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!
Showing posts with label circuit. Show all posts
Showing posts with label circuit. Show all posts
Thursday, December 25, 2008
Wednesday, December 24, 2008
Cisco CCNA Certification: The (Many) Different Kinds Of Switching
When you're studying for your CCNA exam, whether you're taking the Intro-ICND path or the single-exam path, you're quickly introduced to the fact that switching occurs at Layer 2 of the OSI model. No problem there, but then other terms involving switching are thrown in, and some of them can be more than a little confusing. What is "cell switching"? What is "circuit switching"? Most confusing of all, how can you have "packet switching"? Packets are found at Layer 3, but switching occurs at Layer 2. How can packets be switched?
Relax! As you'll see in this article, the terms aren't that hard to keep straight. Packet switching, for example, describes a protocol that divides a message into packets before they're sent. The packets are then sent individually, and may take different paths to the same destination. Once the packets arrive at the final destination, they are reassembled.
Frame switching follows the same process, but at a different layer of the OSI model. When the protocol runs at Layer 2 rather than Layer 3, the process is referred to as frame switching.
Cell switching also does much the same thing, but as the name implies, the device in use is a cell switch. Cell-switched packets are fixed in length. ATM is a popular cell-switching technology.
The process of circuit switching is just a bit different, in that the process of setting up the circuit itself is part of the process. The channel is set up between two parties, data is transmitted, and the channel is then torn down. The circuit-switching technology most familiar to CCNA candidates is ISDN.
Don't let these terms confuse you. The four different terms are describing much the same process. The main difference is that they are occurring at different levels of the OSI model, and using a different transport method to get the data where it needs to go.
Relax! As you'll see in this article, the terms aren't that hard to keep straight. Packet switching, for example, describes a protocol that divides a message into packets before they're sent. The packets are then sent individually, and may take different paths to the same destination. Once the packets arrive at the final destination, they are reassembled.
Frame switching follows the same process, but at a different layer of the OSI model. When the protocol runs at Layer 2 rather than Layer 3, the process is referred to as frame switching.
Cell switching also does much the same thing, but as the name implies, the device in use is a cell switch. Cell-switched packets are fixed in length. ATM is a popular cell-switching technology.
The process of circuit switching is just a bit different, in that the process of setting up the circuit itself is part of the process. The channel is set up between two parties, data is transmitted, and the channel is then torn down. The circuit-switching technology most familiar to CCNA candidates is ISDN.
Don't let these terms confuse you. The four different terms are describing much the same process. The main difference is that they are occurring at different levels of the OSI model, and using a different transport method to get the data where it needs to go.
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