Cisco Certification: Introduction To ISDN

From the CCNA to the CCIE, ISDN is one of the most important technolgies you'll work with. It's also very common in the field ISDN is frequently used as a backup connection in case an organization's Frame Relay connections go down. Therefore, it's important to know ISDN basics not only for your particular exam, but for job success.

ISDN is used between two Cisco routers that have BRI or PRI interfaces. Basically, with ISDN one of the routers places a phone call to the other router. It is vital to understand not only what causes one router to dial another, but what makes the link go down.

Why? Since ISDN is basically a phone call from one router to another, you're getting billed for that phone call -- by the minute. If one of your routers dials another, and never hangs up, the connection can theoretically last for days or weeks. The network manager then receives an astronomical phone bill, which leads to bad things for everyone involved!

Cisco routers use the concept of interesting traffic to decide when one router should call another. By default, there is no interesting traffic, so if you don't define any, the routers will never call each other.

Interesting traffic is defined with the dialer-list command. This command offers many options, so you can tie interesting traffic down not only to what protocols can bring the link up, but what the source, destination, or even port number must be for the line to come up.

One common misconception occurs once that link is up. Interesting traffic is required to bring the link up, but by default, any traffic can then cross the ISDN link.

What makes the link come down? Again, the concept of interesting traffic is used. Cisco routers have an idle-timeout setting for their dialup interfaces. If interesting traffic does not cross the link for the amount of time specified by the idle-timeout, the link comes down.

To summarize: Interesting traffic brings the link up by default, any traffic can cross the link once it's up a lack of interesting traffic is what brings the link down.

Just as important is knowing what keeps the link up once it is dialed. Why? Because ISDN acts as a phone call between two routers, and it's billed that way to your client. The two routers that are connected by this phone call may be located in different area codes, so now we're talking about a long distance phone call.



If your ISDN link does not have a reason to disconnect, the connection could theoretically last for days or weeks before someone realizes what's going on. This is particularly true when the ISDN link is used as a backup for another connection type, as is commonly the case with Frame Relay. When the Frame Relay goes down, the backup ISDN link comes up when the Frame Relay link comes back not billed for all that time.<




To understand why an ISDN link stays up when it's not needed, we have to understand why it stays up period. Cisco's ISDN interfaces use the idle-timeout to determine when an ISDN link should be torn down. By default, this value is two minutes, and it also uses the concept of interesting traffic.



Once interesting traffic brings the link up, by default all traffic can cross the link. However, only interesting traffic resets the idle-timeout. If no interesting traffic crosses the link for two minutes, the idle-timer hits zero and the link comes down.



If the protocol running over the ISDN link is RIP version 2 or EIGRP, the most efficient way to prevent the routing updates from keeping the line up is expressly prohibiting their multicast routing update address in the access-list that is defining interesting traffic. Do not prevent them from crossing the link entirely, or the protocol obviously won't work correctly.



With OSPF, Cisco offers the ip ospf demand-circuit interface-level command. The OSPF adjacency will form over the ISDN link, but once formed, the Hello packets will be suppressed. However, the adjacency will not be lost. A check of the adjacency table with show ip ospf adjacency will show the adjacency remains at Full, even though Hellos are no longer being sent across the link. The ISDN link can drop without the adjacency being lost. When the link is needed, the adjacency is still in place and data can be sent without waiting for OSPF to go through the usual steps of forming an adjacency.



This OSPF command is vital for Cisco certification candidates at every level, but is particularly important for CCNA candidates. Learn this command now, get used to the fact that the adjacency stays up even though Hellos are suppressed, and add this valuable command to your Cisco toolkit.

One myth about ISDN is that Cisco Discovery Packets keep an ISDN link up. CDP is a Cisco-proprietary protocol that runs between directly connected Cisco devices. There is a school of thought that CDP packets have to be disabled on a BRI interface in order to prevent the link from staying up or dialing when it's not really needed. I've worked with ISDN for years in the field and in the lab, and I've never seen CDP bring up an ISDN link. Try it yourself the next time you're working on a practice rack!

Chris Bryant
CCIE #12933

Cisco CCNA - CCNP Home Lab Tutorial: The (Many) Cable Types And Their Purposes

Straight-through cables have quite a few uses in a CCNA / CCNP home lab. You'll need them to connect a switch port to an AUI port on a router (and you'll need a transceiver for that as well). If you have an ISDN simulator, straight-through cables can be used to connect a router's BRI port to the simulator.

Crossover cables are used to connect switches and allow them to trunk. If at all possible, get two switches in your home lab. This will allow you to gain valuable experience in manipulating root bridge election, working with STP, and creating EtherChannels.

DTE/DCE cables are used to connect two routers via their serial cables. If you are planning on using a frame relay switch in your lab, you'll need several of these. You can also get some great practice in by directly connecting two routers and bringing the connection up (and making sure it stays up!). This is valuable practice for your CCNA exam.

Octal cables are used to connect an access server to each of the other routers and switches in your lab.

Finally, there's that precious blue cable, the rollover cable. Rollover cables (sometimes called "rolled cables") allow you to connect a host device directly to a router or switch's console port. These cables have a way of disappearing around an IT shop, so make sure to take one home - and leave it there!

Cisco CCNA - CCNP Certification Exam: Same Command, Different Results

As a CCNA or CCNP, one thing you've got to get used to is that change is constant. Cisco regularly issues new IOS versions, not to mention the many different kinds of hardware they produce! While it's always nice to have "the latest and the greatest" when it comes to routers, switches, firewalls, etc., we have to be prepared for the fact that not all our clients are going to have that latest and greatest!

For instance, there are still quite a few Catalyst 5000 switches out there humming away, and if you're used to working on IOS-driven switches like the 2950, the same command can have dramatically different results.

Let's say you're going to examine the spanning tree protocol (STP) setup of a new client. You're used to working with newer 2950 switches, and you've always run show span on those switches to display spanning-tree information. Then, you run show span on a Catalyst 5000 - and something like this shows:

switch (enable) show span

Destination : Port 6/1

Admin Source : Port 6/2

Oper Source : Port 6/2

Direction : transmit/receive

Incoming Packets: disabled

Learning : enabled

Multicast : enabled

Filter : -

Status : active

Total local span sessions: 1

What's going on here?

The command show span on a 5000 will not show spanning tree stats - instead, what you're going to see are statistics relating to Switched Port ANalyzer (SPAN). Surprise!

Consider an example where you're used to running show span on 5000 switches to see SPAN information. When you run that on a 2950, you know now what you're going to get - spanning tree information! On a 2950, you'll need to run show monitor session, followed by the SPAN session number.

SW1#show monitor session 1

Session 1

---------

Type : Local Session

Source Ports :

Both : Fa0/1

Destination Ports : Fa0/2

Encapsulation : Native

Ingress: Disabled

As a CCNA and CCNP, this is one of those things you just have to get used to. Commands are going to be different, sometimes radically so, between models. That's why you need to be adept with both IOS Help and Cisco's online documentation site. IOS Help is easy, but the online doc site take a little getting used to. Once you learn how to navigate that site, a world of Cisco knowledge is at your fingertips.

Besides, when you sit for the CCIE lab exam, that will be the only friend you have! And a valuable friend it can be - you're just going to have to trust me on that one. :)

Microsoft Certification: The New MCTS Tracks And Exams

Microsoft is revamping its certification tracks, and will eventually retire the familiar MCSE certificatons. The new certification setup is much like Cisco's, where there are specialist certifications to go along with the more general CCNA, then mid-level certifications such as the CCNP, and then a more-advanced practical exam modeled somewhat after the coveted CCIE certification.

Microsoft's new specialist exams are the Microsoft Certified Technology Specialist (MCTS) exams. As of March 2006, there were five separate MCTS Tracks. Three of them deal with .NET Framework 2.0; these are the Web Application, Windows Applications, and Distributed Applications certifications. Each of these separate certifications requires the candidate to pass two exams. There is also a one-exam SQL Server 2005 certification, as well as a BizTalk Server 2006 single-exam certification.

The initial question is "Why is Microsoft doing this?" According to their website, MS feels that IT hiring managers today have a tough time deciding which computer certifications best identify job candidates who best meet their needs. I know it's easy to take verbal shots at Microsoft (it'll be an Olympic sport one day), but this new series of certs does have appeal for hiring managers, which can only help qualified candidates. Instead of the more-general MCSE, which does still suffer from the overcertification of NT 4.0 MCSEs back in the day, these more-specific certifications will make it easier for the job candidate to prove that they can do the job - and easier for the hiring manager to make an informed decision.

Microsoft hasn't announced the track that will eventually replace the MCSE, but this track will be revealed with the next client-server Windows release. It's up to you to stay informed of these changes, so I recommend you visit Microsoft's certification website often. "I didn't know" isn't much help once a certification expires!