OSPF Hub-And-Spoke Details You Must Know!
CCNA exam success depends greatly on knowing the details, and if theres one protocol that has a lot of details, its OSPF!
This is true particularly of hub-and-spoke networks, so in this CCNA OSPF tutorial well 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 elections deciding value is the OSPF interface priority, and the default value is 1. Its not enough to set the hubs 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-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
Its 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 dont 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 its only needed on one side. It doesnt hurt anything to put it on both sides, though.)
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 126.96.36.199 name R1 broadcast 5551111
R2(config-if)#frame map ip 188.8.131.52 221 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!
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage (www.thebryantadvantage.com), home of FREE CCNA and CCNP tutorials and daily exam questions, as well as The Ultimate CCNA and CCNP Study Packages.
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