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CCNA Certification Training: Configuring OSPF Hub-And-Spoke

By Chris Bryant, CCIE #12933

OSPF is probably the toughest protocol you'll have to deal with in your CCNA studies. OSPF hub-and-spoke networks are common in production networks, so Cisco is going to make sure you understand the details that have to be attended to in order to correctly configure a hub-and-spoke OSPF network.

CCNA Certification OSPF Hub And Spoke

This is a typical OSPF hub-and-spoke network. R1 is serving as the hub, with R2 and R3 as the spokes. R2 and R3 can communicate, but any traffic going from one spoke to another must go through the hub. Since R1 will not forward broadcasts or multicasts between the spokes - including OSPF Hellos - there are some special considerations to take into account when configuring this OSPF network type.

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.

R2(config)#int s0
R2(config-if)#ip ospf priority 0

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.)

R1(config)#router ospf 1
R1(config-router)#neighbor 172.12.123.2
R1(config-router)#neighbor 172.12.123.3

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.

R1(config)#interface serial0
R1(config-if)#frame map ip 172.12.123.2 122 broadcast
R1(config-if)#frame map ip 172.12.123.3 123 broadcast

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

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