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Table Of Contents
Troubleshooting Dialup Connections
Using the modem autoconfigure Command
Establishing a Reverse Telnet Session to a Modem
Gathering Modem Performance Information
modem call-record terse (11.3AA/12.0T)
Gathering Client-Side Performance Data
Problems with Particular Server Modems
A Few of the More Common Trends Seen by Cisco's TAC
Interpreting show isdn status Output
Dial-on-Demand Routing: Dialer Interface Operations
Annotated Example of PPP Negotiation
Before Calling Cisco Systems's TAC Team
Troubleshooting Dialup Connections
This chapter introduces and explains some of the technologies used in dialup networks. Configuration tips and interpretations of some of the show commands, which are useful for verifying correct operation of the network, are found in this chapter as well. Actual troubleshooting will be found in Chapter 17, "Troubleshooting ISDN Connections."
This chapter focuses on four principal areas:
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Modem Operations
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Modem Operations
This section discusses issues related specifically to the setup, verification, and use of modems with Cisco routers.
Using the modem autoconfigure Command
If you are using Cisco Internetwork Operating System (Cisco IOS) Release 11.1 or later, you can configure your Cisco router to communicate with and configure your modem automatically.
Use the following procedure to configure a Cisco router to automatically attempt to discover what kind of modem is connected to the line and then to configure the modem:
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If you want to display the list of modems for which the router has entries, use the show modemcap modem-name. If you want to change a modem value that was returned from the show modemcap command, use the modemcap edit modem-name attribute value line configuration command.
For complete information on the use of these commands, refer to the Cisco IOS documentation Dial Solutions Configuration Guide and Dial Solutions Command Reference.
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Establishing a Reverse Telnet Session to a Modem
For diagnostic purposes, or to initially configure the modem if you are running Cisco IOS Release 11.0 or earlier, you must establish a reverse Telnet session to configure a modem to communicate with a Cisco device. As long as you lock the speed of the data terminal equipment (DTE) of the modem (see Table 16-5 for information on locking the modem speed), the modem will always communicate with the access server or router at the desired speed. Be certain that the speed of the Cisco device is configured before issuing commands to the modem via a reverse Telnet session. (See Table 16-5 for information on configuring the speed of the access server or router.)
To configure the modem for a reverse Telnet session, use the line configuration command transport input telnet. To set up a rotary group (in this case on port 1), enter the line configuration command rotary 1. Placing these commands under the line configuration causes the IOS to allocate IP listeners for incoming connections at port ranges starting with the following base numbers:
2000 Telnet protocol
3000 Telnet protocol with rotary
4000 Raw TCP protocol
5000 Raw TCP protocol with rotary
6000 Telnet protocol, Binary Mode
7000 Telnet protocol, Binary Mode with rotary
9000 XRemote Protocol
10000 XRemote Protocol with rotary
To initiate a reverse Telnet session to your modem, perform the following steps:
Step 1
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Step 2
Step 3
Step 4
Table 16-1 outlines the problems that might cause a modem to router connectivity problem symptom and describes solutions to those problems.
Using Rotary Groups
For some applications, the modems on a given router need to be shared by a group of users. The Cisco Dialout Utility would be a good example of this application. The general idea is to have one port for users to connect into that will connect them to whichever modem happens to be available. To add an async line to a rotary group, simply enter rotary n, where n is the number of the rotary group in the configuration for the async line. For example, the following line configuration would allow users to connect to the rotary group by (referencing the previous example) telnet 192.169.53.52 3001 for normal Telnet:
line 1 16modem InOuttransport input allrotary 1speed 115200flowcontrol hardwareAlternatives include ports 5001 for Raw TCP, 7001 for binary Telnet (which Cisco Dialout Utility uses), and 10001 for Xremote connections.
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Interpreting show line Output
The output from the show line line-number exec command is useful when troubleshooting a modem-to-access server or router connection. Figure 16-1 shows the output from the show line command.
Figure 16-1 show line Command Output
When connectivity problems occur, important output appears in the Modem State and the Modem Hardware State fields.
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Table 16-2 shows typical Modem State and Modem Hardware State strings from the output of the show line command, and explains the meaning of each state.
Table 16-2 Modem and Modem Hardware States in show line Output
Idle
CTS noDSR DTR RTS
These are the proper modem states for connections between an access server or router and a modem (when there is no incoming call). Output of any other kind generally indicates a problem.
Ready
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If the modem state is ready instead of idle, there are three possibilities:
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Ready (continued)
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If your software does not support modem control, you must configure the access server line to which the modem is connected with the no exec line configuration command. Clear the line with the clear line privileged exec command, initiate a reverse Telnet session with the modem, and reconfigure the modem so that DCD is high only on CD.
End the Telnet session by entering disconnect, and reconfigure the access server line with the exec line configuration command.
Ready
noCTS noDSR DTR RTS
There are four possibilities for the noCTS string appearing in the Modem Hardware State field:
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Re-enable hardware flow control on the access server with the flowcontrol hardware line configuration command.
Ready
CTS DSR DTR RTS
There are two possibilities for the presence of the DSR string instead of the noDSR string in the Modem hardware state field:
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Configure the access server line to which the modem is connected with the no exec line configuration command. Clear the line with the clear line privileged exec command, initiate a reverse Telnet session with the modem, and reconfigure the modem so that DCD is high only on CD.
End the Telnet session by entering disconnect. Reconfigure the access server line with the exec line configuration command.
Ready
CTS* DSR* DTR RTS2
If this string appears in the Modem Hardware State field, modem control is probably not enabled on the access server. Use the modem inout line configuration command to enable modem control on the line.
See Table 16-1 for more information on configuring modem control on an access server or router line.
1 CD = carrier detect.
2 An * next to a signal indicates one of two things: The signal has changed within the past few seconds, or the signal is not being used by the modem control method selected.
Gathering Modem Performance Information
This section explains ways to gather performance data on MICA digital modems found in the Cisco AS5x00 family of access servers. The data can be used for trend analysis and is useful in troubleshooting performance problems that might be encountered. When looking at the numbers presented here, bear in mind that perfection is not possible in the real world. The modem call success rate (CSR) possible will be a function of the quality of the circuits, the client modem user base, and the set of modulations being used. A typical CSR percentage for V.34 calls is 95 percent. V.90 calls can be expected to connect successfully 92 percent of the time. Premature drops are likely to happen 10 percent of the time.
The tools used to gain an overall view of modem behavior on the access server are:
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If troubleshooting an individual modem connection or gathering data for trend analysis, the following information will be useful:
debug modem csmmodem call-record terseshow modem op (MICA) / AT@E1 (Microcom) while connectedshow modem log - for the session of interest after disconnect.ANI (caller's number)Time of dayClient modem hardware / firmware revisionInteresting info from client (after disconnect)-ATI6, ATI11, AT&V, AT&V1, etc.An audio record (.wav file) of the trainup attempt from the client modemThe commands will be explained further in the following sections, and some common trends will be discussed.
show modem/show modem summary
The show modem command gives a view of individual modems. From these numbers, the health of individual modems can be viewed.
router# show modemCodes:* - Modem has an active callC - Call in setupT - Back-to-Back test in progressR - Modem is being Resetp - Download request is pending and modem cannot be used for taking callsD - Download in progressB - Modem is marked bad and cannot be used for taking callsb - Modem is either busied out or shut-downd - DSP software download is required for achieving K56flex connections! - Upgrade request is pendingInc calls Out calls Busied Failed No SuccMdm Usage Succ Fail Succ Fail Out Dial Answer Pct.* 1/0 17% 74 3 0 0 0 0 0 96%* 1/1 15% 80 4 0 0 0 1 1 95%* 1/2 15% 82 0 0 0 0 0 0 100%1/3 21% 62 1 0 0 0 0 0 98%1/4 21% 49 5 0 0 0 0 0 90%* 1/5 18% 65 3 0 0 0 0 0 95%To see the aggregate numbers for all the modems on the router, use the show modem summary command.
router#show modem summaryIncoming calls Outgoing calls Busied Failed No SuccUsage Succ Fail Avail Succ Fail Avail Out Dial Ans Pct.0% 6297 185 64 0 0 0 0 0 0 97%Table 16-3 provides descriptions of the various show modem fields.
show modem call-stats
The show modem call-stats command offers a view of past performance for the modems. This is useful in trend analysis and can help the administrator to identify possible problems.
compress retrain lostCarr rmtLink trainup hostDrop wdogTimr inacTout
Mdm # % # % # % # % # % # % # % # %
Total 9 41 271 3277 7 2114 0 0
Table 16-4 provides descriptions of the most common show modem call-stats fields.
show modem connect-speeds
As a way for an access server's administrator to maintain a watch on the user community's connection speeds, the show modem connect command allows the admin to see how many people are getting each rate of speed. This is useful in trend analysis.
router>show modem connect 33600 0Mdm 26400 28000 28800 29333 30667 31200 32000 33333 33600 TotCntTot 614 0 1053 0 0 1682 0 0 822 6304router>show modem connect 56000 0Mdm 48000 49333 50000 50666 52000 53333 54000 54666 56000 TotCntTot 178 308 68 97 86 16 0 0 0 6304Expect to see a healthy distribution of V.34 speeds. There should be a peak at 26.4 if the T1s use channel associated signaling (CAS). For ISDN (PRI) T1s, the peak should be at 31.2. Also, look for a smattering of K56Flex, V.90 speeds. If there are no V.90 connections, there may be a network topology problem.
modem call-record terse (11.3AA/12.0T)
Rather than an exec command, this is a configuration command placed at the system level of the access server in question. When a user disconnects, a message similar to the following will be displayed:
*May 31 18:11:09.558: %CALLRECORD-3-MICA_TERSE_CALL_REC: DS0 slot/contr/chan=2/0/18, slot/port=1/29, call_id=378, userid=cisco, ip=0.0.0.0, calling=5205554099,called=4085553932, std=V.90, prot=LAP-M, comp=V.42bis both, init-rx/tx b-rate=26400/41333, finl-rx/tx brate=28800/41333, rbs=0, d-pad=6.0 dB, retr=1, sq=4, snr=29, rx/tx chars=93501/94046,bad=5, rx/tx ec=1612/732, bad=0, time=337, finl-state=Steady, disc(radius)=Lost Carrier/Lost Carrier, disc(modem)=A220 Rx (line to host) data flushing - not OK/EC condition - locally detected/receivedDISC frame -- normal LAPM terminationshow modem operational-status
The exec command show modem operational-status shows the current (or last) parameters pertaining to the modem's connection.
The documentation entry for this command is found in the Cisco IOS Release 12.0 Dial Solutions Command Reference. show modem operational-status is only for MICA modems. The equivalent command for Microcom modems is modem at-mode / AT@E1. Use the modem at-mode <slot>/<port> command to connect to the modem, and issue the AT@E1 command. Complete documentation for the modem at-mode command can be found in the Cisco AS5300 Software Configuration Guide, and documentation for the AT@E1 command is in the AT Command Set and Register Summary for Microcom Modem Modules Command Reference.
Use these two steps to find out what modems a user is coming in on.
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Gathering Client-Side Performance Data
For trend analysis, it's very important to gather client-side performance data. Good information to get includes this:
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Analyze the Performance Data
When you have collected and understood the performance data for your modem system, it's time to look at any remaining patterns/components that may have room for improvement.
Problems with Particular Server Modems
Use show modem or show modem call-stats, and look for any modems with abnormally high rates of trainup failure or bad disconnect rates. If adjacent pairs of modems are having problems, the problem is likely a hung or dead DSP. Use copy flash modem to the affected HMM to recover. Make sure that the modems are running the latest version of portware.
Verify that all modems are correctly configured. To make sure that the modems are correctly configured, use the configuration command modem autoconfigure type <mica/microcom_server> in the line configuration. To make sure that the modems are being autoconfigured whenever a call is hung up, use the exec command debug confmodem. In some cases, it may require a reverse Telnet to fix modems that are badly misconfigured.
Problems with Particular DS0s
Bad DS0s are rare but possible. To find out if one is present, use the command show controller t1 call-counters. Look for any DS0s with abnormally high TotalCalls and abnormally low TotalDuration. To target suspected DS0s, it is sometimes necessary to take out of service other DS0s with the configuration command isdn service dsl, ds0 busyout under the serial interface for the T1. The output from show controller t1 call-counters look like this:
TimeSlot Type TotalCalls TotalDuration1 pri 873 1w6d2 pri 753 2w2d3 pri 4444 00:05:22Obviously, time slot 3 is the suspect channel in this case.
A Few of the More Common Trends Seen by Cisco's TAC
The following are some of the more commonly seen problems. Each problem's telltale signs are listed.
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If . . .
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You might be getting bad circuit paths through the Public Switched Telephone Network.
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Long distance calls are bad but local calls are good.
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Calls from specific geographical regions/exchanges tend to have problems
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Learn the network topology from the telco. If multiple analog-to-digital conversions (caused by nonintegrated SLCs or analog switches) are used to serve an area, V.90/K56flex modem connects will be impossible, and V.34 may be somewhat degraded.
ISDN Operations
Integrated Services Digital Network (ISDN) refers to a set of digital services that is available to end users. ISDN involves the digitization of the telephone network so that voice, data, text, graphics, music, video, and other source material can be provided to end users from a single end-user terminal over existing telephone wiring. Proponents of ISDN imagine a worldwide network much like the present telephone network, but with digital transmission and a variety of new services.
ISDN is an effort to standardize subscriber services, user/network interfaces, and network and internetwork capabilities. Standardizing subscriber services attempts to ensure a level of international compatibility. Standardizing the user/network interface stimulates development and marketing of these interfaces by third-party manufacturers. Standardizing network and internetwork capabilities helps achieve the goal of worldwide connectivity by ensuring that ISDN networks easily communicate with one another.
ISDN applications include high-speed image applications (such as Group IV facsimile), additional telephone lines in homes to serve the telecommuting industry, high-speed file transfer, and videoconferencing. Voice, of course, will also be a popular application for ISDN.
Many carriers are beginning to offer ISDN under tariff. In North America, large local exchange carriers (LECs) are beginning to provide ISDN service as an alternative to the T1 connections (digital carrier facilities provided by telephone companies) that currently carry bulk wide-area telephone service (WATS) services.
ISDN Components
ISDN components include terminals, terminal adapters (TAs), network-termination devices, line-termination equipment, and exchange-termination equipment. ISDN terminals come in two types. Specialized ISDN terminals are referred to as terminal equipment type 1 (TE1). Non-ISDN terminals such as DTE that predate the ISDN standards are referred to as terminal equipment type 2 (TE2). TE1s connect to the ISDN network through a four-wire, twisted-pair digital link. TE2s connect to the ISDN network through a terminal adapter. The ISDN TA can be either a standalone device or a board inside the TE2. If the TE2 is implemented as a standalone device, it connects to the TA via a standard physical layer interface. Examples include EIA/TIA-232-C (formerly RS-232-C), V.24, and V.35.
Beyond the TE1 and TE2 devices, the next connection point in the ISDN network is the network termination type 1 (NT1) or network termination type 2 (NT2) device. These are network-termination devices that connect the four-wire subscriber wiring to the conventional two-wire local loop. In North America, the NT1 is a customer premises equipment (CPE) device. In most other parts of the world, the NT1 is part of the network provided by the carrier. The NT2 is a more complicated device, typically found in digital private branch exchanges (PBXs), that performs Layer 2 and 3 protocol functions and concentration services. An NT1/2 device also exists; it is a single device that combines the functions of an NT1 and an NT2.
A number of reference points are specified in ISDN. These reference points define logical interfaces between functional groupings such as TAs and NT1s. ISDN reference points include the following:
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A sample ISDN configuration is shown in Example 16-1. This example shows three devices attached to an ISDN switch at the central office. Two of these devices are ISDN-compatible, so they can be attached through an S reference point to NT2 devices. The third device (a standard, non-ISDN telephone) attaches through the R reference point to a TA. Any of these devices could also attach to an NT1/2 device, which would replace both the NT1 and the NT2. And, although they are not shown, similar user stations are attached to the far-right ISDN switch.
ISDN Services
The ISDN Basic Rate Interface (BRI) service offers two B channels and one D channel (2B+D). BRI B-channel service operates at 64 kbps and is meant to carry user data; BRI D-channel service operates at 16 kbps and is meant to carry control and signaling information, although it can support user data transmission under certain circumstances. The D-channel signaling protocol comprises Layers 1 through 3 of the OSI reference model. BRI also provides for framing control and other overhead, bringing its total bit rate to 192 kbps. The BRI physical layer specification is International Telecommunication Union-Telecommunications Standards Sector (ITU-T; formerly the Consultative Committee for International Telegraph and Telephone [CCITT]) I.430.
ISDN Primary Rate Interface (PRI) service offers 23 B channels and one D channel in North America and Japan, yielding a total bit rate of 1.544 Mbps (the PRI D channel runs at 64 kbps). ISDN PRI in Europe, Australia, and other parts of the world provides 30 B plus one 64-kbps D channel and a total interface rate of 2.048 Mbps. The PRI physical layer specification is ITU-T I.431.
Layer 1
ISDN physical layer (Layer 1) frame formats differ depending on whether the frame is outbound (from terminal to network) or inbound (from network to terminal). Both physical layer interfaces are shown in Figure 16-2.
Figure 16-2 ISDN Physical Layer Frame Formats
The frames are 48 bits long, of which 36 bits represent data. The bits of an ISDN physical layer frame are used as follows:
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Multiple ISDN user devices can be physically attached to one circuit. In this configuration, collisions can result if two terminals transmit simultaneously. ISDN, therefore, provides features to determine link contention. When an NT receives a D bit from the TE, it echoes back the bit in the next E-bit position. The TE expects the next E bit to be the same as its last transmitted D bit.
Terminals cannot transmit into the D channel unless they first detect a specific number of ones (indicating "no signal") corresponding to a pre-established priority. If the TE detects a bit in the echo channel that is different from its D bits, it must stop transmitting immediately. This simple technique ensures that only one terminal can transmit its D message at one time. After successful D message transmission, the terminal has its priority reduced by being required to detect more continuous ones before transmitting. Terminals cannot raise their priority until all other devices on the same line have had an opportunity to send a D message. Telephone connections have higher priority than all other services, and signaling information has a higher priority than nonsignaling information.
Layer 2
Layer 2 of the ISDN signaling protocol is Link Access Procedure on the D channel, also known as LAPD. LAPD is similar to High-Level Data Link Control (HDLC) and Link Access Procedure, Balanced (LAPB). As the expansion of the LAPD abbreviation indicates, it is used across the D channel to ensure that control and signaling information flows and is received properly. The LAPD frame format (see Figure 16-3) is very similar to that of HDLC; like HDLC, LAPD uses supervisory, information, and unnumbered frames. The LAPD protocol is formally specified in ITU-T Q.920 and ITU-T Q.921.
Figure 16-3 LAPD Frame Format
The LAPD Flag and Control fields are identical to those of HDLC. The LAPD Address field can be either 1 or 2 bytes long. If the extended address bit of the first byte is set, the address is 1 byte; if it is not set, the address is 2 bytes. The first Address field byte contains the service access point identifier (SAPI), which identifies the portal at which LAPD services are provided to Layer 3. The C/R bit indicates whether the frame contains a command or a response. The Terminal Endpoint Identifier (TEI) field identifies either a single terminal or multiple terminals. A TEI of all ones indicates a broadcast.
Layer 3
Two Layer 3 specifications are used for ISDN signaling: ITU-T (formerly CCITT) I.450 (also known as ITU-T Q.930) and ITU-T I.451 (also known as ITU-T Q.931). Together, these protocols support user-to-user, circuit-switched, and packet-switched connections. A variety of call establishment, call termination, information, and miscellaneous messages are specified, including SETUP, CONNECT, RELEASE, USER INFORMATION, CANCEL, STATUS, and DISCONNECT. These messages are functionally similar to those provided by the X.25 protocol (see Chapter 19, "Troubleshooting X.25 Connections," for more information). Figure 16-4, from ITU-T I.451, shows the typical stages of an ISDN circuit-switched call.
Figure 16-4 ISDN Circuit-Switched Call Stages
Interpreting show isdn status Output
To find out what the current condition of the ISDN connection is between the router and the telco switch, use the command show isdn status. The two kinds of interfaces that are supported by this command are the Basic Rate Interface (BRI) and the Primary Rate Interface (PRI) (Tables 16-5 and 16-6).
3620-2#show isdn statusGlobal ISDN Switchtype = basic-niISDN BRI0/0 interfacedsl 0, interface ISDN Switchtype = basic-niLayer 1 Status:ACTIVELayer 2 Status:TEI = 88, Ces = 1, SAPI = 0, State = MULTIPLE_FRAME_ESTABLISHEDTEI = 97, Ces = 2, SAPI = 0, State = MULTIPLE_FRAME_ESTABLISHEDSpid Status:TEI 88, ces = 1, state = 5(init)spid1 configured, no LDN, spid1 sent, spid1 validEndpoint ID Info: epsf = 0, usid = 0, tid = 1TEI 97, ces = 2, state = 5(init)spid2 configured, no LDN, spid2 sent, spid2 validEndpoint ID Info: epsf = 0, usid = 1, tid = 1Layer 3 Status:0 Active Layer 3 Call(s)Activated dsl 0 CCBs = 0The Free Channel Mask: 0x800000035200-1# show isdn statusGlobal ISDN Switchtype = primary-5essISDN Serial0:23 interfacedsl 0, interface ISDN Switchtype = primary-5essLayer 1 Status:ACTIVELayer 2 Status:TEI = 0, Ces = 1, SAPI = 0, State = MULTIPLE_FRAME_ESTABLISHEDLayer 3 Status:0 Active Layer 3 Call(s)Activated dsl 0 CCBs = 0The Free Channel Mask: 0x807FFFFFTotal Allocated ISDN CCBs = 05200-1#If the show isdn status command does not work or does not show the PRI, try using the show isdn service command. Make sure that the pri-group command appears in the configuration under the T1/E1 controller in the configuration. If the command is not present, configure the controller with the pri-group command.
Following is an example of a configuration for a Cisco router with a channelized T1 card:
controller t1 0framing esfline code b8zspri-group timeslots 1-24Table 16-6 details the fields for the show isdn status command.
Dial-on-Demand Routing: Dialer Interface Operations
Dial-on-demand routing (DDR) is a method of providing WAN connectivity on an economical as-needed basis, either as a primary link or as backup for a nondial serial link.
A dialer interface is defined as any router interface capable of placing or receiving a call. This generic term should be distinguished from the term Dialer interface (with a capital D), which refers to a logical interface configured to control one or more physical interfaces of a router and which is seen in a router configuration as interface Dialer X. From this point forward, unless otherwise stated, we will be using the term dialer in its generic sense.
Dialer interface configuration comes in two flavors: dialer map-based (sometimes referred to as legacy DDR) and dialer profiles. Which method you use depends on the circumstances under which you need dial connectivity. Dialer map-based DDR was first introduced in IOS version 9.0, and dialer profiles were introduced in IOS version 11.2.
Triggering a Dial
At its heart, DDR is just an extension of routing wherein interesting packets are routed to a dialer interface, triggering a dial attempt. We will attempt here to explain the concepts involved in defining interesting traffic, and to explain the routing used for DDR connections.
Interesting Packets
Interesting is the term used to describe packets or traffic that either will trigger a dial attempt or, if a dial link is already active, will reset the idle timer on the dialer interface. For a packet to be considered interesting, it must have these characteristics:
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By default, no packets are considered to be interesting. Interesting packet definitions must be explicitly declared in a router or access server configuration.
Dialer Group
In the configuration of each dialer interface on the router or access server, there must be a dialer-group command. If the dialer-group command is not present, there is no logical link between the interesting packet definitions and the interface. The command syntax is as follows:
dialer-group [group number]
The group-number is the number of the dialer access group to which the specific interface belongs. This access group is defined with the dialer-list command. Acceptable values are nonzero, positive integers between 1 and 10.
An interface can be associated with a single dialer access group only; multiple dialer-group assignment is not allowed. A second dialer access group assignment will override the first. A dialer access group is defined with the dialer-group command. The dialer-list command associates an access list with a dialer access group.
Packets that match the dialer group specified trigger a connection request.
The destination address of the packet is evaluated against the access list specified in the associated dialer-list command. If it passes, either a call is initiated (if no connection has already been established) or the idle timer is reset (if a call is currently connected).
Dialer List
The dialer-list global configuration command is used to define a DDR dialer list to control dialing by protocol, or by a combination of protocol and access list. Interesting packets are those that match the protocol level permit or which are permitted by the list in the dialer-list command:
dialer-list dialer-group protocol protocol-name {permit | deny | list access-list-number | access-group}
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Access List
For each networking protocol that is to be sent across the dial connection, an access list may be configured. For purposes of cost control, it is usually desirable to configure an access list to prevent certain traffic—such as routing updates—from bringing up or keeping up a connection. Note that when we create access lists for the purpose of defining interesting and uninteresting traffic, we are not declaring that uninteresting packets cannot cross the dial link—only that they will not reset the idle timer, nor will they bring up a connection on their own. As long as the dial connection is up, uninteresting packets will still be allowed to flow across the link.
For example, a router running EIGRP as its routing protocol can have an access list configured to declare EIGRP packets uninteresting and all other IP traffic interesting:
access-list 101 deny eigrp any any
access-list 101 permit ip any any
Access lists can be configured for all protocols that might cross the dial link. Remember that for any protocol, the default behavior in the absence of an access list permit statement is to deny all traffic. If there is no access list and if there is no dialer-list command permitting the protocol, then that protocol will be uninteresting. In actual practice, if there is no dialer list for a protocol, those packets will not flow across the link at all.
Example: Putting It All Together
With all the elements in place, it is possible to examine the complete process by which the "interesting" status of a packet is determined. In this example, IP and IPX are the protocols that may cross the dial link, but the user wants to prevent broadcasts and routing updates from initiating a call or keeping the link up.
!interface async 1dialer-group 7!access-list 121 deny eigrp any anyaccess-list 121 deny ip any host 255.255.255.255access-list 121 permit ip any anyaccess-list 9
