DDR consists of two portions: logical and physical. Network layer address, encapsulation, and dialer parameters are part of the logical portion of DDR. The interface that places and receives calls is the physical portion. When dialer profiles are implemented, the physical interfaces comprise a dialer pool and are allocated from this pool on an as-needed basis. A physical interface is borrowed from the dialer pool when a call is made. It is returned to the pool when the call is complete. Dialer profiles dynamically bind logical and physical configurations for each call. This allows the physical interface to take on different characteristics according to the requirements of an incoming or outgoing call. Remember that the combination of physical and logical characteristics is only temporary and lasts as long as the call.

Advantages of Dialer Profiles

In addition to the aforementioned perks, dialer profiles provide the ability to separate the logical portion of DDR from the physical interface, allowing you to

• Enable concurrent bridging over DDR interfaces to multiple sites

• Limit the number of minimum or maximum connections taking place on a DDR interface

• Assign different Layer 3 network addresses to different members of a physical interface

• Specify different encapsulations for different B channels

• Configure different members of a DDR interface with different DDR parameters

Dialer profiles support only PPP or HDLC encapsulation. PPP encapsulation is the most popular choice because it's nonproprietary and offers authentication options.

Dialer Profile Components

A dialer profile is a combination of the following components:

• Dialer interface A logical portion of a dialer profile. The dialer interface governs all configuration settings for a destination. Each dialer interface can contain multiple dialer maps. Furthermore, different per-call parameters can be assigned to each dialer map defined in a dialer map class. The dialer interface defines the destination network protocol address, encapsulation type, type of PPP authentication, and dialer remote name for PPP PAP or CHAP. Other specified parameters include the dialer string/map, dialer pool number, interesting traffic lists, Multilink PPP, and optional timeouts.

• Dialer map class An optional portion of a dialer profile that defines call characteristics for a specified destination. Map classes are designed to avoid having to identify the same call characteristics repeatedly for multiple interfaces. If a map class isn't used, a separate call characteristics definition is required for each dialer interface, even if those characteristics are identical for several dialer interfaces. The information included in a map class is tuned for each destination. This information can specify an ISDN speed of 56 kbps, whether it is a semipermanent connection, optional dialer timers such as dialer fast idle, dialer idle timeout, and dialer wait-for-carrier time.

• Dialer pool A group of one or more physical interfaces of which each dialer interface is a member. Each dialer interface is associated with a dialer pool. A physical interface can be part of more than one dialer pool. You can also configure an optional priority, which determines outbound dialing contention for specific physical interfaces in the pool.

• Physical interfaces Members of one or more pools. The configuration of a physical interface is limited to the encapsulation parameters. If required, Multilink PPP and PPP authentication are specified to enable identification of the dialer pools to which the interface belongs. The encapsulation method of the physical interface must match that of the dialer interface, which belongs to the same pool as the physical interface.

Dialer Profile Binding Sequence

As you know, dialer profiles specify the technique of dynamically binding the logical and physical configuration. It is the job of the NAS to associate dialer information with a physical port to accommodate the needs of a particular user dialing in to or out of the NAS. When multiple dialer profiles are configured on the NAS, it must determine which profile to bind for every call. The following two sections describe the binding sequence for dialing out and dialing in.

Dialing Out

The binding process for the outgoing calls works as follows:

1. When an outgoing packet arrives at the NAS, a route table lookup is performed, and the incoming packet from the network arrives. A route table lookup points to the destination via the dialer interface.

2. When it is noted that the dialer interface is a dialer profile, the IOS determines whether an existing connection for this profile exists. If there is none, the software identifies the pool to which the dialer interface belongs.

3. The NAS searches for the first available physical interface of the pool that has the highest pool priority. When it is located, this interface is identified for use in dialing. It is then bound to the dialer interface, taking on the configuration of that dialer interface.

4. The telephone number for the dialer profile is dialed, and the regular DDR process takes place.

Dialing In

What makes the incoming call-binding process more complex than that for the outgoing calls is the fact that the called physical interface may be a member of multiple pools, and the pools, in turn, may be associated with multiple dialer profiles. The incoming call-binding process is as follows:

• If the physical interface belongs to only one pool, which is associated with one dialer profile, the bind occurs between the physical interface and this dialer profile. If this isn't possible, the next step is a further attempt at binding known as an approximate match.

• This attempt looks for a match of the Call Line ID (CLID) from the call with the dialer number from a dialer profile. However, the search involves only the profiles associated with the pool to which the dialed physical interface belongs. If there is a match, the physical interface is bound to the dialer profile that returned a match. If this step fails as well, proceed with the further binding attempt known as a complete match.

• If PPP authentication is configured on the physical interface, the call is answered, and the caller is authenticated. In this case, the authenticated name is used to match the dialer profile that contains the same name in its configuration. Again, the only profiles that are checked are those that are associated with the same pools of which the called physical interface is a member. If the check returns a match, the physical interface is bound to the found dialer interface. If the complete match fails, the binding cannot occur, and the call is disconnected.

You might have realized that for the last binding attempt to be successful, the physical interface needs to have PPP encapsulation and PPP authentication enabled. Also, the physical interface engages in PPP Link Control Protocol (LCP) layer negotiations before binding to a profile. This means that if a dialer profile is using Multilink PPP, the physical interface must be configured for Multilink PPP as well because LCP negotiations might take place before the dialer profile is located.

After the bind has occurred, this does not mean that the connection has occurred as well. Just because the physical interface found the logical configuration to use, this does not imply that the call cannot be disconnected for other reasons. One such reason can be the maximum threshold configured for inbound calls. When the NAS locates an appropriate profile for an incoming call, it checks whether the profile has reached its maximum connection limit. If the current incoming call puts the profile's connection limit over its configured maximum, the call is disconnected.

Dialer Profile Limitations

Dialer profiles have certain limitations:

• Dialer profiles do not support dynamic encapsulation.

• The only supported encapsulation types are PPP and HDLC. X.25 and Frame Relay are not currently supported.

• The physical and dialer interfaces both require PPP authentication to be enabled.

• The maximum threshold for incoming calls is checked only after the call has been answered, so the charge applies regardless of whether the call is later disconnected because of the exceeded limit.

• Each dialer interface takes up an interface description block (IDB). IDB is an internal structure that manages an interface. Because a limited number of IDBs are available (the exact number depends on the hardware platform), dialer profiles might have certain scalability constraints.