Cisco Systems 78-11741-02 manual Changing the Ospf Administrative Distances, IPC-233

Configuring OSPF

Changing the OSPF Administrative Distances

Changing the OSPF Administrative Distances

An administrative distance is a rating of the trustworthiness of a routing information source, such as an individual router or a group of routers. Numerically, an administrative distance is an integer from 0 to

255.In general, the higher the value, the lower the trust rating. An administrative distance of 255 means the routing information source cannot be trusted at all and should be ignored.

OSPF uses three different administrative distances: intra-area, interarea, and external. Routes within an area are intra-area; routes to another area are interarea; and routes from another routing domain learned via redistribution are external. The default distance for each type of route is 110.

To change any of the OSPF distance values, use the following command in router configuration mode:

Command	Purpose
Router(config-router)#distance ospf {[intra-area dist1] [inter-area	Changes the OSPF distance values.
dist2] [external dist3]}

For an example of changing administrative distance, see the section “Changing OSPF Administrative Distance Example” at the end of this chapter.

Configuring OSPF on Simplex Ethernet Interfaces

Because simplex interfaces between two devices on an Ethernet represent only one network segment, for OSPF you must configure the sending interface to be a passive interface. This configuration prevents OSPF from sending hello packets for the sending interface. Both devices are able to see each other via the hello packet generated for the receiving interface.

To configure OSPF on simplex Ethernet interfaces, use the following command in router configuration mode:

Command	Purpose
Router(config-router)# passive-interface	Suppresses the sending of hello packets through the specified
interface-type interface-number	interface.

Configuring Route Calculation Timers

You can configure the delay time between when OSPF receives a topology change and when it starts a shortest path first (SPF) calculation. You can also configure the hold time between two consecutive SPF calculations. To do so, use the following command in router configuration mode:

Command	Purpose
Router(config-router)# timers spf spf-delay	Configures route calculation timers.
spf-holdtime

Cisco IOS IP Configuration Guide

IPC-233