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What Are OSPF Areas and Virtual Links?
Document ID: 13703

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Contents
Introduction Prerequisites Requirements Components Used Conventions What Are Areas, Stub Areas, and Not-So-Stubby Areas? Define a Stub Area Define a Totally Stub Area Define a Not-So-Stubby Area Define an NSSA Totally Stub Area Normal, Stub, Totally Stub and NSSA Area Differences What Are Virtual Links? NetPro Discussion Forums - Featured Conversations Related Information

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Introduction
This document describes Open Shortest Path First (OSPF) areas and virtual links.

Prerequisites
Requirements
There are no specific requirements for this document.

Components Used
This document is not restricted to specific software and hardware versions.

Conventions
For more information on document conventions, refer to the Cisco Technical Tips Conventions.

http://www.cisco.com/warp/customer/104/8.html

12/15/2005

Each non-backbone area must be directly connected to the backbone area (though this connection might be a simple logical connection through a virtual link. route summarization and filtering is possible between different areas.0) is inserted into the stub area in order to reach these external routes. You can define totally stubby areas. Routers in totally stubby areas keep their LSDB-only information about routing within their area. Stub Areas. If there are multiple areas in your network. An area border router is in the OSPF boundary between two areas.0 is equal to area 0.0. and you'll need to configure virtual links to repair the partition. plus the default route.Cisco . A router with interfaces in two (or more) different areas is an area border router. it must be a contiguous area. external link states make up a large percentage of the link states in the databases of every router. you need to name the backbone area "area 0". It is not possible to do route update filtering within an area. however.. Each OSPF network that is divided into different areas must follow these rules: A backbone area—which combines a set of independent areas into a single domain—must exist. A stub area is an area in which you don't allow advertisements of external routes. a default summary route (0. thereby reducing the size of its database. and Not-So-Stubby Areas? An OSPF network can be divided into sub-domains called areas. Stub areas are shielded from external routes but receive information about networks that belong to other areas of the same OSPF domain. An area is a logical collection of OSPF networks. Caution: Some of the routers in your network can have partial routing information. The router doesn't have detailed information about network topology outside of its area. Both sides of any link always belong to the same OSPF area. The backbone area must not be partitioned—divided into smaller pieces—under any failure conditions. Instead. Areas are identified by an area ID. routers.0. Since this backbone connects the areas in your network. External routes are the routes redistributed into OSPF from any other protocol.0.com/warp/customer/104/8. Cisco IOS® software supports area IDs expressed in IP address format or decimal format. The link-state database (LSDB) of routers within the same area must be synchronized and be exactly the same.cisco. which negatively compromises your network. and links that have the same area identification. If the backbone is partitioned. In many cases. area 0. An autonomous system boundary router (ASBR) advertises external destinations throughout the OSPF autonomous system. for example. http://www.0. such as link or router down events. then you have no need to define stub areas. thus reducing the size of the database even more. if you do not follow these rules. parts of the autonomous system will be unreachable. Areas limit the scope of route information distribution.html 12/15/2005 . A router within an area must maintain a topological database for the area to which it belongs.What Are OSPF Areas and Virtual Links? Page 2 of 6 What Are Areas. ). The main benefit of creating areas is a reduction in the number of routes to propagate—by the filtering and the summarization of routes. If you have no external routes in your network.

except that they can only be flooded into the NSSA. In the topology shown below. To define area 7 as a stub area. routers in area 7 do not need to know about all the external destinations. NSSAs are more flexible than stub areas in that an NSSA can import external routes into the OSPF routing domain. in the topology below. configure the area 7 stub no summary command on the ABR. Like stub areas. In the previous network diagram. NSSAs (like stub areas) must be placed at the edge of an OSPF routing domain.com/warp/customer/104/8. The routers in area 7 must send a packet to the ABR to reach the ASBR. Area 7 can be defined as a stub area. they prevent the flooding of AS-external link-state advertisements (LSAs) into NSSAs. no matter what the external destination. Define a Not-So-Stubby Area Use the OSPF router configuration area xx nssa command to define an NSSA. Area 7 can be defined as a totally stub area. In order to further propagate the NSSA external information. but still allows for the attachment of the non-OSPF router.cisco. the Type-7 LSA must be translated to a Type-5 AS-external-LSA by the NSSA ABR. we configured Area 2 as an NSSA by entering the area 2 nssa command on all routers in the area. Type-7 LSAs are similar to Type-5 ASexternal LSAs. configure the area 7 stub command in all routers in that area. relying instead on default routing to external destinations. As a result.2 and later. Define a Stub Area Use the area xx stub command in every router in the area to define an area as a stub area. This protects Area 2's internal routers from all the AS-external LSAs imported by the OSPF ASBR. thereby providing transit service to small routing domains that are not part of the OSPF routing domain. For example.html 12/15/2005 . NSSA is supported in Cisco IOS 11. Define a Totally Stub Area Use the OSPF router configuration area xx stub no-summary command to define a totally stub area. Refer to the OSPF Database Explanation Guide to learn more about the OSPF database. External routing information is imported into an NSSA in Type-7 LSAs. routers in area 7 do not need to know about all external destinations or any summary LSA from the backbone for other areas.Cisco .What Are OSPF Areas and Virtual Links? Page 3 of 6 Not-so-stubby areas (NSSAs) are an extension of OSPF stub areas. http://www. The routers in area 7 must send packets to the ABR to reach any destination outside the area 7. To define area 7 as a totally stub area.

Normal. As mentioned above. In the previous network diagram. http://www.com/warp/customer/104/8.Cisco . 4 or 5 LSAs except the default summary route.What Are OSPF Areas and Virtual Links? Page 4 of 6 Define an NSSA Totally Stub Area Use the OSPF router configuration command area xx nssa no-summary command to define an NSSA totally stub area. What Are Virtual Links? All areas in an OSPF autonomous system must be physically connected to the backbone area (area 0). we configured area 2 as NSSA totally stub by entering the area 2 nssa no-summary command on the NSSA ABR. The area through which you configure the virtual link.cisco. known as a transit area. 4 or 5 LSAs allowed except the default summary route No Type 5 AS-external LSAs allowed. In some cases where this physical connection is not possible. but Type 7 LSAs that convert to Type 5 at the NSSA ABR can traverse No Type 3.html 12/15/2005 . Stub. The transit area cannot be a stub area. you can also use virtual links to connect two parts of a partitioned backbone through a non-backbone area. but Type 7 LSAs that convert to Type 5 at the NSSA ABR are allowed Restriction Refer to the Types of OSPF Areas section of How Does OSPF Generate Default Routes? to learn more about different types of areas. Totally Stub and NSSA Area Differences This table shows the differences between the types of areas defined in this document: Area Normal Stub Totally Stub NSSA NSSA Totally Stub None No Type 5 AS-external LSA allowed No Type 3. This keeps any Type 5 ASexternal or Type 3 summary routes from leaking in area 2. must have full routing information. you can use a virtual link to connect to the backbone through a non-backbone area.

2. NetPro Discussion Forums .Dec 14. where the area-id is the area ID assigned to the transit area (this can be either a valid IP address or a decimal value).Dec 8.What Are OSPF Areas and Virtual Links? Page 5 of 6 Use the area area-id virtual-link router-id command to configure a virtual link.Cisco .1. 2005 Connecting to a different subnet when using vpn . 2005 Unable to receive print requests using VPN .2. and information about networking solutions. Refer to Configuring OSPF Authentication on a Virtual Link for more information. and technologies. 2005 Unable to telnet inside routers from pptp client .Dec 14. .1.2.2.html 12/15/2005 .Dec 14.2.Dec 8.2 subcommand on router 1. suggestions.Featured Conversations Networking Professionals Connection is a forum for networking professionals to share questions.Dec 14. 2005 MPLS : IGP route aggregation and broken LSPs.Dec 14.1.2. 2005 VRF traffic via MPLS TE tunnel .2.2.1 subcommand on router 2.Featured Conversations for RP Service Providers: MPLS MPLS-VPN.cisco. 2005 Cisco SSL WebVPN Windows Domain/AD Logon .. 2005 VPN-Client and Aladdin USB Token . PE-CE L3 routing Info doubt ? . NetPro Discussion Forums .1 and router ID 2. configure the area 5 virtual-link 2.com/warp/customer/104/8.1.Dec 14. 2005 Vpn client can access internet but cannot access internal network .Dec 13. In this example. To create the virtual link. . 2005 http://www.Dec 14.Dec 14.1 and the area 5 virtual-link 1. 2005 Virtual Private Networks: Security how can i use asdm access ssm . 2005 Vrf-Lite with MPLS requires a PE at the customer side? . the virtual link is created between the routers with router ID 1.Dec 14. the virtual link connects area 7 to the backbone through area 5: In this example.1. 2005 Virtual Private Networks: Services VPN error message . The featured links are some of the most recent conversations available in this technology.Dec 14. products.Dec 11. and where router-id is the router ID associated with the virtual link neighbor. 2005 Router As VPN server and Multicasting.Dec 13. 2005 Why do providers like BGP for CE-PE connectivity . 2005 Fixup for VPN getting error .1.

Important Notices and Privacy Statement. Updated: Aug 10.cisco.What Are OSPF Areas and Virtual Links? Page 6 of 6 Related Information OSPF Support Page OSPF Database Explanation Guide Technical Support .html 12/15/2005 .Cisco .com/warp/customer/104/8. Inc. 2005 Document ID: 13703 http://www.Cisco Systems All contents are Copyright © 1992-2005 Cisco Systems. All rights reserved.

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