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OSPF Area Transit Capability

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OSPF Area Transit Capability

Contents

Information About OSPF Area Transit Capability

How the OSPF Area Transit Capability Feature Works

How to Disable OSPF Area Transit Capability

Disabling OSPF Area Transit Capability on an Area Border Router

Additional References

Related Documents

Standards

MIBs

RFCs

Technical Assistance

Command Reference

capability transit

Feature Information for OSPF Area Transit Capability


OSPF Area Transit Capability

First Published: January 27, 2004
Last Updated: July 13, 2007

The OSPF Area Transit Capability feature provides an OSPF Area Border Router (ABR) with the ability to discover shorter paths through the transit area for forwarding traffic that would normally need to travel through the virtual-link path. This functionality allows Cisco IOS software to be compliant with RFC 2328.

Finding Feature Information in This Module

Your Cisco IOS software release may not support all of the features documented in this module. To reach links to specific feature documentation in this module and to see a list of the releases in which each feature is supported, use the "Feature Information for OSPF Area Transit Capability" section.

Finding Support Information for Platforms and Cisco IOS and Catalyst OS Software Images

Use Cisco Feature Navigator to find information about platform support and Cisco IOS and Catalyst OS software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Contents

•Information About OSPF Area Transit Capability

•How to Disable OSPF Area Transit Capability

•Additional References

•Command Reference

•Feature Information for OSPF Area Transit Capability

Information About OSPF Area Transit Capability

To use the OSPF Area Transit Capability feature, you should understand the concept in the following section:

•How the OSPF Area Transit Capability Feature Works

How the OSPF Area Transit Capability Feature Works

The OSPF Area Transit Capability feature is enabled by default. RFC 2328 defines OSPF area transit capability as the ability of the area to carry data traffic that neither originates nor terminates in the area itself. This capability enables the OSPF ABR to discover shorter paths through the transit area and forward traffic along those paths rather than using the virtual link or path, which are not as optimal.

For a detailed description of OSPF area transit capability, see RFC 2328, OSPF Version 2, at the following URL:

http://www.faqs.org/rfcs/rfc2328.html

How to Disable OSPF Area Transit Capability

This section contains the following procedure:

•Disabling OSPF Area Transit Capability on an Area Border Router (required)

Disabling OSPF Area Transit Capability on an Area Border Router

This task describes how to disable the OSPF Area Transit Capability feature on an OSPF ABR.

SUMMARY STEPS

1. enable

2. configure terminal

3. router ospf process-id [vrf vpn-name]

4. no capability transit

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

•Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

router ospf process-id [vrf vpn-name]

Example:

Router(config)# router ospf 100

Enables OSPF routing and enters router configuration mode.

•The process-id argument identifies the OSPF process.

Step 4 

no capability transit

Example:

Router(config-router)# no capability transit

Disables OSPF area capability transit on all areas for a router process.

Additional References

The following sections provide references related to the OSPF Area Transit Capability feature.

Related Documents


Standards

Standard
Title

None

—


MIBs

MIB
MIBs Link

None

To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:

http://www.cisco.com/go/mibs


RFCs

RFC
Title

RFC 2328

OSPF Version 2


Technical Assistance

Description
Link

The Cisco Support website provides extensive online resources, including documentation and tools for troubleshooting and resolving technical issues with Cisco products and technologies.

To receive security and technical information about your products, you can subscribe to various services, such as the Product Alert Tool (accessed from Field Notices), the Cisco Technical Services Newsletter, and Really Simple Syndication (RSS) Feeds.

Access to most tools on the Cisco Support website requires a Cisco.com user ID and password.

http://www.cisco.com/techsupport


Command Reference

This section documents only commands that are new or modified.

•capability transit

capability transit

To reenable Open Shortest Path First (OSPF) area capability transit after it has been disabled, use the capability transit command in router configuration mode. To disable OSPF area capability transit on all areas for a router process, use the no form of this command.

capability transit

no capability transit

Syntax Description

This command has no arguments or keywords.

Command Default

OSPF area capability transit is enabled.

Command Modes

Router configuration

Command History

Release
Modification

12.0(27)S

This command was introduced.

12.3(7)T

This command was integrated into Cisco IOS Release 12.3(7)T.

12.2(25)S

This command was integrated into Cisco IOS Release 12.2(25)S.

12.2(27)SBC

This command was integrated into Cisco IOS Release 12.2(27)SBC.

12.2(33)SRA

This command was integrated into Cisco IOS Release 12.2(33)SRA.

12.2(33)SXH

This command was integrated into Cisco IOS Release 12.2(33)SXH.


Usage Guidelines

OSPF area capability transit is enabled by default, allowing the OSPF Area Border Router to install better-cost routes to the backbone area through the transit area instead of the virtual links. If you want to retain a traffic pattern through the virtual-link path, you can disable capability transit by entering the no capability transit command. If paths through the transit area are discovered, they are most likely to be more optimal paths, or at least equal to, the virtual-link path. To reenable capability transit, enter the capability transit command.

If you need to verify whether OSPF area capability transit is enabled for a specific routing process, enter the show ip ospf command.

Examples

The following example shows how to disable OSPF area capability transit on all areas for a router process named ospf 1. A show ip ospf command is issued first to display the current areas that have area capability transit enabled. The no capability transit command is then entered to disable OSPF area capability transit on all areas for the router process ospf 1. 

Router# show ip ospf


 Routing Process "ospf 1" with ID 10.1.1.1

 Supports only single TOS(TOS0) routes

 Supports opaque LSA

 Supports Link-local Signaling (LLS)

!Supports area transit capability

It is an area border router

 Initial SPF schedule delay 5000 msecs

 Minimum hold time between two consecutive SPFs 10000 msecs

 Maximum wait time between two consecutive SPFs 10000 msecs

 Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs

 LSA group pacing timer 240 secs

 Interface flood pacing timer 33 msecs

 Retransmission pacing timer 66 msecs

 Number of external LSA 8. Checksum Sum 0x02853F

 Number of opaque AS LSA 0. Checksum Sum 0x000000

 Number of DCbitless external and opaque AS LSA 0

 Number of DoNotAge external and opaque AS LSA 0

 Number of areas in this router is 2. 2 normal 0 stub 0 nssa

!Number of areas transit capable is 1

External flood list length 0

    Area BACKBONE(0)

        Number of interfaces in this area is 3

        Area has no authentication

        SPF algorithm last executed 00:02:21.524 ago

        SPF algorithm executed 11 times

        Area ranges are

        Number of LSA 49. Checksum Sum 0x19B5FA

        Number of opaque link LSA 0. Checksum Sum 0x000000

        Number of DCbitless LSA 0

        Number of indication LSA 0

        Number of DoNotAge LSA 38

        Flood list length 0

    Area 1

        Number of interfaces in this area is 3

       !This area has transit capability: Virtual Link Endpoint

        Area has no authentication

        SPF algorithm last executed 00:02:36.544 ago

        SPF algorithm executed 9 times

        Area ranges are

        Number of LSA 42. Checksum Sum 0x1756D5

        Number of opaque link LSA 0. Checksum Sum 0x000000

        Number of DCbitless LSA 0

        Number of indication LSA 0

        Number of DoNotAge LSA 0

        Flood list length 0


Router(config)# router ospf 1

Router(router-config)# no capability transit

Related Commands

Command
Description

show ip ospf

Displays general information about OSPF routing processes.


Feature Information for OSPF Area Transit Capability

Table 1 lists the release history for this feature.

Not all commands may be available in your Cisco IOS software release. For release information about a specific command, see the command reference documentation.

Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which Cisco IOS and Catalyst OS software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Note Table 1 lists only the Cisco IOS software release that introduced support for a given feature in a given Cisco IOS software release train. Unless noted otherwise, subsequent releases of that Cisco IOS software release train also support that feature.

Note Software images for Cisco 12000 series Internet routers have been deferred to Cisco IOS Release 12.0(27)S1.

Table 1 Feature Information for OSPF Area Transit Capability 

Feature Name
Releases
Feature Information

OSPF Area Transit Capability

12.0(27)S
12.3(7)T
12.2(25)S
12.2(27)SBC
12.2(33)SRA
12.2(33)SXH

The OSPF Area Transit Capability feature provides an OSPF Area Border Router (ABR) the ability to discover shorter paths through the transit area for forwarding traffic that would normally need to travel through the virtual-link path. This functionality allows Cisco IOS software to be compliant with RFC 2328.

This feature was introduced in Cisco IOS
Release 12.0(27)S.

This feature was integrated into Cisco IOS
Release 12.3(7)T.

This feature was integrated into Cisco IOS
Release 12.2(25)S.

This feature was integrated into Cisco IOS
Release 12.2(27)SBC.

This feature was integrated into Cisco IOS
Release 12.2(33)SRA.

This feature was integrated into Cisco IOS
Release 12.2(33)SXH.


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Any Internet Protocol (IP) addresses used in this document are not intended to be actual addresses. Any examples, command display output, and figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses in illustrative content is unintentional and coincidental.

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