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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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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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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BECNs and FECNs aren’t just important to know for your Cisco CCNA and CCNP certification exams – they’re an important part of detecting congestion on a Frame Relay network and allowing the network to dynamically adjust its transmission rate when congestion is encountered.

The Forward Explicit Congestion Notification (FECN, pronounced “feckon”) bit is set to zero by default, and will be set to 1 if congestion was experienced by the frame in the direction in which the frame was traveling. A DCE (frame relay switch) will set this bit, and a DTE (router) will receive it, and see that congestion was encountered along the frame’s path.

If network congestion exists in the opposite direction in which the frame was traveling, the Backward Explicit Congestion Notification (BECN, pronounced “beckon”) will be set to 1 by a DCE.

If this is your first time working with BECNs and FECNs, you might wonder why the BECN even exists – after all, why send a “backwards” notification? The BECN is actually the most important part of this entire process, since it’s the BECN bit that indicates to the sender that it needs to slow down!

For example, frames sent from Kansas City to Green Bay encounter congestion in the FR cloud. A Frame Switch sets the FECN bit to 1. In order to alert KC that it’s sending data too fast, GB will send return frames with the BECN bit set. When KC sees the BECN bit is set to 1, the KC router knows that the congestion occurred when frames were sent from KC to GB.

Frame Relay BECN Adaptive Shaping allows a router to dynamically throttle back on its transmission rate if it receives frames from the remote host with the BECN bit set. In this case, KC sees that the traffic it’s sending to GB is encountering congestion, because the traffic coming back from GB has the BECN bit set. If BECN Adaptive Shaping is running on KC, that router will adjust to this congestion by slowing its transmission rate. When the BECNs stop coming in from GB, KC will begin to send at a faster rate.

BECN Adaptive Shaping is configured as follows:

KC(config)#int s0

KC(config-if)#frame-relay adaptive-shaping becn

To see how many frames are coming in and going out with the BECN and FECN bits set, run show frame pvc.

R3#show frame pvc

< some output removed for clarity >

input pkts 306 output pkts 609 in bytes 45566

out bytes 79364 dropped pkts 0 in FECN pkts 0

in BECN pkts 0 out FECN pkts 0 out BECN pkts 0

in DE pkts 0 out DE pkts 0

out bcast pkts 568 out bcast bytes 75128

pvc create time 01:26:27, last time pvc status changed 01:26:27

Just watch the “in”s and “out”s of BECN, FECN, and DE in both the exam room and your production networks!

Whether you’re just starting to think about passing the CCNA or CCNP exams, or you’ve been on the certification track for a while, you’ve got to have a plan for success. If you wanted to drive your car from Florida to California, you’d create a plan to get there. You’d get a map and decide how far you wanted to drive per day, and maybe even make some hotel reservations in advance. You certainly wouldn’t get in your car, just drive it randomly down the nearest highway, and hope you ended up in California, would you?

Certainly not. Earning your CCNA certification is the same way. It’s not enough to just study a few minutes “when you feel like it”, or tell yourself that you’ll start studying for the exams “when I get such-and-such done”. The perfect time to start on the road to Cisco certification is not tomorrow, and it’s not next week. It’s today.

You’re much better off with one hour of solid study than three hours of interrupted, unfocused study. Here are a few ways to go about getting the kind of quality study time that will get you to the CCNA or CCNP (or any Cisco certification, for that matter!).

Schedule your study time, and regard this study time as you would an appointment with a client. If you were to meet a customer at 10:00 to discuss a network install, would you just decide not to show up and watch television instead? Not if you wanted the job. The same goes for your study time. That’s an appointment with the most important customer of all – YOU.

Turn your cell, iPod, TV, instant messenger, and all other electronic collars off for the duration of your study time. I know those of us in information technology don’t like to say this, but we can actually exist without being in touch with the world for a little while. You may even get to like it! Having uninterrupted study time is key to CCNA and CCNP exam success.

Finally, schedule your exam before you start studying. Contrary to what many people think, “deadline” is not a dirty word. We do our best work when we have a deadline and a schedule to keep. Make out your study schedule, schedule your exam, and get to work just as you would a network project for a customer. The project you’re working on is your career and your life, and by following these simple steps you can make it a highly successful project – by passing your CCNA and CCNP exam!

When you’re studying to pass the Cisco CCNA and CCNP certification exams, you quickly learn that there’s always something else to learn. (You’ll really pick up on this in your CCIE studies, trust me!) Today we’ll take a look at an often-overlooked topic in Frame Relay, the encapsulation type. You don’t exactly change this on a daily basis in production networks (not if you want to stay employed, anyway!), but it’s an important exam topic that you must be familiar with.

The DCE and DTE must agree on the LMI type, but there’s another value that must be agreed upon by the two DTEs serving as the endpoints of the VC. The Frame encapsulation can be left at the default of Cisco (which is Cisco-proprietary), or it can be changed to the industry-standard IETF, as shown below. If a non-Cisco router is the remote endpoint, IETF encapsulation must be used. Note that the default of Cisco isn’t listed as an option by IOS Help, so you better know that one by heart!

R1(config)#int s0

R1(config-if)#encap frame ?

ietf Use RFC1490/RFC2427 encapsulation

R1(config-if)#encap frame ietf

What if a physical interface is in use and some remote hosts require Cisco encapsulation and others require IETF? The encapsulation type can be configured on a per-PVC basis as well. One encap type can be used on the interface, and any map statements that require a different encap type can have that specified in the appropriate map statement. In the following example, all PVCs will use the default Cisco encapsulation type except for PVC 115. The frame map statement using that PVC has ietf specified.

R1(config)#int s0/0

R1(config-if)#encap frame

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

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

show frame map shows us that the mapping to DLCI 123 is using Cisco encapsulation, and DLCI 122 is using IETF.

R1#show frame map

Serial0 (up): ip 172.12.123.3 dlci 123(0×7B,0×1CB0), static

broadcast, CISCO, status defined, active

Serial0 (up): ip 172.12.123.2 dlci 122(0×7B,0×1CB0), static

broadcast, ietf, status defined, active

Just remember that Cisco is the default, and all PVCs will use Cisco unless you specify IETF in the frame map statement itself. You could also change the entire interface to use IETF for all mappings with the frame-relay encapsulation IETF command. For Cisco exams, as well as work on production networks, it’s always a good idea to know more than one way to do something!

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!