Advanced OSPF Design Concepts

Previous Table of Contents Next

Design Scenario #4: Site Router Is in Two Routing Domains

This approach relies on one-way redistribution of multiple instances of a separate routing protocol into OSPF as shown in Figure 6-13. Auto-summarization must also be disabled in this scenario. Administrative distances should be tweaked to ensure that OSPF is the favored routing protocol. This approach has the advantage that interfaces may be shared among areas; that is, a dedicated set of interfaces for each area is not required.

Figure 6-13  Site Router in two routing domains.

Included are some sample configurations for this design scenario.

Router A Configuration

    interface ethernet 0      ip address    interface serial 0      ip address    backup interface serial 1      backup delay 0 5    interface serial 1      ip address    router ospf 1      network area 1      network area 1    router eigrp 1      network distance 200 

Router B Configuration

    interface fddi 0      ip address    interface serial 0      ip address    router ospf 1      network area 1      area 1 range      network area 0 

Router C Configuration

    interface fddi 0      ip address    interface serial 0      ip address    router ospf 1      network area 0      redistribute eigrp 1 subnets metric 32000    router eigrp 1      network    passive-interface fddi 0      distance 200 

The preceding examples work with dial-on-demand routing as well as dial-backup. One should set the metric on the backup interface to be less favorable than that on the primary. Also, be certain to set the administrative distance on the backup routing protocol to be greater than that of the primary (both of these so as to allow the idle timer to work). The redistribution of a static route for the backed-up site is absolutely necessary. Not only does it speed convergence somewhat, it is the controlling factor in directing traffic at the upstream dial-on-demand interface to trigger dialing. After the primary interface goes down, all knowledge of the site LAN is lost. If the backup server is originating routes for the site LANs at a much higher cost, these will now come into effect.

OSPF Configuration Commands

There are literally thousands of documented and undocumented commands that can be used on Cisco equipment. These commands are found in several different areas and, of course, differ by IOS. These differences might not be major in the case of OSPF, but I recommend you have a set for each version of IOS in your network.

  IOS Documentation Set (a minimum of 16 books)
  Technical Compact Disc (updated every month)
  World Wide Web (
  Command-line help (effectiveness varies by equipment)

At first glance, it might seem that the variety of information sources available is wonderful. But what the uninitiated does not realize is that the various commands are spread out within each source. There is no one link that takes you to all the OSPF commands that Cisco routers have available.

This chapter is concerned about OSPF and only the commands associated with its configuration, design, and operation. Specifically, this chapter deals with the various commands that can set up or alter the performance of OSPF within a network. This section will provide you with an exhaustive list of Cisco router configuration commands dealing specifically with the OSPF protocol. Hopefully, you will find this section to be a valuable resource. Of course, all of this information can be found with various Cisco sources as detailed above, but not all in one place!

The following section covers the various commands used within a Cisco router to configure OSPF authentication. These entries follow a specific format (where applicable):

  Command syntax
  Syntax description
  Default settings
  Command mode
  Usage guidelines
  Related commands

area authentication

To enable authentication for an OSPF area, use the area authentication router configuration command. To remove an area’s authentication specification or a specified area from the configuration, use the no form of this command.

 area area-id authentication [message-digest] no area area-id authentication no area area-id 

Syntax Description:

area-id. Identifier of the area for which authentication is to be enabled. The identifier can be specified as either a decimal value or an IP address.
message-digest (optional). This enables MD5 authentication on the area specified by area-ID.

When dealing with a multi-vendor network, you might have to disable the MD5 OSPF Authentication for proper operation. You will also want to remember that the authentication must be configured on the opposing side’s router for proper operation.

area default-cost

To specify a cost for the default summary route sent into a stub area, use the area default-cost router configuration command. To remove the assigned default route cost, use the no form of this command. The syntax for this command (and the no form) is as follows:

 area area-id default-cost cost no area area-id default-cost cost 

Syntax Description:

area-id. Identifier for the stub area. The identifier can be specified as either a decimal value or as an IP address.
cost. Cost for the default summary route used for a stub area. The acceptable value is a 24-bit number.

Default: Cost of 1.

Command Mode: Router configuration.

Previous Table of Contents Next

OSPF Network Design Solutions
OSPF Network Design Solutions
ISBN: 1578700469
EAN: 2147483647
Year: 1998
Pages: 200
Authors: Tom Thomas © 2008-2017.
If you may any questions please contact us: