Cisco Systems 78-11741-02 Configuring Lookup of DNS Names, Controlling Default Metrics, IPC-232

Configuring OSPF

Configuring Lookup of DNS Names

Configuring Lookup of DNS Names

You can configure OSPF to look up Domain Naming System (DNS) names for use in all OSPF show EXEC command displays. This feature makes it easier to identify a router, because the router is displayed by name rather than by its router ID or neighbor ID.

To configure DNS name lookup, use the following command in global configuration mode:

Command	Purpose
Router(config)# ip ospf name-lookup	Configures DNS name lookup.

Forcing the Router ID Choice with a Loopback Interface

OSPF uses the largest IP address configured on the interfaces as its router ID. If the interface associated with this IP address is ever brought down, or if the address is removed, the OSPF process must recalculate a new router ID and resend all its routing information out its interfaces.

If a loopback interface is configured with an IP address, the Cisco IOS software will use this IP address as its router ID, even if other interfaces have larger IP addresses. Because loopback interfaces never go down, greater stability in the routing table is achieved.

OSPF automatically prefers a loopback interface over any other kind, and it chooses the highest IP address among all loopback interfaces. If no loopback interfaces are present, the highest IP address in the router is chosen. You cannot tell OSPF to use any particular interface.

To configure an IP address on a loopback interface, use the following commands beginning in global configuration mode:

	Command	Purpose
Step 1
	Router(config)# interface loopback 0	Creates a loopback interface, which places the router
		in interface configuration mode.
Step 2
	Router(config-if)# ip address ip-address mask	Assigns an IP address to this interface.

Controlling Default Metrics

In Cisco IOS Release 10.3 and later releases, by default OSPF calculates the OSPF metric for an interface according to the bandwidth of the interface. For example, a 64-kbps link gets a metric of 1562, while a T1 link gets a metric of 64.

The OSPF metric is calculated as the ref-bwvalue divided by the bandwidth value, with the ref-bwvalue equal to 108 by default, and the bandwidth value determined by the bandwidth interface configuration command. The calculation gives FDDI a metric of 1. If you have multiple links with high bandwidth, you might want to specify a larger number to differentiate the cost on those links. To do so, use the following command in router configuration mode:

Command		Purpose
Router(config-router)#auto-cost reference-bandwidth		Differentiates high bandwidth links.
ref-bw
	Cisco IOS IP Configuration Guide

IPC-232