Understanding Dial-on-Demand Routing Backup

Dial-on-demand routing (DDR) backup is a feature that provides a J Series device with full-time connectivity across an ISDN line. You configure dial-on-demand routing using the following features:

• Dialer filters—A dialer filter is a stateless firewall filter that enables dial-on-demand routing backup when applied to a physical ISDN interface and its dialer interface configured as a passive static route. The passive static route has a lower priority than dynamic routes, meaning that the static route is used only when the dynamic routes are no longer available.

  When a floating static route is configured on an interface with a dialer filter, the interface can be used for backup. A floating static route is a route with an administrative distance greater than the administrative distance of the dynamically learned versions of the same route.

  If all dynamic routes on a primary serial T1, E1, T3, E3, Fast Ethernet, Gigabit Ethernet, or PPPoE interface to an address are lost from the routing table and the device receives a packet for that address, the dialer interface initiates an ISDN backup connection. The dialer interface then sends the packet over the backup connection. To save connection time costs, the device drops the ISDN connection after a configured period of inactivity.

  This dial-on-demand routing backup method allows an ISDN line to be activated only when network traffic configured as an “interesting packet” arrives on the network. Once the network traffic is sent, an inactivity timer is triggered and the connection is closed after the timer expires. You define an interesting packet as part of the dialer filter feature configuration.

• Demand circuits—A demand circuit is a network segment whose cost varies with usage, according to a service level agreement with a service provider. Demand circuits limit traffic based on either bandwidth (bites or packets transmitted) or access time. For example, ISDN interfaces can be configured for dial-on-demand routing backup. In OSPF, the demand circuit reduces the amount of OSPF traffic by removing all OSPF protocols when the routing domain is in a steady state.

  Two types of routing protocol traffic are used by OSPF to establish and maintain network structure. First, periodic hello packets are sent over each link for neighbor discovery and maintenance. Second, OSPF protocol traffic achieves and maintains link-state database synchronization between devices. The OSPF demand circuit feature removes the periodic nature of both traffic types and reduces the amount of OSPF traffic by removing all OSPF protocol traffic from a demand circuit when the routing domain is in a steady state. This feature allows the underlying data-link connections to be closed when no application traffic is on the network.

Related Topics

• Junos OS Feature Support Reference for SRX Series and J Series Devices