Multi-Area OSPF Multi-area OSPF networks can be difficult to design, and typically demand more administrative attention than any other popular interior gateway protocol.

OSPF Advantages OSPF routers in a heavily populated OSPF network can be overwhelmed by: high demand for router processing and memory resources, large routing tables, and large topology tables. OSPF allows splitting the network into manageable pieces, or areas, thereby allowing tremendous scalability.

Hierarchical Routing OSPF's capability to separate a large internetwork into multiple smaller internetworks, called areas, is referred to as hierarchical routing. With this technique interarea routing occurs, which is simply the process of exchanging routing information between OSPF areas. Interarea routing allows OSPF to summarize and contain area-specific information so that many of the smaller internal routing operations are restricted to one area.

Hierarchical Routing Advantages The hierarchical topology possibilities of OSPF yield several important advantages: Reduced frequency of SPF calculations Smaller routing tables Reduced LSU overhead

OSPF Router Types There are four different types of OSPF Routers : Internal router - routers that have all their interfaces within the same area with identical Link State databases and running the same routing algorithm. Backbone router - Routers attached to the backbone area of the OSPF network with at least one interface connected to Area 0. Area Border Router - routers with interfaces attached to multiple areas that maintain separate link-state databases for each area they are connected to. Areas can have one or more ABRs. Autonomous System Boundary Router - ASBRs - routers that have at least one interface connected to an external internetwork (another autonomous system), such as a non- OSPF network. A router can be more than one router type. If a router interconnects to Area 0 and Area 1, as well as to a non-OSPF network, it would be both an ABR and an ASBR.

OSPF Area Types Backbone area (Area 0) – A transit area, the central entity to which all other areas connect, all other areas must connect to this area to exchange route information. Accepts all LSAs. Stub area - Area that does not accept information about routes external to the autonomous system (AS), such as routes from non-OSPF sources. If routers need to reach networks outside the AS, they use a default route. Does not accept LSAs. Totally stubby area - Area that does not accept external AS routes and summary routes from other areas internal to the AS. Does not accept external or summary LSAs. A totally proprietary Cisco feature that uses a default route to reach networks outside the AS. Not-so-stubby areas - Area that is similar to a stub area but allows for importing external routes as Type 7 LSAs and translation of specific Type 7 LSA routes into Type 5 LSAs.

LSA types LSA1s build the Router Link States LSA2s build the Net Link States (O) LSA3s build the Summary Net Link States (O IA) LSA4s build the AS Summary Net Link States (IA) LSA5s build external route information (E1 or E2)

Stub and Totally Stub Areas You can configure an OSPF router interface to either operate in a stub area (does not accept information about routes external to the AS) or as a totally stubby area (does not accept external AS routes and summary routes from other areas internal to the AS). By configuring an area as stub, you can greatly reduce the size of the link-state database inside that area and, as a result, reduce the memory requirements of area routers. Stub areas are typically created when you have a hub- and-spoke topology, with the spokes (such as branch offices) configured as stub areas. Branch offices may not need to know about every network at the headquarters site but can instead use default routes to get there.

Is it Stubby? Or Totally Stubby? An area qualifies as stub or totally stubby when it meets the following criteria: There is a single exit point from that area. The area is not needed as a transit area for virtual links. No ASBR is internal to the stub area. The area is not the backbone area (Area 0). All these criteria are important because a stub/totally stubby area is configured primarily to exclude external routes.

Configuring Stub/Totally Stub Areas (See curriculum and 5.4.5) To configure a stub or totally stubby area, use the following on all router interfaces in that area: Router(config-router)#area area-id stub An optional no-summary keyword is added only on ABRs. This configures the ABR to block interarea routes (Type 3 and Type 4 LSAs). The no-summary creates a totally stubby area. The area stub command is configured on each router in the stub location and is essential for the routers to become neighbors and exchange routing information. On ABRs only, there is an option of defining a cost of the default route to be automatically injected in the stub/totally stubby area: Router(config-router)#area area-id default-cost cost

Not-so-stubby Areas (NSSAs) NSSAs are a relatively new, standards-based OSPF enhancement. Generally, NSSAs are the area located between a RIP/IGRP network and Area 0 of an OSPF network. This area allows LSA5s in, in a limited fashion. Data updates about the RIP or IGRP networks come in as LSA7s and are then switched into Area 0 via an ABR as LSA5s.

Configuring NSSAs To configure an area as a NSSA, you must configure all OSPF router interfaces that belong to the area using the following command syntax: Router(config-router)#area area-id nssa [no-summary] The no-summary keyword is used on the ABR and typically makes the NSSA totally stubby. To control the summarization or filtering during the translation using the following syntax: Router(config)#summary-address prefix mask [not- advertise] [tag tag] The not-advertise keyword is used to suppress routes that match the prefix/mask pair. This keyword applies to OSPF only. The tag value can also be assigned but is not required.

Configuring NSSAs (continued) The ABR does not generate default routes in an NSSA. To force the ABR to generate the default route, use this command only on the ABR for the NSSA: router ospf pid area id nssa default-information originate To verify that NSSA is defined on a given router, you can use the show ip ospf command

Virtual Links If a new area is added after the OSPF internetwork has been designed, and it is not possible to provide that new area with direct access to the backbone, a virtual link can be defined to provide the needed connectivity to the backbone area. Because all areas must be connected to Area 0, the virtual link provides the disconnected area a logical path to the backbone. The virtual link has the two requirements: It must be established between two routers that share a common area. One of these two routers must be connected to the backbone.

Reasons for Virtual Links They can link an area that does not have a physical connection to the backbone. For example, when two organizations merge. They can patch the backbone if discontinuity in Area 0 occurs. For example, if two companies merge their two separate OSPF networks into a single one with a common Area 0, the only alternative for the companies is to redesign the entire OSPF network and create a unified backbone. Virtual links can add redundancy in cases when router failure might cause the backbone to be split into two.

Configuring Virtual Links On each router that will use the virtual link, create the "virtual link" configuration. The routers that make the links are the ABR that connects the remote area to the transit area and the ABR that connects the transit area to the backbone area: router(config-router)#area area-id virtual-link router-id

Summarization Summarization is the consolidation of multiple routes into one single, supernet advertisement to conserve bandwidth, CPU, and memory resources. Summarization is done only on the ABR and ASBR.

Summarization commands: On the ABR – (config-router)#area id range address mask On the ASBR – (config-router)#summary-address address mask