To cross your CCNA exam and acquire this coveted certification, you've got to grip the seven layers of the OSI mannequin and what each layer does. For those of you catching the two-examination route, you possibly can calculate on quite a couple of OSI prototype answers on the Intro exam. In this seven-part series, we'll spend a while seeing at every of the OSI model layers, beginning with the Bodily layer.
Often, CCNA nominees ask if the OSI mannequin has any advisable uses for network supervisors. I used to prodigy the same factor, and I can immediately let you know thatthe respond is definitely yes!
The OSI model is not someone you need to remember later which forget approximately, as using the OSI mannequin provides you a structured means for troubleshooting. At any time while a community machine is not working properly, I all the time say to "start at the physical layer". The Bodily layer is Layer One of many OSI mannequin, and that is where troubleshooting should all start. Is the machine on? Is it correctly related? If everything is ok at Layer One, you simply convey up to Layer Two, and proceed on this structured manner till the problem is identified.
The Physical layer is the layer at which bits are transmitted over the bodily media. There isn't a routing or switching occurring at this layer. The info has been damaged down into more manageable items till the info takes the form of ones and zeroes on the Physical layer.
Though there is no routing or switching at the Physical layer, CCNA candidates ought to be adapted to a few network devices that work at Layer One. A repeater is a apparatus that regenerates an electrical sign, granting the whistle to junket longer distances without fading. (The process of an electrical semaphore step by step fading in strength over distance is "diminution".) A hub is basically a multiport repeater, and both of these units are thought of Bodily layer devices. Ethernet and Token Ring both feature on the Bodily layer as well.
Learning the OSI model's Bodily membrane isn't simply needful in your CCNA exam research, it is the first tread in anyone community troubleshooting. After all, your network's end customers are going apt have a tough period sending publish jobs apt a printer that namely rotated off!
OSPF is a serious heading on your CCNA examination, as well it should be. OSPF is a widely-used WAN protocol, and that you must be taught the fundamentals earlier than moving on to extra complicated configurations. One such detail is the OSPF Router ID, or RID.
The RID is the dotted decimal value by which additional OSPF routers will resolve a given OSPF router. There are some fascinating defaults for this value, and a command it is best to know to hardcode the RID. You had yet better understand what has to occur for this command to take effect, so let's take an accessory detailed look at the OSPF RID.
On this example, R1 has one adjacency with R2 and R3 over the 172.12.123.zero/24 skeleton network. R1 is the hub, with R2 and R3 as the spokes. No assorted interfaces are OSPF-enabled on any of the routers. Working present ip ospf adjoin on R1, we discern some unusual values below "Neighbor ID", which is different appoint for the OSPF RID.
R1show ip ospf neighbor
Neighbor ID Pri State Dead Time Deal with Interface
3.3.3.3 0 FULL/DROTHER 00:01:57 172.12.123.3 Serial0
2.2.2.2 zero FULL/DROTHER 00:01:57 172.12.123.2 Serial0
Notice the Neighbor ID of each distant tackle is the loopback address. How can that be whether they're not OSPF-enabled?
When figuring out the Router ID (RID) of an OSPF-enabled router, OSPF will all the time use the numerically highest IP address on the router's loopback interfaces, regardless of if that loopback is OSPF-enabled.
What if there is no loopback? OSPF will then use the numerically highest IP address of the physical interfaces, no material whether that interface is OSPF-enabled.
BOTTOM LINE: An interface does not must be working OSPF to have its IP tackle used because the OSPF RID.
The OSPF RID will be changed, but it surely requires a restart or to reinitialize the OSPF routing process. Use the router-id command to alter the default RID of every router as shown, and clear the OSPF process to do so.
R1conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)router ospf 1
R1(config-router)router-id 11.11.11.11
Reload or use "clear ip ospf process" command, for this to take effect
R1clear ip ospf course of
Reset ALL OSPF processes? [not]: yes
1d05h: %OSPF-5-ADJCHG: Course of 1, Nbr 3.3.3.3 on Serial0 from 2WAY to
DOWN, Neighbor Down: Interface down or indifferent
1d05h: %OSPF-5-ADJCHG: Course of 1, Nbr 2.2.2.2 on Serial0 from 2WAY to
DOWN, Neighbor Down: Interface down or detached
After entering the router-id bid, the router console knowledgeable you that it's a have to to reload the router or reset the OSPF processes as this to take effect. You enter the remove ip ospf course of mandate to do this. Notice that when you're requested for those who really need to do that, the hint is "no"? That's as a result of all of the OSPF adjacencies above this router might be lost and ought begin the process again. That's OKAY on a practice rack, not agreeable in a product web. Don't use that 1 at work.
The OSPF RID shouldn't be a complicated conception, but the fact that an interface does not need to be OSPF-enabled as a way to have its IP tackle perform because the RID takes some getting accustom to. And reserve in idea - when the router or change asks you a question and the prompted reply is "no", take one step again and assure you really wish to do what you are
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