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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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!

Earning your Cisco CCNA and CCNP is a tough proposition, and part of that is the fact that you quickly learn that there’s usually more than one way to do things with Cisco routers – and while that’s generally a good thing, you better know the ins and outs of all options when it comes to test day and working on production networks. Working with Frame Relay subinterfaces and split horizon is just one such situation.

One reason for the use of subinterfaces is to circumvent the rule of split horizon. You recall from your CCNA studies that split horizon dictates that a route cannot be advertised out the same interface upon which it was learned in the first place. In the following example, R1 is the hub and R2 and R3 are the spokes. All three routers are using their physical interfaces for frame relay connectivity, and they are also running RIPv2 172.12.123.0 /24. Each router is also advertising a loopback interface, using the router number for each octet.

R1(config)#int s0

R1(config-if)#ip address 172.12.123.1 255.255.255.0

R1(config-if)#no frame inverse

R1(config-if)#frame map ip 172.12.123.2 122 broadcast

R1(config-if)#frame map ip 172.12.123.3 123 broadcast

R1(config-if)#no shut

R2(config)#int s0

R2(config-if)#encap frame

R2(config-if)#no frame inver

R2(config-if)#frame map ip 172.12.123.1 221 broadcast

R2(config-if)#frame map ip 172.12.123.3 221 broadcast

R2(config-if)#ip address 172.12.123.2 255.255.255.0

R3(config)#int s0

R3(config-if)#encap frame

R3(config-if)#no frame inver

R3(config-if)#frame map ip 172.12.123.1 321 broadcast

R3(config-if)#frame map ip 172.12.123.2 321 broadcast

R3(config-if)#ip address 172.12.123.3 255.255.255.0

R1#show ip route rip

2.0.0.0/32 is subnetted, 1 subnets

R 2.2.2.2 [120/1] via 172.12.123.2, 00:00:20, Serial0

3.0.0.0/32 is subnetted, 1 subnets

R 3.3.3.3 [120/1] via 172.12.123.3, 00:00:22, Serial0

R2#show ip route rip

1.0.0.0/32 is subnetted, 1 subnets

R 1.1.1.1 [120/1] via 172.12.123.1, 00:00:06, Serial0

R3#show ip route rip

1.0.0.0/32 is subnetted, 1 subnets

R 1.1.1.1 [120/1] via 172.12.123.1, 00:00:04, Serial0

The hub router R1 has a route to both loopbacks, but neither spoke has a route to the other spoke’s loopback. That’s because split horizon prevents R1 from advertising a network via Serial0 if the route was learned on Serial0 to begin with.

We’ve got two options here, one of which is to disable spilt horizon on the interface. While doing so will have the desired effect in our little network, disabling split horizon is not a good idea and should be avoided whenever possible. We’re not going to do it in this lab, but here is the syntax to do so:

R1(config)#interface serial0

R1(config-if)#no ip split-horizon

A better solution is to configure subinterfaces on R1. The IP addressing will have to be revisited, but that’s no problem here. R1 and R2 will use 172.12.123.0 /24 to communicate, while R1 and R3 will use 172.12.13.0 /24. R3’s serial0 interface will need to be renumbered, so let’s look at all three router configurations:

R1(config)#interface serial0

R1(config-if)#encap frame

R1(config-if)#no frame inverse-arp

R1(config-if)#no ip address

R1(config-if)#interface serial0.12 multipoint

R1(config-subif)#ip address 172.12.123.1 255.255.255.0

R1(config-subif)#frame map ip 172.12.123.2 122 broadcast

R1(config-subif)#interface serial0.31 point-to-point

R1(config-subif)#ip address 172.12.13.1 255.255.255.0

R1(config-subif)#frame interface-dlci 123

R2(config)#int s0

R2(config-if)#ip address 172.12.123.2 255.255.255.0

R2(config-if)#encap frame

R2(config-if)#frame map ip 172.12.13.3 221 broadcast

R2(config-if)#frame map ip 172.12.123.1 221 broadcast

R3(config)#int s0

R3(config-if)#ip address 172.12.13.3 255.255.255.0

R3(config-if)#encap frame

R3(config-if)#frame map ip 172.12.13.1 321 broadcast

R3(config-if)#frame map ip 172.12.123.2 321 broadcast

A frame map statement always names the REMOTE IP address and the LOCAL DLCI. Don’t forget the broadcast option!

Show frame map shows us that all the static mappings on R1 are up and running. Note the “static” output, which indicates these mappings are a result of using the frame map command. Pings are not shown, but all three routers can ping each other at this point.

R1#show frame map

Serial0 (up): ip 172.12.123.2 dlci 122(0×7A,0×1CA0), static,

broadcast, CISCO, status defined, active

Serial0 (up): ip 172.12.13.3 dlci 123(0×7B,0×1CB0), static,

broadcast, CISCO, status defined, active

After the 172.12.13.0 /24 network is added to R1 and R3’s RIP configuration, R2 and R3 now have each other’s loopback network in their RIP routing tables.

R2#show ip route rip

1.0.0.0/32 is subnetted, 1 subnets

R 1.1.1.1 [120/1] via 172.12.123.1, 00:00:20, Serial0

3.0.0.0/32 is subnetted, 1 subnets

R 3.3.3.3 [120/1] via 172.12.123.1, 00:00:22, Serial0

R3#show ip route rip

1.0.0.0/32 is subnetted, 1 subnets

R 1.1.1.1 [120/1] via 172.12.13.1, 00:00:20, Serial0

2.0.0.0/32 is subnetted, 1 subnets

R 2.2.2.2 [120/1] via 172.12.13.1, 00:00:22, Serial0

While turning split horizon off is one way to achieve total IP connectivity, doing so can have other unintended results. The use of subinterfaces is a more effective way of allowing the spokes to see the hub’s loopback network.