To pass Cisco certification exams, you have to know more than one way to do things. Learn another method of configuring ISDN callback from Chris Bryant, CCIE #12933.

Cisco CCNA / CCNP Certification Exam: Caller ID Screening And Callback

Cisco CCNA / CCNP Certification Exam:  Caller ID Screening And CallbackAs a CCNA and/or CCNP candidate, you've got to be able to spot situations where Cisco router features can save your client money and time. For example, if a spoke router is calling a hub router and the toll charges at the spoke site are higher than that of the hub router, having the hub router hang up initially and then call the spoke router back can save the client money (and make you look good!) A popular method of doing this is using PPP callback, but as we all know, it's a good idea to know more than one way to do things in Cisco World! A lesser-known but still effective method of callback is Caller ID Screening & Callback. Before we look at the callback feature, though, we need to know what Caller ID Screening is in the first place!This feature is often referred to simply as "Caller ID", which can be a little misleading if you've never seen this service in operation before. To most of us, Caller ID is a phone service that displays the source phone number of an incoming call. Caller ID Screening has a different meaning, though. Caller ID Screening on a Cisco router is really another kind of password - it defines the phone numbers that are allowed to call the router. The list of acceptable source phone numbers is created with the isdn caller command. Luckily for us, this command allows the use of x to specify a wildcard number. The command isdn caller 555xxxx results in calls being accepted from any 7-digit phone number beginning with 555, and rejected in all other cases. We'll configure R2 to do just that and then send a ping from R1 to R2. To see the results of the Caller ID Screening, debug dialer will be run on R1 before sending the ping. Ive edited this output, since the output you see here will be repeated fire times once for each ping packet.R2(config-if)#isdn caller 555xxxxR1#debug dialerDial on demand events debugging is onR1#ping escape sequence to abort.Sending 5, 100-byte ICMP Echos to, timeout is 2 seconds:03:30:25: BR0 DDR: Dialing cause ip (s=, d= BR0 DDR: Attempting to dial 8358662.Success rate is 0 percent (0/5)R1 doesn't give us any hints as to what the problem is, but we can see that the pings definitely aren't going through. On R2, show dialer displays the number of screened calls.R2#show dialerBRI0 - dialer type = ISDNDial String Successes Failures Last DNIS Last status8358661 1 0 00:03:16 successful7 incoming call(s) have been screened.0 incoming call(s) rejected for callback.The callback option mentioned in the last line shown above enables the router to reject a phone call, and then call that router back seconds later.R2 will now be configured to initially hang up on R1, and then call R1 back. R2(config-if)#isdn caller 8358661 callbackR1 will now ping R2. The pings aren't returned, but seconds later R2 calls R1 back.R1#ping rate is 0 percent (0/5)R1#03:48:12: BRI0: wait for isdn carrier timeout, call id=0x8023R1#03:48:18: %LINK-3-UPDOWN: Interface BRI0:1, changed state to upR1#03:48:18: BR0:1 DDR: dialer protocol upR1#03:48:19: %LINEPROTO-5-UPDOWN: Line protocol on Interface BRI0:1, changed state to upR1#03:48:24: %ISDN-6-CONNECT: Interface BRI0:1 is now connected to 8358662 R2show dialer on R2 shows the reason for the call to R1 is a callback return call.R2#show dialerBRI0 - dialer type = ISDNDial String Successes Failures Last DNIS Last status8358661 3 0 00:00:48 successful7 incoming call(s) have been screened.10 incoming call(s) rejected for callback.BRI0:1 - dialer type = ISDNIdle timer (120 secs), Fast idle timer (20 secs)Wait for carrier (30 secs), Re-enable (15 secs)Dialer state is data link layer upDial reason: Callback return callTime until disconnect 71 secsConnected to 8358661 (R1)The drawback to Caller ID Callback is that not all telco switches support it, so if you have the choice between this and PPP Callback, you're probably better off with PPP Callback. However, it's always a good idea to know more than one way to get things done with Cisco!

Cisco CCNA Certification Exam Tutorial: Prefix Notation

Cisco CCNA Certification Exam Tutorial:  Prefix Notation

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: /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!

CCNP Certification / BCMSN Exam Tutorial: HSRP MAC Addresses And Timers

CCNP Certification / BCMSN Exam Tutorial:  HSRP MAC Addresses And Timers

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 configured Active router is, 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 configured Active router is local Standby router is 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 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!

Cisco CCNA Certification: Error Detection vs. Error Recovery

Cisco CCNA Certification:  Error Detection vs. Error Recovery

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!

Cisco CCNA Certification Exam Tutorial: Keep Your Most Important Appointment

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!

CCNP Certification / BCMSN Exam Tutorial: Server Load Balancing (SLB)

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 hosts will seek to communicate with the server at, 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 to the server farm:MLS(config)# ip slb serverfarm ServFarmMLS(config-slb-sfarm)# 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 inserviceFrom the top down, the vserver was named VIRTUAL_SERVER, which represents the server farm ServFarm. The virtual server is assigned the IP address, 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 /24 to connect to the virtual server.MLS(config-slb-vserver)# client 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.


To pass Cisco certification exams, you have to know more than one way to do things. Learn another method of configuring ISDN callback from Chris Bryant, CCIE #12933.