Demand-Dial Routing

Types of Demand-Dial Connections

Demand-dial connections can be characterized in the following ways:

On-demand or persistent
Two-way initiated or one-way initiated

The decision to use one or the other impacts the configuration of the demand-dial interface.

On-Demand and Persistent Connections

On-demand connections are used when the cost of using the communications link is time-sensitive. For example, long distance analog phone calls are charged on a per-minute basis. With on-demand connections, the connection is made when traffic is forwarded, and the connection is terminated after a configured amount of idle time.

Idle disconnect behavior can be configured on the calling router and the answering router:

To configure idle disconnect on the calling router, the router initiating the connection, set an idle disconnect time on the General tab of the properties of the demand-dial interface.
To configure idle disconnect on the answering router, the router accepting the connection attempt, set an idle disconnect time on the Dial-In Constraints tab of the profile properties of remote access policy being used by the demand-dial connection.

Persistent demand-dial connections use a dial-up WAN technology when the cost of the link is fixed and the connection can be active 24 hours a day. Examples of WAN technologies for persistent demand-dial connections include local calls that use analog phone lines, leased analog lines, and flat-rate ISDN. If a persistent connection is lost, the calling router immediately attempts to reestablish the connection.

Persistent connection behavior must be configured on the calling router and the answering router.

To configure connection persistence on the calling router

In the Routing and Remote Access snap-in, right-click the demand-dial interface under Routing Interfaces, and then click Properties.
On the Options tab, select Persistent connection.

To configure connection persistence on the answering router

In the Routing and Remote Access snap-in, right-click the remote access policy being used by the demand-dial connection under Remote Access Policies, and then click Properties.
Click Edit profile.
On the Dial-in Constraints tab, clear the Disconnect if idle for check box.

Both the calling router and the answering router must be configured for connection persistence. If the calling router is configured for a persistent connection and the answering router is configured for an idle disconnect, then the answering router terminates the connection after the specified idle time. The calling router must then reestablish the demand-dial connection, causing a pause in the forwarding of packets equal to the connection establishment delay.

Two-Way and One-Way Initiated Connections

With two-way initiated connections, either router can be the answering router or the calling router depending on who is initiating the connection. Both routers must be configured to initiate and accept a demand-dial connection. You use two-way initiated connections when traffic from either router can create the demand-dial connection. Two-way initiated demand-dial connections require that:

Both routers are configured as LAN and WAN routers.
User accounts are added for both routers so that the authentication credentials of the calling router are accessed and validated by the answering router.
Demand-dial interfaces, with the same name as the user account that is used by the calling router, are fully configured at both routers, including the phone number of the answering router and user account credentials to authenticate the calling router.
Static routes are configured on both routers.

For two-way initiated demand-dial routing to work properly, the user name of the calling router must match the name of a demand-dial interface on both sides of the connection. Table 6.1 shows an example of this configuration.

Table 6.1 Example of Two-Way Initiated Connection Configuration

Router	Demand-Dial Interface Name	User Account Name

Corporate office router	NewYorkRouter	CorpHub
Branch office router	CorpHub	NewYorkRouter

With one-way initiated connections, one router is always the answering router and one router is always the calling router. In one-way initiated connections, the routing configuration is simplified because user accounts, demand-dial interfaces, and static IP routes do not need to be fully configured on both sides of the connection. Instead of configuring a demand-dial interface and static routes on the answering router, static routes are added to the dial-in properties of the user account of the calling router.

When the connection is made, the static routes on the user account of the calling router are added to the IP routing table of the answering router. If routing protocols are used to propagate the new static routes, then there is a delay between the time the connection is made and the time when all of the routers on the intranet of the answering router are aware of the new route. Therefore, hosts on the intranet of the calling router might experience a delay between the time that the connection is made and the time when they begin to receive traffic back from hosts on the intranet of the answering router.

If your answering router is in a Windows 2000 mixed-mode domain or in a Microsoft® Windows NT® version 4.0 domain, static routes on the user account are not available. In this case, one-way initiated connections require that:

Both routers are configured as LAN and WAN routers.
A user account is added for the authentication credentials of the calling router.
A demand-dial interface is configured at the calling router with the user credentials of the user account. A demand-dial interface is configured at the answering router with the same name as the user account that is used by the calling router. The demand-dial interface of the answering router is not used to dial out, therefore it is not configured with the phone number of the calling router or with valid user credentials.