Question:At my work, we have multiple networks set up across the world and I was looking over a listing of how our networks are subnetted. Here’s an example of how we’re set up on one of our networks:
Subnet id of 18.104.22.168
Range of .97 – .110
Broadcast of .111 on a subnet mask of /28 and the next subnet is
It does work, but it seems contradictory to what’s taught in the basic CCNA courses. Is this typical advanced subnetting that I just haven’t learned or read about?
Based on your IP subnets there, it looks like you’re running into the logical dilemma of mixing different types of subnets all next to each other.
If we used only /28s, we would expect:
22.214.171.124 (Giving hosts from .1 through .14 with a broadcast of .15.)
126.96.36.199 (Giving hosts from .17 through .30 with a broadcast of .31.)
188.8.131.52 (You get the point, so I won’t list them all!)
Now, if we had /27s, we would expect:
184.108.40.206 (Giving hosts from .1 through .30 with a broadcast of .31.)
220.127.116.11 (Giving hosts from .33 through .62 with a broadcast of .63.)
18.104.22.168 (Again, you get the point.)
I believe that’s the way we try to learn things in the CCNA/CCENT training. Mostly, we do that in order to be able to grasp the concepts of binary and how the router looks at things without running the risk of our heads exploding. What the router really looks at is pure binary groups, and as long as there’s no overlap, we’re good to go.
So notice the groupings we can have in the two lists. We all should’ve learned why we cannot have 22.214.171.124/27 as an example. While it makes perfect sense from a counting standpoint that this would allow from .81 to .110 as host addresses, it doesn’t work because of “crossing a bit boundary” (of .96) in the middle of it.
As long as we don’t cross any bit boundaries, though, we’re able to mix and match however we’d like to see things done and in any particular order (get ready for a little headache).
On a single router, or anywhere within my network, I could have:
126.96.36.199/26 (Gives us from .1 to .62 as hosts and .63 as broadcast.)
188.8.131.52/28 (Gives us from .65 to .78 as hosts and .79 as broadcast.
184.108.40.206/29 (Gives us from .81 to .86 as hosts and .87 as broadcast.)
220.127.116.11/30 (Gives us from .89 to .90 as hosts and .91 as broadcast.)
18.104.22.168/31 (Gives us a point to point link with .92 and .93 usable.)
22.214.171.124/31 (Gives us a point to point link with .94 and .95 usable.)
126.96.36.199/27 (Gives us from .97 to .126 as hosts and .127 as broadcast.)
188.8.131.52/25 (Gives us from .129 to .254 as hosts and .255 as broadcast.)
We could have interfaces/networks with all of those subnets, all at the same time, because each and every one of them is contained within a bit boundary based on its netmask.
The router works in a purely binary world. So as long as each separate thing doesn’t violate any rule (like overlap another interface, or cross a bit boundary), then life is good!
CCNA/CCENT training gives us the building blocks by which we can make everything possible, but it’s often not the only way that we have to do things. I know that some people get very irritated by that, but think about it a different way.When you were first learning the concepts of multiplication and division, would it have been nice for your teacher to make you divide two fractions? Or give some long, heinous math problem involving parentheses and stuff? We certainly know now that it’s possible, but at that point in time, our heads would have exploded!
As you progress in networking, and certainly as you get into the world of CCIE, you’ll get to discover all sorts of things that make you go “hmm.” But I hope this at least helps give you a better understanding of the way routers think about things!
With all the changes and advances in IP telephony, Cisco announced that it’s “enhancing” its Cisco Certified Voice Professional (CCVP) certification.
This certification requires five exams beyond the Cisco Certified Network Associate (CCNA) and focuses on integrating IP telephony solutions into underlying network architectures, as well as the ability to implement, configure and troubleshoot. Those who earn the certification are expected to know PSTN, VoIP, signaling protocols, voice gateways, gatekeepers and the Cisco Unified Border Element (CUBE).
Read the full article here.
Ambition without knowledge is like a boat on dry land. If you truly want to set sail and reach your goal of Cisco certification, you will need several keys to help you reach it. In order to gain the knowledge required to pass the Cisco Certified Network Associate (CCNA) exam, you have to familiarize yourself with various mediums of information.Before I discuss those mediums, however, I want to mention what is most important for success. What is truly required to obtain any Cisco certification is not a tangible item (such as the plethora of books, manuals, PDF documents, Web sites and countless other commercial tools that are out there). It is the psychological fuel that motivates you to follow through with those tools and implement them into your understanding of the exam.
A few years ago, I had lost my driver’s license and with it my job. After a few months of searching, I eventually found a new job, but getting there and back meant an hour and a half each day on the train and a two mile walk to the office from the station.
I quickly realized I needed a way of killing time until I got to work — there was no way I was going to spend 3+ hours of my day staring out of a train window at a blur and having no choice but to listen to someone else’s music blaring out of their earphones.
My network manager at the time had mentioned studying for my CCNA, as there were only a few people there with any real routing/switching knowledge. So I decided that day to go out and buy one of the many CCNA books that were available.
Read the full CCNA certification: When to study, what to read article.
CCNP certification continues to hold its value in the marketplace,and Cisco Press continues to put out high-quality study guides for the exams.
One of the newest entries in the line is Amir Ranjbar’s “CCNP ONT Official Exam Certification Guide.” Targeted at exam 642-845, this book walks through the objectives in an understandable manner and condenses what you need to know into a manageable package.
EIGRP is a major topic for your CCNA and CCNP studies, and one basic skill you’ll need to pass your Cisco certification exams is to identify situations where you need to enable or disable split horizon.
EIGRP commands tend to be a little different than those used with other protocols, so let’s take a look at how EIGRP and split horizon interoperate.
R1 is our hub router, with R2 and R3 as the spokes. There are no subinterfaces, and each router is advertising a single loopback network using its router number for each octet. R1 will see both R2 and R3’s loopback network, but the spokes will not have a route to the other spoke’s loopback. “show ip route eigrp” verifies this.
EIGRP runs split horizon by default, making it impossible for R1 to forward an advertisement to R2 regarding R3’s loopback. Likewise, R1 cannot advertise R2’s loopback address to R3. We could configure two subinterfaces on R1 to resolve this issue, but here we’re going to disable split horizon instead.
Note that disabling split horizon resulted in the EIGRP adjacencies being torn down. They came back up 20 – 35 seconds after being torn down according to the timestamps, but that’s a good detail to keep in mind!
The routing tables of each spoke should now show the loopback network configured on the remote spoke.
Be careful when disabling split horizon. In this scenario, R1 can and will advertise routes out Serial0 that were learned about on that interface in the first place, and that’s not always desirable. Split horizon is enabled by default for a reason, so be careful when disabling it!
Knowing when and how to create an OSPF virtual link is an essential skill for BSCI and CCNP exam success, not to mention how important it can be on your job!
As a CCNA and CCNP candidate, you know the theory of virtual links, so let’s take a look at how to configure a virtual link, as well as some real-world tips that many CCNA and CCNP study guides leave out!
In this configuration, no router with an interface in Area 4 has a physical interface in Area 0. This means a logical connection to Area 0, a virtual link, must be built.
Tn the following example, R1 and R3 are adjacent and both have interfaces in Area 0. R4 has an adjacency with R3 via Area 34, but R4 has no physical interface in Area 0 and is advertising its loopback 184.108.40.206 into OSPF. R1 doesn’t have the route to that loopback.
R1#show ip route ospf
220.127.116.11/32 is subnetted, 1 subnets
O 18.104.22.168 [110/11] via 10.1.1.5, 01:05:45, Ethernet0
172.23.0.0/27 is subnetted, 1 subnets
O IA 172.23.23.0 [110/74] via 22.214.171.124, 00:04:14, Serial0
126.96.36.199/32 is subnetted, 1 subnets
O 188.8.131.52 [110/11] via 10.1.1.5, 01:05:45, Ethernet0
To resolve this, a virtual link will be built between R3 and R4 through Area 34. The area through which the virtual link is built, the transit area, cannot be a stub area of any kind.
R4(config)#router ospf 1
R4(config-router)#area 34 virtual-link 184.108.40.206
R3(config)#router ospf 12d07h: %OSPF-4-ERRRCV: Received invalid packet: mismatch area ID, from backbone area must be virtual-link but not found from 172.23.23.4, Ethernet0
R3(config)#router ospf 1
R3(config-router)#area 34 virtual-link 220.127.116.11
2d07h: %OSPF-5-ADJCHG: Process 1, Nbr 18.104.22.168 on OSPF_VL0 from LOADING to FULL, Loading Done
A few details worth noting… the virtual link command uses the remote device’s RID, not necessarily the IP address on the interface that’s in the transit area. Also, don’t worry about that error message you see in the output from R3 that is normal and you’ll see it until you finish building the virtual link.
Always confirm the virtual link with show ip ospf virtual-link. If you’ve configured it correctly, the VL should come up in a matter of seconds.
R3#show ip ospf virtual-link
Virtual Link OSPF_VL0 to router 22.214.171.124 is up
Run as demand circuit
DoNotAge LSA allowed.
Transit area 34, via interface Ethernet0, Cost of using 10
Transmit Delay is 1 sec, State POINT_TO_POINT,
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:00
Adjacency State FULL (Hello suppressed)
Index 2/4, retransmission queue length 1, number of retransmission 1
First 0x2C8F8E(15)/0x0(0) Next 0x2C8F8E(15)/0x0(0)
Last retransmission scan length is 1, maximum is 1
Last retransmission scan time is 0 msec, maximum is 0 msec
Link State retransmission due in 3044 msec
Viirtual links are actually simple to configure, but for some reason they seem to intimidate people. It’s my experience that the error message highlighted in R3’s output above causes a lot of panic, but the only thing that message means is that you’re not finished configuring the virtual link yet.
There are three main misconfigurations that cause 99% of virtual link configuration issues:
- Using the wrong OSPF RID value
- Trying to use a stub area as the transit area
- Failure to configure link authentication on the virtual link when Area 0 is running authentication
That last one is the one that gets forgotten! A virtual link is really an extension of Area 0, and if Area 0 is running link authentication, the virtual link must be configured for it as well. Pay attention to the details. don’t panic when you see the error message on the second router you configure with the virtual link, and you’ll be ready for any virtual link situation on the job or in the CCNA / CCNP exam room!
To pass the BSCI exam and become a CCNP, you have to be aware of the proper use of passive interfaces.
You learned about passive interfaces in your CCNA studies, but here we’ll review the basic concept and clear up one misconception regarding passive interfaces and OSPF.
Configuring an interface as passive will still allow the interface to receive routing updates, but the interface will no longer transmit them.
While the command itself would make you think this command will be applied at the interface level, that is not the case. Below, we’ll configure ethernet0 as a RIP passive interface.
Ethernet0 will no longer send RIP routing updates, but will accept them.
The passive interface concept is clear enough with RIP, IGRP, and EIGRP – all rotocols that send routing update packets. But OSPF doesn’t send routing update ackets – OSPF sends link state advertisements.
It’s the inability of the passive interface command to stop LSAs that lead many o think that passive interfaces cannot be used with OSPF.
Even though OSPF does not sent “routing updates” in the form that RIP, IGRP, and IGRP do, you can still configure an OSPF-enabled interface as passive in order o prevent OSPF traffic from exiting or entering that interface.
No OSPF adjacency can be formed if one of the interfaces involved is a passive nterface, and if you configure an OSPF-enabled interface as passive where an djacency already exists, the adjacency will drop almost immediately.
Let’s see that in action. R1 and R2 have an existing OSPF adjacency over their thernet interfaces. In an effort to reduce routing traffic, R1’s e0 interface s configured as passive. The adjacency drops right away.
R1(config)#router ospf 1
18:31:11: %OSPF-5-ADJCHG: Process 1, Nbr 126.96.36.199 on Ethernet0 from FULL to DOWN,
Neighbor Down: Interface down or detached
Knowing how to use the passive interface command is a vital part of being a CNP, and of being a master networker. Good luck to you in both of these pursuits!
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 188.8.131.52 name R2 broadcast 5555555
The 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 CISCO
Must set encapsulation to PPP before using PPP subcommands
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)#ppp authentication pap
R1(config-if)#ppp pap sent-username R1 password CISCO
But 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-ni
R1#show isdn status
Global 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
CCNA exam success depends on mastering the fundamentals, and two important fundamentals are knowing exactly what the terms “collision domain” and “broadcast domain” mean.
In this free Cisco tutorial, we’ll take a look at the term “collision domain” and how a collision domain is defined.
A collision domain is an area in which a collision can occur. Fair enough, but what “collision” are we talking about here?
We’re talking about collisions that occur on CSMA/CD segments, or Carrier Sense Multiple Access with Collision Detection.
If two hosts on an Ethernet segment transmit data at exactly the same time, the data from the two hosts will collide on the shared segment.
CSMA/CD exists to lessen the chances of this happening, but collisions can still occur. To lessen the chances of collisions occurring, we may decide to create multiple, smaller collision domains.
Lets say we have four hosts on a single Ethernet segment. The entire segment is a collision domain; any data sent by one of the hosts can collide with data sent by any of the other hosts. We have one collision domain containing four devices.
To create smaller collision domains, we’ll need to introduce some type of networking device into this example.
Hubs and repeaters have their place as far as extending the reach of a network segment and cutting down on attenuation, but these OSI Layer One devices do nothing to define collision domains. We could connect each host into a separate port on a hub (a hub is basically a multiport repeater) and we’d still have one single collision domain with four hosts in it.
The most common and most effective way to create multiple collision domains is to use a switch.
If we connect each of these four hosts to their own separate switch port, we would now have four separate collision domains, each with one host; each switch port actually acts as a single collision domain, making collisions between these four hosts impossible.
Passing the CCNA is all about knowing the details of how things work, and knowing CSMA/CD theory and how to define collision domains is one of the many details youve got to master.
In the next part of this CCNA tutorial, we’ll take a look at broadcast domains, and how defining broadcast domains in the right places can dramatically cut down on unnecessary traffic on your network.