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Multiple OSPF Areas

Multiple OSPF Areas. Just how many routers can an OSPF area support? Field studies have shown that a single OSPF area should not stretch beyond 50 routers, although there is no concrete limit. OSPF's capability to separate a large internetwork into multiple areas is referred to as hierarchical rout

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Multiple OSPF Areas

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    1. Multiple OSPF Areas Three issues can overwhelm an OSPF router in a heavily populated OSPF network: high demand for router processing and memory resources, large routing tables, and large topology tables. Fortunately, OSPF allows large areas to be separated into smaller, more manageable areas that can exchange summaries of routing information rather than exchange every detail.

    2. Multiple OSPF Areas Just how many routers can an OSPF area support? Field studies have shown that a single OSPF area should not stretch beyond 50 routers, although there is no concrete limit. OSPF's capability to separate a large internetwork into multiple areas is referred to as hierarchical routing. Hierarchical routing enables you to separate large internetworks into smaller internetworks that are called areas.

    3. Multiple OSPF Areas Interarea routing is the process of exchanging routing information between OSPF areas. The hierarchical topology possibilities of OSPF have several important advantages: Reduced frequency of SPF calculations. Smaller routing tables. Reduced link-state update (LSU) overhead.

    4. Multiple OSPF Areas Hierarchical routing increases routing efficiency because it allows you to control the type of routing information that flows into and out of an area.

    5. OSPF Routing Types Four different types of OSPF routers exist, Internal router- routers that have all their interfaces within the same area are called internal routers. Internal routers in the same area have identical link-state databases and run a single copy of the routing algorithm.

    6. OSPF Routing Types Backbone router- Routers that are attached to the backbone area of the OSPF network are called backbone routers. They have at least one interface connected to Area 0 (the backbone area). These routers maintain OSPF routing information using the same procedures and algorithms as internal routers.

    7. OSPF Routing Types Area Border Router (ABR) - ABRs are routers with interfaces attached to multiple areas. They maintain separate link-state databases for each area to which they are connected, and they route traffic destined to or arriving from other areas. ABRs are exit points for the area, which means that routing information destined for another area can travel there only via the local area's ABR.

    8. OSPF Routing Types ABRs summarize information about the attached areas from their link-state databases and distribute the information into the backbone. The backbone ABRs then forward the information to all other connected areas. An area can have one or more ABRs.

    9. OSPF Routing Types Autonomous System Boundary Router (ASBR) - ASBRs are routers that have at least one interface connected to an external internetwork (another autonomous system), such as a non-OSPF network. These routers can import non-OSPF network information to the OSPF network, and vice versa (this is referred to as redistribution.

    10. OSPF Routing Types A router can be more than one router type. For example, 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.

    11. OSPF Area Types Multiarea OSPF is scalable because a router's link-state database can include multiple types of LSAs. DRs (Designated Routers) and routers that reside in multiple areas or autonomous systems use special LSAs to send or summarize routing information.The characteristics that you assign to an area control the type of route information that it can receive.

    12. OSPF Area Types For example, you may want to minimize the size of routing tables in an OSPF area, in which case you can configure the routers to operate in an area that does not accept external routing information (Type 5 LSAs). Read about the 7 different LSA’s.

    13. OSPF Area Types Standard area - A standard area can accept link updates and route summaries. Backbone area (transit area) - When interconnecting multiple areas, the backbone area is the central entity to which all other areas connect. The backbone area is always Area 0. All other areas must connect to this area to exchange route information. The OSPF backbone has all the properties of a standard OSPF area.

    14. OSPF Area Types Stub area - A stub area is an area that does not accept information about routes external to the autonomous system (the OSPF internetwork), such as routes from non-OSPF sources. If routers need to reach networks outside the autonomous system, they use a default route. A default route is noted as 0.0.0.0/0.

    15. OSPF Area Types Totally stubby area - A totally stubby area is an area that does not accept external autonomous system (AS) routes and summary routes from other areas internal to the autonomous system. Instead, if the router needs to send a packet to a network external to the area, it sends it using a 0.0.0.0/0 default route. Totally stubby areas are a Cisco proprietary feature.

    16. OSPF Area Types Not-so-stubby area (NSSA) - An NSSA is an 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. 

    17. OSPF Area Types A key difference among these OSPF area types is the way they handle external routes. External routes are injected into OSPF by an ASBR. The ASBR may learn these routes from RIP or some other routing protocol. You can configure an ASBR to send out two types of external routes into OSPF: Type 1 (denoted in the routing table as E1) and Type 2 (E2).

    18. OSPF Area Types Depending on the type, OSPF calculates the cost of external routes differently, as follows: E1 - If a packet is an E1, then the metric is calculated by adding the external cost to the internal cost of each link that the packet crosses. You use this packet type when you have multiple ASBRs advertising a route to the same autonomous system.

    19. OSPF Area Types E2 - If a packet is an E2, then the packet will always have the external cost assigned, no matter where in the area it crosses (this is the default setting on ASBRs). You use this packet type if only one router is advertising a route to the autonomous system. Type 2 routes are preferred over Type 1 routes unless two equal cost routes exist to the destination.

    20. Configuring OSPF Across Multiple Areas This section summarizes how the different types of OSPF routers flood information and how they build their routing tables when operating within a multiarea environment. In Chapter 4, you saw that a packet destined for a network within an area is merely forwarded from one internal router to another until it reaches the destination network.

    21. Configuring OSPF Across Multiple Areas However, what if a packet must traverse multiple areas? For the OSPF routers to make routing decisions, they must build sufficient routing tables by exchanging LSUs. The LSU exchange process within a single OSPF area relies on just two LSA types-Type 1 and Type 2. To distribute routing information to multiple areas efficiently, Type 3 and Type 4 LSAs must be used by ABRs.

    22. Flooding LSU’s to Multiple Areas An ABR is responsible for generating routing information about each area to which it is connected and flooding the information through the backbone area to the other areas to which the backbone is connected. The general process for flooding follows these steps:

    23. Flooding LSU’s to Multiple Areas The routing processes occur within the area. The entire area must be synchronized before the ABR can begin sending summary LSAs to other areas.

    24. Flooding LSU’s to Multiple Areas The ABR reviews the resulting link-state database and generates summary LSAs (Type 3 or Type 4). By default, the ABR sends summary LSAs for each network that it knows about. To reduce the number of summary LSA entries, you can configure route summarization so that a single IP address can represent multiple networks. To use route summarization, your areas need to use contiguous IP addressing.

    25. Flooding LSU’s to Multiple Areas The summary LSAs are placed in an LSU and distributed through all ABR interfaces, with the following exceptions: If the interface is connected to a neighboring router that is in a state below the exchange state, then the summary LSA is not forwarded.

    26. Flooding LSU’s to Multiple Areas If the interface is connected to a totally stubby area, then the summary LSA is not forwarded. If the summary LSA includes a Type 5 (external) route and the interface is connected to a stub or totally stubby area, then the LSA is not sent to that area.

    27. Configuring OSPF Across Multiple Areas After an ABR or ASBR receives summary LSAs, it adds them to its link-state databases and floods them to the local area. The internal routers then assimilate the information into their databases.

    28. Configuring OSPF Across Multiple Areas Remember that OSPF enables you to configure different area types so that you can reduce the number of route entries that internal routers maintain. To minimize routing information, you can define the area as a stub area, a totally stubby area, or an NSSA.

    29. Updating the Routing Tables The order in which paths are calculated is as follows: All routers first calculate the paths to destinations within their area and add these entries into the routing table. These are learned via Type 1 and Type 2 LSAs.

    30. Updating the Routing Tables All routers then calculate the paths to the other areas within the internetwork. These paths are learned via interarea route entries, or Type 3 and Type 4 LSAs. If a router has an interarea route to a destination and an intra-area route to the same destination, the intra-area route is kept.

    31. Updating the Routing Tables All routers, except those that are in any of the stub area types, then calculate the paths to the AS external (Type 5) destinations.

    32. Configuring OSPF Components Configuring an ABR There are no special commands to make a router an ABR or an ASBR. The router becomes an ABR as soon as you configure two of its interfaces to operate in different areas.

    33. Configuring OSPF Components Configuring an ASBR ASBRs are created when you configure OSPF to import, or redistribute, external routes into OSPF. Ex. Redistribute Rip, This command tells OSPF to import RIP routing information.

    34. OSPF Route Summarization Recall that summarization is the consolidation of multiple routes into one single, supernet advertisement. Proper summarization requires contiguous (sequential) addressing (for example, 200.10.0.0, 200.10.1.0, 200.10.2.0, and so on). OSPF routers can be manually configured to advertise a supernet route, which is different from an LSA summary route.

    35. OSPF Route Summarization OSPF supports two types of summarization: Interarea route summarization - Interarea route summarization is done on ABRs and applies to routes from within each area. It does not apply to external routes injected into OSPF via redistribution. To take advantage of summarization, network numbers within areas should be contiguous.

    36. OSPF Route Summarization External route summarization - External route summarization is specific to external routes that are injected into OSPF via redistribution. Here again, it is important to ensure that external address ranges that are being summarized are contiguous. Summarization of overlapping ranges from two different routers could cause packets to be sent to the wrong destination. Only ASBRs can summarize external routes.

    37. OSPF Route Summarization To configure an ABR to summarize routes for a specific area before injecting them into a different area, you use the following syntax: Router(config-router)# area area-id range address mask. To perform interarea summarization:

    38. OSPF Route Summarization RTB(config)# router ospf 1 RTB(config-router)# area 1 range 192.168.16.0 255.255.252.0. Note that the area 1 range command in this example specifies the area containing the range to be summarized before being injected into Area 0.

    39. OSPF Route Summarization OSPF Route Summarization To configure an ASBR to summarize external routes before injecting them into the OSPF domain, you use the following syntax: Router(config-router)# summary-address address mask

    40. OSPF Route Summarization RTA(config)# router ospf 1 RTA(config-router)# summary-address 200.9.0.0 255.255.0.0

    41. OSPF Route Summarization Also, note that, depending on your network topology, you may not want to summarize area 0 networks. If you have more than one ABR between an area and the backbone area, for example, sending a summary LSA with the explicit network information will ensure that the shortest path is selected. If you summarize the addresses, a suboptimal path selection may occur.

    42. Using Stub and Totally Stubby 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).

    43. Using Stub and Totally Stubby Areas 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. Remember that stub areas do not accept Type 5 (that is, external) LSAs.

    44. Using Stub and Totally Stubby Areas Because OSPF routers internal to a stub area will not learn about external networks, routing to the outside world is based on a 0.0.0.0/0 default route. When you configure a stub area, the stub's ABR automatically propagates a 0.0.0.0/0 default route within the area.

    45. Using Stub and Totally Stubby Areas Stub areas are typically created when you have a hub-and-spoke topology, with the spokes (such as branch offices) configured as stub areas.

    46. Using Stub and Totally Stubby Areas To further reduce the number of routes in a table, you can create a totally stubby area, which is a Cisco-specific feature. A totally stubby area is a stub area that blocks external Type 5 LSAs and summary (that is, Type 3 and Type 4) LSAs from entering the area. This way, intra-area routes and the default of 0.0.0.0/0 are the only routes known to the stub area. ABRs inject the default summary link 0.0.0.0/0 into the totally stubby area.

    47. Using Stub and Totally Stubby Areas This is typically a better solution than creating stub areas, unless the target area uses a mix of Cisco and non-Cisco routers.

    48. Stub and Totally Stub Criteria An area can be qualified as a 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. (Virtual links are discussed at the end of this chapter.).

    49. Stub and Totally Stub Criteria No ASBR is internal to the stub area. The area is not the backbone area (Area 0). These criteria are important because a stub/totally stubby area is configured primarily to exclude external routes. If these criteria are not met, external links may be injected into the area, invalidating their stubby nature.

    50. Stub and Totally Stub Criteria To configure an area as a stub or totally stubby area, use the following syntax on all router interfaces that are configured to belong to that area: Router(config-router)#area area-id stub  

    51. Stub and Totally Stub Criteria The optional no-summary keyword is used only on ABRs. This keyword configures the ABR to block interarea summaries (Type 3 and Type 4 LSAs). The no-summary keyword creates a totally stubby area.

    52. Stub and Totally Stub Criteria The area stub command is configured on each router in the stub location, which is essential for the routers to become neighbors and exchange routing information. When this command is configured, the stub routers exchange hello packets with the E bit set to 0. The E bit is in the Options field of the hello packet. It indicates that the area is a stub area.

    53. Stub and Totally Stub Criteria On ABRs only, you also have the option of defining the cost of the default route that is automatically injected in the stub/totally stubby area. You use the following syntax to configure the default route's cost:

    54. Stub and Totally Stub Criteria Router(config-router)#area area-id default-cost cost

    55. Meeting the Backbone Requirements OSPF has certain restrictions when multiple areas are configured. One area must be defined as Area 0, the backbone area. It is called the backbone because all inter-area communication must go through it.

    56. Meeting the Backbone Requirements Thus, all areas should be physically connected to Area 0 so that the routing information injected into this backbone can be disseminated to other areas. The backbone area must always be configured as Area 0. You cannot make any other area ID function as the backbone.

    57. Virtual Links There are situations, however, when 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. In these cases, a virtual link can be defined to provide the needed connectivity to the backbone area.

    58. Virtual Links The virtual link provides the disconnected area a logical path to the backbone. All areas must connect directly to the backbone area or through a transit area. The virtual link has the following two requirements:

    59. Virtual Links It must be established between two routers that share a common area. One of these two routers must be connected to the backbone. Virtual links serve the following purposes: They can link an area that does not have a physical connection to the backbone. This linking could occur, for example, when two organizations merge.

    60. Virtual Links They can patch the backbone if discontinuity in Area 0 occurs. Discontinuity of the backbone might occur, 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.

    61. Virtual Links Another reason for creating a virtual link is to add redundancy in cases when router failure might cause the backbone to be split into two.

    62. Virtual Links To configure a virtual link, perform the following steps: router(config-router)#area area-id virtual-link router-id If you do not know the neighbor's Router ID, you can Telnet to it and type the show ip ospf command.

    63. Virtual Links Area 3 does not have a direct physical connection to the backbone (Area 0), which is an OSPF requirement because the backbone is a collection point for LSAs. ABRs forward summary LSAs to the backbone, which in turn forwards the traffic to all areas. All interarea traffic transits the backbone.

    64. Virtual Links To provide connectivity to the backbone, a virtual link must be configured between R2 and R1. Area 1 will be the transit area and R1 will be the entry point into area 0. R2 will have a logical connection to the backbone through the transit area.

    65. Virtual Links Both sides of the virtual link must be configured, as follows: R2(config-router)#area 1 virtual-link 10.3.10.5 --- With this command, area 1 is defined to be the transit area and the router ID of the other side of the virtual link is configured

    66. Virtual Links R1(config-router)#area 1 virtual-link 10.7.20.123 --- With this command, area 1 is defined to be the transit area and the router ID of the other side of the virtual link is configured.

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