When you're preparing to pass the CCNA exam and earn this coveted Cisco certification, you've got to be totally prepared for the many kinds of binary and subnetting questions Cisco may throw at you. You also have to be familiar with the different manners in which a subnet mask can be expressed, as in the following:255.255.255.0 /24Believe it or not, those two values are exactly the same. The first mask is written out in the more familiar dotted decimal format, and you know by looking at those first three octets that every bit is set to "1", since the maximum value of such an octet is 255.The second value represents the exact same mask, only this value is expressed in prefix notation. This particular value would be pronounced "slash twenty-four", and the 24 represents the number of consecutive ones that are set in the subnet mask.Those of us who hate to type numbers are particularly appreciative of this, since it means you'll have to type a lot less numbers to represent a subnet mask. In addition, it's a lot easier to discuss masks in prefix notation than dotted decimal. ("I thought about using a two-fifty-five two-fifty-five two-fifty-five zero mask ,but then decided to use a two-fifty-five two-fifty-five two-fifty-five one-twenty-eight mask...")Be sure you're comfortable with prefix notation before taking your CCNA exam. As with Cisco documentation, you'll most likely see masks expressed in both dotted decimal and prefix notation, and you've got to be ready to use the both as well!
To earn your CCNP certification and pass the BCMSN exam, you've got to know what HSRP does and the many configurable options. While the operation of HSRP is quite simple (and covered in a previous tutorial), you also need to know how HSRP arrives at the MAC address for the virtual router - as well as how to configure a new MAC for this virtual router. This puts us in the unusual position of creating a physical address for a router that doesn't exist!The output of show standby for a two-router HSRP configuration is shown below.R2#show standbyEthernet0 - Group 5 Local state is Standby, priority 100 Hellotime 3 sec, holdtime 10 sec Next hello sent in 0.776 Virtual IP address is 126.96.36.199 configured Active router is 188.8.131.52, priority 100 expires in 9.568 Standby router is local 1 state changes, last state change 00:00:22R3#show standbyEthernet0 - Group 5 Local state is Active, priority 100 Hellotime 3 sec, holdtime 10 sec Next hello sent in 2.592 Virtual IP address is 184.108.40.206 configured Active router is local Standby router is 220.127.116.11 expires in 8.020 Virtual mac address is 0000.0c07.ac05 2 state changes, last state change 00:02:08R3 is in Active state, while R2 is in Standby. The hosts are using the 18.104.22.168 address as their gateway, but R3 is actually handling the workload. R2 will take over if R3 becomes unavailable.An IP address was assigned to the virtual router during the HSRP configuration process, but not a MAC address. However, there is a MAC address under the show standby output on R3, the active router. How did the HSRP process arrive at a MAC of 00-00-0c-07-ac-05?Well, most of the work is already done before the configuration is even begun. The MAC address 00-00-0c-07-ac-xx is reserved for HSRP, and xx is the group number in hexadecimal. That's a good skill to have for the exam, so make sure you're comfortable with hex conversions. The group number is 5, which is expressed as 05 with a two-bit hex character. If the group number had been 17, we'd see 11 at the end of the MAC address - one unit of 16, one unit of 1.The output of the show standby command also tells us that the HSRP speakers are sending Hellos every 3 seconds, with a 10-second holdtime. These values can be changed with the standby command, but HSRP speakers in the same group should have the same timers. You can even tie down the hello time to the millisecond, but it's doubtful you'll ever need to do that.R3(config-if)#standby 5 timers ? <1-254> Hello interval in seconds msec Specify hello interval in millisecondsR3(config-if)#standby 5 timers 4 ? <5-255> Hold time in secondsR3(config-if)#standby 5 timers 4 12Another important HSRP skill is knowing how to change the Active router assignment. I'll show you how to do that, and how to configure HSRP interface tracking, in the next part of my CCNP / BCMSN exam tutorial!
Passing the CCNA, Intro, and ICND exam is all about knowing and noticing the details. (Which makes perfect sense, since becoming a master networking administrator or engineer is also about noticing the details!) One such detail knows the difference between error detection and error recovery. While the terms are sometimes used interchangeably, they are not the same thing. Error detection is just that - error detection only. Two common error detection methods are found at the Data Link layer of the OSI model, the FCS (Frame Check Sequence) and CRC (Cyclical Redundancy Check). A mathematical equation is run against the data in the frame, and the result is sent along with the data. The receiver runs the equation again, but this time. If the result is the same, the frame is considered valid; if the result is different, the frame is considered corrupt and is discarded.Note that the FCS and CRC do nothing in regards to retransmission. They are strictly error detection schemes.For an example of error recovery, we look to the Transport layer, where TCP runs. TCP performs reliable delivery, and the reason we call it "reliable" is that TCP uses sequence numbers to detect missing segments. If the sender determines from the sequence numbers that the remote host did not receive transmitted segments, the sender will retransmit the missing segments.The key to keeping the terms straight in your head is to remember that while both error detection and error recovery both detect problems, only error recovery does anything about it. It's also worth reading an exam question twice when you see either term!
Imagine this. You have an appointment with a client to work on a server or router install. A few minutes before you're scheduled to be there, you decide there's something really good on TV you'd like to watch. Or you decide to go to the gym, or play a game, or do anything else except go see the client. Even if you weren't going to get fired for not showing up, it's certainly unfair to the client. You've got a professional obligation, and you should be there on time. Now, what's this got to do with you becoming a CCNA or CCNP? Plenty. Because when it comes to your study time, you're the client. You owe it to yourself to show up. You would never blow off an appointment to meet a client to get some important work done. First, though, you have to make that appointment with yourself! Schedule your CCNA / CCNP study time, and keep that appointment as you would with a client. Turn off the TV, your cell, your iPod, and everything else electronic that you carry around. Believe it or not, the world can survive with being in contact with you for an hour or so! You might even like it!Getting certified isn't about how many hours, days, or weeks you spend studying. It's about how much quality time you put in. Be honest with yourself and realize that you're better off with 45 minutes of uninterrupted study as you would be with three hours of constantly interrupted study.Don't blow off an appointment to yourself, either. Schedule the time, be there on time, get your study done, and you're one step closer to your CCNA and CCNP!
When you're working on your BCMSN exam on your way to CCNP certification, you'll read at length about how Cisco routers and multilayer switches can work to provide router redundancy - but there's another helpful service, Server Load Balancing, that does the same for servers. While HSRP, VRRP, and CLBP all represent multiple physical routers to hosts as a single virtual router, SLB represents multiple physical servers to hosts as a single virtual server.In the following example, three physical servers have been placed into the SRB group ServFarm. They're represented to the hosts as the virtual server 22.214.171.124.The hosts will seek to communicate with the server at 126.96.36.199, not knowing that they're actually communicating with the routers in ServFarm. This allows quick cutover if one of the physical servers goes down, and also serves to hide the actual IP addresses of the servers in ServFarm.The basic operations of SLB involves creating the server farm, followed by creating the virtual server. We'll first add 188.8.131.52 to the server farm:MLS(config)# ip slb serverfarm ServFarmMLS(config-slb-sfarm)# real 184.108.40.206MLS(config-slb-real)# inserviceThe first command creates the server farm, with the real command specifying the IP address of the real server. The inservice command is required by SLB to consider the server as ready to handle the server farm's workload. The real and inservice commands should be repeated for each server in the server farm. To create the virtual server:MLS(config)# ip slb vserver VIRTUAL_SERVERMLS(config-slb-vserver)# serverfarm ServFarmMLS(config-slb-vserver)# virtual 220.127.116.11MLS(config-slb-vserver)# inserviceFrom the top down, the vserver was named VIRTUAL_SERVER, which represents the server farm ServFarm. The virtual server is assigned the IP address 18.104.22.168, and connections are allowed once the inservice command is applied.You may also want to control which of your network hosts can connect to the virtual server. If hosts or subnets are named with the client command, those will be the only clients that can connect to the virtual server. Note that this command uses wildcard masks. The following configuration would allow only the hosts on the subnet 22.214.171.124 /24 to connect to the virtual server.MLS(config-slb-vserver)# client 126.96.36.199 0.0.0.255SLB is the server end's answer to HSRP, VRRP, and GLBP - but you still need to know it to become a CCNP! Knowing redundancy strategies and protocols is vital in today's networks, so make sure you're comfortable with SLB before taking on the exam.