ISDN is a vital topic for today’s CCNA and CCNP candidates, especially for the ICND and Intro exams – you’ve got to know ISDN inside and out to pass those exams. Naturally you want to include it in your home lab. What many candidates don’t realize is that you can’t connect two Cisco routers directly via their Basic Rate Interface (BRI) interfaces you’ve got to have another device between them called an ISDN simulator.

An ISDN simulator is not one of those software programs pretending to be routers (”router simulators”) this is a piece of hardware that acts as the telephone company in your home lab. Older simulators come with preprogrammed phone numbers and SPIDs, where newer ones let you program the phone numbers you want to use. Either way, an ISDN simulator is great for your CCNA/CCNP home lab, because you can practice dial scenarios that actually work. And you get to troubleshoot the ones that don’t, which is also important to learn! )

You don’t need any special cables or connectors you just connect both of your routers’ BRI interfaces to the ISDN simulator with a straight-through cable and you’re ready to go.
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To earn your Cisco CCNA and CCNP certifications, you’ve got to master ISDN – and despite what some people say, there’s still a lot of ISDN out there that needs to be supported. And when it comes to troubleshooting ISDN, there’s a lot to look at. Is the correct ISDN switchtype configured? Are the dialer map statements correct? What about the dialer-group and dialer-list commands? And that’s just the start.

I always say that all troubleshooting starts at Layer 1, the Physical layer of the OSI model. The usual method of troubleshooting ISDN is sending pings across the link, but the connection can be tested without using pings or even before assigning IP addresses to the BRI interfaces!

It’s a good idea to place these test calls before configuring the interfaces – that way, you know you’ve got a valid connection before beginning the configuration (and there’s a lot of config to go along with ISDN!)

To place a test call without using pings, use the isdn call interface command.
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To pass the BSCI exam and move one step closer to CCNP certification success, you’ve got to know how and when to use debug commands to troubleshoot and verify network operations. While you should never practice debug commands on a production network, it’s important to get some hands-on experience with them and not rely on “router simulators” and books to learn about them.

When it comes to RIP, “debug ip rip” is the primary debug to use. This debug will show you the contents of the routing update packets, and is vital in diagnosing RIP version mismatches and routing update authentication issues.

You know how to use the variance command to configure unequal-cost load-sharing with IGRP, but IGRP has no topology table that will give you the feasible successor metrics you need. With IGRP, you need to use the “debug ip igrp transactions” command to get these vital metrics.
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One of the first things you learned about Frame is that the LMI also serves as a keepalive, or a heartbeat – and if three consecutive LMIs are missed, the line protocol goes down. There’s a limitation to LMI as a keepalive, though. The LMI is exchanged only between the DTE and the closest DCE. The LMI is therefore a local keepalive that does not reflect any possible issues on the remote end of the virtual circuit.

Taking the LMI concept to the next logical level, Frame Relay End-To-End Keepalives (FREEK, one of the least-heard Cisco acronyms for some reason) are used to verify that endpoint-to-endpoint communications are functioning properly.

What you have to keep in mind about FREEK is that each and every PVC needs two separate keepalive processes. Remember, with a PVC, there’s no guarantee that the path taking through the frame relay cloud to get from R1 to R2 is going to be the same path taken to go back from R2 to R1. One process will be used to send requests for information and handle the responses to these requests; this is the send side. When the send side transmits a keepalive request, a response is expected in a certain number of seconds. If one is not received, an error event is noted. If enough error events are recorded, the VC’s keepalive status is marked as down.

The process that responds to the other side’s requests is the receive side.

This being Cisco, we’ve got to have some modes, right? FREEK has four operational modes.

Bidirectional mode enables both the send and receive process enabled on the router, meaning that the router will send requests and process responses (send side) and will also respond to remote requests for information (receive side).

Request mode enables only the send process. The router will send requests and process responses to those requests, but will not answer requests from other routers.
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Dialer Watch is a vital part of your CCNA and CCNP studies, particularly for the BCRAN exam, but it’s one of the most misunderstood technologies as well. To help you pass the CCNA and CCNP certification exams, here’s a detailed look at Dialer Watch.

Dialer Watch allows you to configure a route or routes as “watched” when the watched route leaves the routing table and there is no other valid route to that specific destination, the ISDN link will come up. In the following example, R1 and R2 are connected by both a Frame Relay cloud over the 172.12.123.0 /24 network and an ISDN cloud using the 172.12.12.0 /24 network. The routers are running OSPF over the Frame cloud, and R1 is advertising its loopback of 1.1.1.1/32 as well as an Ethernet segment, 10.1.1.0/24, via OSPF. R2 has both of these routes in its OSPF table, as shown below.

R2#show ip route ospf

1.0.0.0/32 is subnetted, 1 subnets

O 1.1.1.1 [110/65] via 172.12.123.1, 00:00:07, Serial0

10.0.0.0/24 is subnetted, 1 subnets

O 10.1.1.0 [110/128] via 172.12.123.1, 00:00:08, Serial0

We want R2 to place a call to R1 if either the loopback or Ethernet networks leave R2’s routing table, but we don’t want to have to depend on interesting traffic. That dictates the use of Dialer Watch.

First, configure the list of watched routes with dialer watch-list. Only one of the watched routes needs to leave the routing table for the ISDN link to come up. In this example, R2 will watch both routes from its OSPF routing table.
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You may run into situations where a router in a remote location needs to dial in to a central router, but the toll charges are much higher if the remote router makes the call. This scenario is perfect for PPP Callback, where the callback client places a call to a callback server, authentication takes place, and the server then hangs up on the client! This ensures that the client isn’t charged for the call. The server then calls the client back.

In the following example, R2 has been configured as the client and R1 is the callback server. Let’s look at both configurations and the unique commands PPP Callback requires.
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ISDN is a huge topic on both your Cisco CCNA and BCRAN CCNP exams. While many ISDN topics seem straightforward, it’s the details that make the difference in the exam room and working with ISDN in production networks. Configuring and troubleshooting multilink PPP is just one of the skills you’ll need to pass both of these demanding exams.

With BRI, we’ve got two B-channels to carry data, and both of them have a 64-kbps capacity. You might think it would be a good idea to have both channels in operation before one reaches capacity, and it is a great idea Problem is, it’s not a default behavior of ISDN. The second b-channel will not begin to carry traffic until the first one reaches capacity.

With Multilink PPP (MLP), a bandwidth capacity can be set that will allow the second b-channel to bear data before the first channel reaches capacity. The configuration for MLP is simple, but often misconfigured. We’ll use our good friend IOS Help to verify the measurement this command uses.
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To earn your Cisco CCNA certification and pass the BSCI CCNP exam, you have to know your protocol basics like the back of your hand! To help you review these important concepts, here’s a quick look at the basics of RIPv1, RIPv2, IGRP, and EIGRP.

RIPv1: Broadcasts updates every 30 seconds to the address 255.255.255.255. RIPv1 is a classful protocol, and it does not recognize VLSM, nor does it carry subnet masking information in its routing updates. Update contains entire RIP routing table. Uses Bellman-Ford algorithm. Allows equal-cost load-balancing by default. Max hop count is 15. Does not support clear-text or MD5 authentication of routing updates. Updates carry 25 routes maximum.

RIPv2: Multicasts updates every 30 seconds to the address 224.0.0.9. RIPv2 is a classless protocol, allowing the use of subnet masks. Update contains entire RIP routing table. Uses Bellman-Ford algorithm. Allows equal-cost load-balancing by default. Max hop count is 15. Supports clear-text and MD5 authentication of routing updates. Updates carry 25 routes maximum.

IGRP: Broadcasts updates every 90 seconds to the address 255.255.255.255. IGRP is a Cisco-proprietary protocol, and is also a classful protocol and does not recognize subnet masking. Update contains entire routing table. Uses Bellman-Ford algorithm. Equal-cost load-balancing on by default; unequal-cost load-sharing can be used with the variance command. Max hop count is 100.

EIGRP: Multicasts full routing table only when an adjacency is first formed. Multicasts updates only when there is a change in the network topology, and then only advertises the change. Multicasts to 224.0.0.10 and allows the use of subnet masks. Uses DUAL routing algorithm. Unequal-cost load-sharing available with the variance command.

By mastering the basics of these protocols, you’re laying the foundation for success in the exam room and when working on production networks. Pay attention to the details and the payoff is “CCNA” and “CCNP” behind your name!