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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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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As 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. I’ve edited this output, since the output you see here will be repeated fire times – once for each ping packet.

R2(config-if)#isdn caller 555xxxx

R1#debug dialer

Dial on demand events debugging is on

R1#ping 172.12.12.2

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 172.12.12.2, timeout is 2 seconds:

03:30:25: BR0 DDR: Dialing cause ip (s=172.12.12.1, d=172.12.12.2)

03:30:25: 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 dialer

BRI0 – dialer type = ISDN

Dial String Successes Failures Last DNIS Last status

8358661 1 0 00:03:16 successful

7 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 callback

R1 will now ping R2. The pings aren’t returned, but seconds later R2 calls R1 back.

R1#ping 172.12.12.2

Success rate is 0 percent (0/5)

R1#

03:48:12: BRI0: wait for isdn carrier timeout, call id=0×8023

R1#

03:48:18: %LINK-3-UPDOWN: Interface BRI0:1, changed state to up

R1#

03:48:18: BR0:1 DDR: dialer protocol up

R1#

03:48:19: %LINEPROTO-5-UPDOWN: Line protocol on Interface BRI0:1, changed state to up

R1#

03:48:24: %ISDN-6-CONNECT: Interface BRI0:1 is now connected to 8358662 R2

show dialer on R2 shows the reason for the call to R1 is a callback return call.

R2#show dialer

BRI0 – dialer type = ISDN

Dial String Successes Failures Last DNIS Last status

8358661 3 0 00:00:48 successful

7 incoming call(s) have been screened.

10 incoming call(s) rejected for callback.

BRI0:1 – dialer type = ISDN

Idle timer (120 secs), Fast idle timer (20 secs)

Wait for carrier (30 secs), Re-enable (15 secs)

Dialer state is data link layer up

Dial reason: Callback return call

Time until disconnect 71 secs

Connected 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!