Your CCNA studies are going to include quite a bit of information about switches, and for good reason. if you don't understand basic switching theory, you can't configure and troubleshoot Cisco switches, either on the CCNA exam or in the real world. That goes double for trunking!Trunking is simply enabling two or more switches to communicate and send frames to each other for transmission to remote hosts. There are two major trunking protocols that we need to know the details of for exam success and real-world success, but before we get to the protocols, let's discuss the cables we need.Connecting two Cisco switches requires a crossover cable. As you know, there are eight wires inside an ethernet cable. In a crossover cable, four of the cables "cross over" from one pin to another. For many newer Cisco switches, all you need to do to create a trunk is connect the switches with a crossover cable. For instance, 2950 switches dynamically trunk once you connect them with the right cable. If you use the wrong cable, you'll be there a while!There are two different trunking protocols in use on today's Cisco switches, ISL and IEEE 802.1Q, generally referred to as "dot1q". There are three main differences between the two. First, ISL is a Cisco-proprietary trunking protocol, where dot1q is the industry standard. (Those of you new to Cisco testing should get used to the phrases "Cisco-proprietary" and "industry standard".) If you're working in a multivendor environment, ISL may not be a good choice. And even though ISL is Cisco's own trunking protocol, some Cisco switches run only dot1q.ISL also encapsulates the entire frame, increasing the network overhead. Dot1q only places a header on the frame, and in some circumstances, doesn't even do that. There is much less overhead with dot1q as compared to ISL. That leads to the third major difference, the way the protocols work with the native vlan.The native vlan is simply the default vlan that switch ports are placed into if they are not expressly placed into another vlan. On Cisco switches, the native vlan is vlan 1. (This can be changed.) If dot1q is running, frames that are going to be sent across the trunk line don't even have a header placed on them; the remote switch will assume that any frame that has no header is destined for the native vlan.The problem with ISL is that is doesn't understand what a native vlan is. Every single frame will be encapsulated, regardless of the vlan it's destined for.Switching theory is a big part of your CCNA studies, and it can seem overwhelming at first. Just break your studies down into smaller, more manageable parts, and soon you'll see the magic letters "CCNA" behind your name!
CCNA and CCNP candidates who have their own Cisco home labs often email me about an odd situation that occurs when they erase a switch's configuration. Their startup configuration is gone, as they expect, but the VLAN and VTP information is still there!Sounds strange, doesn't it? Let's look at an example. On SW1, we run show vlan brief and see in this abbreviated output that there are three additional vlans in use:SW1#show vlan br10 VLAN0010 active20 VLAN0020 active30 VLAN0030 activeWe want to totally erase the router's startup configuration, so we use the write erase command, confirm it, and reload without saving the running config:SW1#write eraseErasing the nvram filesystem will remove all configuration files! Continue? [confirm][OK]Erase of nvram: complete00:06:00: %SYS-7-NV_BLOCK_INIT: Initalized the geometry of nvramSW1#reloadSystem configuration has been modified. Save? [yes/no]: nProceed with reload? [confirm]The router reloads, and after exiting setup mode, we run show vlan brief again. And even though the startup configuration was erased, the vlans are still there!Switch#show vlan br10 VLAN0010 active20 VLAN0020 active30 VLAN0030 activeThe reason is that this vlan and VTP information is actually kept in the VLAN.DAT file in Flash memory, and the contents of Flash are kept on a reload. The file has to be deleted manually.There's a little trick to deleting this file. The switch will prompt you twice to ask if you really want to get rid of this file. Don't type "y" or "yes"; just accept the defaults by hitting the return key. If you type "y", the router attempts to delete a file named "y", as shown here:Switch#delete vlan.datDelete filename [vlan.dat]? yDelete flash:y? [confirm]%Error deleting flash:y (No such file or directory)Switch#delete vlan.datDelete filename [vlan.dat]?Delete flash:vlan.dat? [confirm]Switch#The best way to prepare for CCNA and CCNP exam success is by working on real Cisco equipment, and by performing lab tasks over and over. Repetition is the mother of skill, and by truly erasing your VLAN and VTP information by deleting the vlan.dat file from Flash, you'll be building your Cisco skills to the point where your CCNA and CCNP exam success is a certainty.
RIP isn't exactly the most complex routing protocol on the CCNA exam, but that makes it easy to overlook some of the important details you must keep in mind in order to pass the exam! To help you review for the exam, here are just a few of those details!RIPs default behavior is to send version 1 updates, but to accept both version 1 and 2 routing updates.R2(config)#router ripR2(config-router)#net 172.16.0.0R2(config-router)#^ZR2#show ip protocolsRouting Protocol is "rip" Sending updates every 30 seconds, next due in 6 seconds Invalid after 180 seconds, hold down 180, flushed after 240 Outgoing update filter list for all interfaces is Incoming update filter list for all interfaces is Redistributing: rip Default version control: send version 1, receive any version Interface Send Recv Key-chain Serial0 1 1 2By default, RIP v2 autosummarizes routing updates sent across classful network boundaries. To disable this behavior, run no auto-summary under the RIP process.R1#conf tR1(config)#router ripR1(config-router)#version 2R1(config-router)#no auto-summaryYou do not specify a subnet mask or wildcard mask when configuring RIP just the classful network, even if youre running RIP v2.R1#conf tEnter configuration commands, one per line. End with CNTL/Z.R1(config)#router ripR1(config-router)#version 2R1(config-router)#no auto-summaryR1(config-router)#network 22.214.171.124 ?
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CCNA exam success depends on mastering many technologies that are new to you, and few exam topics have more details than ISDN. ISDN isn't just for your CCNA exam studies, though. While ISDN is dismissed by many, the fact is that there are many small and mid-size networks out there that use ISDN as their backup to frame relay. Some of these companies have spoke networks that use ISDN to connect to their hub as well, so it's a great idea to know ISDN configuration and troubleshooting for your real-world career as well as passing the CCNA. With that in mind, let's take a look at five common ISDN errors and how to avoid them.With dialer map statements, remember that the phone number you put in the dialer map is the phone number of the remote router, not the local one. Look at it this way - if you want to call a friend on your cell, you don't pick up your cell and dial your own number!Speaking of dialer map statements, don't forget the all-important broadcast option at the end of the command:R1(config-if)#dialer map ip 126.96.36.199 name R2 broadcast 5555555The router will accept that command without the "broadcast" option, but routing protocol updates and hellos would not be able to travel across the line. (This command is also needed in frame relay map statements to allow broadcasts and multicasts to be transmitted.)PAP is PPP's clear-text authentication scheme, and clear text is a really bad idea. But if you do have to configure it, don't forget that PAP requires additional configuration -the ppp pap sent-username command.R1(config-if)#ppp pap sent-username R1 password CISCOMust set encapsulation to PPP before using PPP subcommandsR1(config-if)#The error message we got while configuring the sent-username command is another important reminder - by default, a BRI line is running HDLC, not PPP. Since HDLC doesn't allow us to use either PAP or CHAP, we'll need to set the link to PPP with the encapsulation ppp command.R1(config-if)#encapsulation pppR1(config-if)#ppp authentication papR1(config-if)#ppp pap sent-username R1 password CISCOBut before we configure any of this information, we should configure the ISDN switch-type. Why? Because without the switch-type configuration, it doesn't matter that we avoid the other four errors - the line will not come up. Configure the switch-type with the "isdn switch-type" command, and then verify it with "show isdn status".R1(config)#isdn switch-type basic-niR1#show isdn statusGlobal ISDN Switchtype = basic-ni (output of this command cut here for clarity)If you forget this part of the configuration, the output of show isdn status wastes no time in reminding you!R1#show isdn status **** No Global ISDN Switchtype currently defined ****ISDN is an important part of your CCNA studies, and this knowledge still comes in handy in production networks as well. Keep studying, notice the details, run those debugs, and you'll be a CCNA before you know it!