To cross your CCNA examination and earn this coveted certification, you've got to grasp the seven layers of the OSI model and what every layer does. For those of you taking the 2-exam path, you can expect quite a couple of OSI mannequin questions on the Intro exam. In this seven-half sequence, we'll spend a while having a look at each of the OSI model layers, beginning with the Bodily layer. Usually, CCNA candidates ask if the OSI model has any practical makes use of for community administrators. I used to wonder the identical thing, and I can now let you know that the reply is certainly sure! The OSI mannequin isn't one thing you want to memorize and then forget about, as utilizing the OSI model gives you a structured strategy for troubleshooting. Each time a community system is not working properly, I always say to "begin at the physical layer". The Bodily layer is Layer One of the OSI model, and that is the place troubleshooting should all the time start. Is the system on? Is it properly related? If everything is okay at Layer One, you simply move as much as Layer Two, and continue on this structured style till the problem is identified. The Bodily layer is the layer at which bits are transmitted over the bodily media. There is no routing or switching happening at this layer. The data has been damaged down into extra manageable pieces until the info takes the form of ones and zeroes on the Physical layer. Although there is no routing or switching on the Bodily layer, CCNA candidates must be aware of a couple of network devices that work at Layer One. A repeater is a tool that regenerates an electrical sign, permitting the sign to travel longer distances without fading. (The method of an electrical signal steadily fading in energy over distance is "attenuation".) A hub is mainly a multiport repeater, and each of those devices are thought-about Physical layer devices. Ethernet and Token Ring both function on the Physical layer as well. Studying the OSI mannequin's Physical layer is not simply vital in your CCNA exam studies, it is the first step in any network troubleshooting. In spite of everything, your network's finish users are going to have a troublesome time sending print jobs to a printer that's turned off! OSPF is a serious subject in your CCNA exam, as effectively it should be. OSPF is a broadly-used WAN protocol, and you should learn the fundamentals before transferring on to extra complicated configurations. One such detail is the OSPF Router ID, or RID. The RID is the dotted decimal value by which different OSPF routers will establish a given OSPF router. There are some interesting defaults for this value, and a command it's best to know to hardcode the RID. You had also better know what has to occur for this command to take effect, so let's take an extra detailed have a look at the OSPF RID. On this instance, R1 has an adjacency with R2 and R3 over the 172.12.123.zero/24 body network. R1 is the hub, with R2 and R3 because the spokes. No different interfaces are OSPF-enabled on any of the routers. Running present ip ospf neighbor on R1, we see some uncommon values under "Neighbor ID", which is one other title for the OSPF RID. R1show ip ospf neighbor Neighbor ID Pri State Useless Time Address Interface 3.3.3.three 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 Discover the Neighbor ID of every remote tackle is the loopback address. How can that be in the event that they're not OSPF-enabled? When figuring out the Router ID (RID) of an OSPF-enabled router, OSPF will at all times use the numerically highest IP handle on the router's loopback interfaces, regardless of whether that loopback is OSPF-enabled. What if there isn't any loopback? OSPF will then use the numerically highest IP address of the bodily interfaces, regardless of whether or not that interface is OSPF-enabled. BOTTOM LINE: An interface doesn't have to be working OSPF to have its IP deal with used because the OSPF RID. The OSPF RID can be changed, nevertheless it requires a restart or to reinitialize the OSPF routing process. Use the router-id command to change the default RID of each router as proven, and clear the OSPF process to do so. R1conf t Enter configuration instructions, 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? [no]: 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: Process 1, Nbr 2.2.2.2 on Serial0 from 2WAY to DOWN, Neighbor Down: Interface down or detached After coming into the router-id command, the router console knowledgeable you that it's a must to reload the router or reset the OSPF processes for this to take effect. You enter the clear ip ospf process command to do this. Notice that whenever you're asked should you actually wish to do that, the immediate is "no"? That's because all of the OSPF adjacencies on this router will be lost and will have to begin the method again. That's OK on a practice rack, not good in a manufacturing network. Don't use that one at work. The OSPF RID just isn't a complicated concept, but the fact that an interface doesn't need to be OSPF-enabled in an effort to have its IP tackle act as the RID takes some getting used to. And remember - when the router or swap asks you a question and the prompted reply is "no", take one step again and make sure you actually want to do what you are
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To move the CCNA exam and earn this coveted certification, you must master the OSI model. In Half 1 of this seven-part series, learn all in regards to the Physical Layer from Chris Bryant, CCIE 12933.
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