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80 Chapter 4: Configuring Asynchronous Connections with Modems
to the discussion in this section that covers ip address pool). Note that it is quite common for
a large number of asynchronous interfaces to a common physical interface to be unnumbered
and to use an address pool to assign the network-layer addresses to the dial-up users.
The ip tcp header-compression passive command states that if the other DCE device sends
packets with header-compression, the interface understands and sends in kind but does not
initiate the compression.
The encapsulation ppp command declares the encapsulation method for the interface.
The bandwidth 38 command tells the routing protocol and the router (for statistics) the speed
of the line. This command has no affect on the actual negotiated speed of the modem or the
speed at which the DTE talks to the modem.
The async mode interactive command enables, once a connection is made, the dial-up user
access to the EXEC prompt.
The peer default ip address pool remaddpool command specifies that the IP address assigned
to the dial-up user be from the address grouping or pool defined by the label remaddpool. The
syntax for the pool definition, defined in global configuration mode, is as follows:
ip local pool remaddpool low-ip-pool-address high-ip-pool-address.
A unique address from the pool of addresses is given to a dial-up user for the duration of the
session. The address is returned to the pool when the dial-up user disconnects the session. In
this fashion, it is not necessary to associate an IP address with each asynchronous interface.
Each asynchronous interface to another interface on the router is unnumbered and the pool is
created from part of that interface's subnet. For more information and examples on the use of
address pools and unnumbering, refer to Chapter 6, "Using ISDN and DDR Technologies."
The no cdp enable command turns off the Cisco Discovery Protocol for the interface. By
default, this protocol is on, and because the interface is likely connected to a dial-up user who
does not understand CDP, the bandwidth it would use is saved.
The ppp authentication chap command specifies that the Challenge Handshake
Authentication Protocol (CHAP) be used on this link. Failure of the client to honor CHAP
results in the link not being established.
Physical Considerations on the Router
Physical characteristics are configured in line mode. These include the speed, the direction of
the call, modem setup, and so on. Example 4-6 shows a configuration used to connect to a USR
Sportster modem on physical line 2.