3.1 PPP Architecture
3.1.7 Configuring the Interface-Addressing Method for Remote Devices
You can control whether addressing is dynamic (the user specifies the address at the EXEC level when making the connection), or whether default addressing is used (the address is forced by the system). If you specify dynamic addressing, the router must be in interactive mode and the user enters the address at the EXEC level.

It is common to configure an asynchronous interface to have a default address and to allow dynamic addressing. With this configuration, the choice between the default address or dynamic addressing is made by the users when they enter the ppp EXEC level command. If the user enters an address, it is used; if the user enters the default keyword, the default address is used.

You can, therefore, configure the router to do one of the following:

  • Assign a default async address

or

  • Allow an async address to be assigned dynamically by the caller

Assigning a Default Async Address
To assign a predefined default IP address to the remote client node that dials in to the corresponding asynchronous line, use the peer default ip address command as shown in the Figure. Additionally, the pool and dhcp arguments allow address allocation from a local pool of addresses or a Dynamic Host Configuration Protocol (DHCP) server. The peer default ip address command first appeared in IOS 11.0. Before 11.0, the async interface command was used instead (for example, async default ip address address).

Router(config-if)#peer default ip address{ip-address | dhcp | pool poolname}

Additionally, the pool and dhcp options to the peer default ip address command require a global command to create the pool of addresses; for example, ip local pool pool-name starting-address end-address.

DHCP Consideration
If the peer default ip dhcp option is chosen, you have to also configure ip helper address and ip dhcp-server.

Allowing an Async Address to Be Assigned Dynamically
The async dynamic address allows the remote dial-in client to enter its own IP address, if it has one:

Router(config-if)#async dynamic address

When a line is configured for dynamic assignment of asynchronous addresses, the user enters the slip or ppp EXEC command, and is prompted for an address or logical host name. Assigning asynchronous addresses dynamically is also useful when you want to assign set addresses to users. For example, an application on a personal computer that automatically dials in using SLIP and polls for electronic mail messages can be set up to dial in periodically, and then enter the required IP address and password.