experience with configuring OSPF hub-and-spoke before taking the CCNA exam, because it’s by actually performing tasks such as this that makes you supremely confident on CCNA test day.