4.3 DDR Configuration  
  4.3.1 DDR operation  
Dial-on-demand routing (DDR) is triggered when traffic that matches a predefined set of criteria is queued to be sent out a DDR-enabled interface. The traffic that causes a DDR call to be placed is referred to as interesting traffic. Once the router has transmitted the interesting traffic, the call is terminated.

The key to efficient DDR operation is in the definition of interesting traffic. Interesting traffic is defined with the dialer-list command. Dialer lists can allow all traffic from a specific protocol to bring up a DDR link, or they can query an access list to see what specific types of traffic should bring up the link. Dialer lists do not filter traffic on an interface. Even traffic that is not interesting will be forwarded if the connection to the destination is active.

DDR is implemented in Cisco routers in the following steps:

  1. The router receives traffic, performs a routing table lookup to determine if there is a route to the destination, and identifies the outbound interface.
  2. If the outbound interface is configured for DDR, the router does a lookup to determine if the traffic is interesting.
  3. The router identifies the dialing information necessary to make the call using a dialer map to access the next-hop router.
  4. The router then checks to see if the dialer map is in use. If the interface is currently connected to the desired remote destination, the traffic is sent. If the interface is not currently connected to the remote destination, the router sends call-setup information through the BRI using the D channel.
  5. After the link is enabled, the router transmits both interesting and uninteresting traffic. Uninteresting traffic can include data and routing updates.
  6. The idle timer starts and runs as long as no interesting traffic is seen during the idle timeout period and disconnects the call based on the idler timer configuration.

The idle timer setting specifies the length of time the router should remain connected if no interesting traffic has been sent. Once a DDR connection is established, any traffic to that destination will be permitted. However, only interesting traffic resets the idle timer.

 

Web Links

DDR

http://www.cisco.com/en/US/tech/tk801/ tk133/tech_protocol_family_ home.html