Floating Static Routes as Backup

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Floating Static Routing Cisco

Floating static routes are static routes that have an administrative distance greater than the administrative distance of dynamic routes. The administrative distance can be configured on a static route so that the static route is less desirable than a dynamic route, and the static route is not used when the dynamic route is available. However, if the dynamic route is lost, the static route can take over and traffic can be sent through this alternate route. If the alternate route is provided by a DDR interface, the DDR can then be used as a backup mechanism.

Note The administrative distance values of some common Interior Gateway Routing Protocols

(IGRPs) are: EIGRP: 90, IGRP: 100, OSPF: 110, Routing Information Protocol (RIP): 120, and External EIGRP: 170.

In the previous example, the dynamic primary route to the central site Ethernet network, 10.1.2.0, is over the Frame Relay network, 10.1.4.0. A floating static route over the ISDN network, 10.1.5.0, is configured with the administrative distance of 130. However, the route over the ISDN network will only be used to get to network 10.1.2.0 if the Frame Relay network is down because the administrative distance is set higher on the ISDN connection.

Floating static routes are independent of line protocol status. The line protocol of a Frame Relay multipoint interface may not go down if the PVC becomes inactive. This situation defeats the purpose of configuring backup interfaces. A failed PVC may not bring down a line protocol status; thus, dynamic routes will not be flushed from the routing table. The floating static route with a higher administrative distance will not be installed in the routing table of that router.

To configure a floating static route, establish a static route for a designated network by specifying a higher administrative distance than that of the dynamic routing protocol. Use the ip route command to configure a floating static route. The ip route command arguments are listed in the table.

ip route Command Arguments

Command

Description

Network-number

IP address of the target network or subnet

Network-mask

Network mask that lets you mask network and subnetwork bits

IP address

IP address of the next hop that can be used to reach that network in standard IP address notation. Example 1.1.1.1

Interface

Network interface to use

Distance

(Optional) An administrative distance, which is a rating of the trustworthiness of a routing information source, such as an individual router or a group of routers

Dialer Watch as Backup

This topic describes how to use dialer watch as a backup connection that activates upon primary line failures.

Using Dialer Watch as Backup

© 2004 Cisco Systems, In

As an alternative to floating static routes, you can use the dialer watch commands. Dialer watch is a backup feature that integrates dial backup with routing capabilities. Dialer watch provides reliable connectivity without relying solely on defining interesting traffic to trigger outgoing calls to the central site router. Hence, dialer watch can also be considered regular DDR with no requirement for interesting traffic, just lost routes. By configuring a set of watched routes that define the primary interface, you are able to monitor and track the status of the primary interface as watched routes are added and deleted.

The figure shows the configuration of the branch site using dialer watch to monitor the network 10.1.2.0/24 coming from the central site. This network and mask must be an exact match or dialer watch will fail.

With dialer watch, the router monitors the existence of a specified route and if that route is not present, it initiates dialing of the backup link. Unlike the other backup methods (such as backup interface or floating static routes) dialer watch does not require interesting traffic to trigger the dial. Instead it triggers a dial backup call when a watched route is deleted from the routing table.

When a monitored network is deleted from the routing table of a dialer watch router, the router checks for another valid route for the lost network. If an alternate valid route using a nonbackup interface exists for a deleted watched network, the primary link is considered active and the backup link is not initiated. However, if there is no valid route, the primary line is considered down and unusable, and the router then initiates a dial backup call. Upon activation of the secondary link, the router forwards all traffic destined for the remote network over the backup link.

After the dial backup link is initialized, the router checks to see if the primary link has been reestablished after each idle timeout period. If the router finds that the primary link remains down, the idle timer resets and the backup link remains active. As soon as the primary link is re-established, the router updates its routing table and routes traffic over the primary link. Because traffic is no longer routed over the dialup connection, the backup link deactivates as the idle timeout expires.

Note Dialer watch is supported with IGRP, EIGRP, and OSPF routing protocols only.

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