OSPF Support for Variable-Length Subnet Masking (VLSM) and Route Summarization in Multiple Areas

When you start to design your OSPF internetwork, it is important to realize that address assignment and route summarization are fundamentally connected. To build a scalable OSPF internetwork, you simply must implement route summarization. To review, route summarization is the process of aggregating routes to contiguous subnets into a single summary address. Route summarization is performed only on the area border routers because network routes are summarized for the purpose of injecting them into the OSPF backbone area. The result of this aggregation will be smaller routing tables on the ABRs, faster table lookups, and an overall decrease in the size of the link-state advertisements. Instead of advertising many specific routes from an area, the ABR needs to advertise only one or more summary routes. Route summarization will actually diminish the size of the routing tables on all routers in an AS while maintaining route availability.

Deploying an efficient hierarchical addressing scheme will have a huge impact on the performance and scalability of your OSPF internetwork. Variable-length subnet masking provides economical allocation of IP addresses and can be implemented in either single-area or multi-area OSPF routing domains. OSPF is a classless protocol for which route summarization is configured manually at either major or arbitrary bit boundaries.

Despite the flexibility that comes from VLSM and route summarization, you should shoot for making your addressing scheme as straightforward as possible, without oversimplifying the design. Although simple addressing can save some maintenance and troubleshooting time, shortcuts may have certain harsh long- term consequences. You need to design a scalable and structured addressing environment that starts with a large block at the edge, breaks into smaller networks inside, retains contiguous address space along the way, and plans for growth at each layer. This should provide the proper combination of present applicability, flexibility, and room to grow without requiring you to redo your entire scheme. Make sure that route summarization can be accomplished effectively at all your area border routers.

OSPF summary routes are configured on a router in router configuration mode so summary routes are not associated with any specific interface. OSPF summarization is enabled by default. You tell the ABR to perform route summarization for a specific area before injecting the routes (designated as IA routes in the routing table) into a different area with the area range command. You use this command to summarize routes between OSPF areas (summarizing the IA routes). Here is the syntax of the area range command. Notice that you will be in router configuration mode:

 RouterA(config-network)#  area  area-id  range  address  mask 

The area-id parameter identifies the area for which routes will be summarized. The address keyword is the summary address specified for a particular range of addresses. The mask keyword is the subnet mask of the summary route. Figure 5.4 and Listing 5.1 show ABR RouterA summarizing the subnet range 169.254.96.0 through 169.254.127.0 into a single range of 169.254.96.0 with a mask of 255.255.224.0. In addition, Area 2 is summarizing the range 169.254.32.0 with a mask of 255.255.225.0 into Area 0.

Listing 5.1 Configuring Route Summarization on OSPF ABR Routers

 RouterA>en RouterA#config t Enter configuration commands, one per line.  End with CNTL/Z. RouterA(config)#router ospf 10 RouterA(config-router)#network 169.254.32.1 0.0.0.0 area 2 RouterA(config-router)#network 169.254.96.1 0.0.0.0 area 0 RouterA(config-router)#area 0 range 169.254.96.0 255.255.224.0 RouterA(config-router)#area 2 range 169.254.32.0 255.255.224.0 RouterA(config-router)#exit RouterA(config)# RouterB>en RouterB#config t Enter configuration commands, one per line.  End with CNTL/Z. RouterB(config)#router ospf 10 RouterB(config-router)#network 169.254.64.1 0.0.0.0 area 1 RouterB(config-router)#network 169.254.127.1 0.0.0.0 area 0 RouterB(config-router)#area 0 range 169.254.96.0 255.255.224.0 RouterB(config-router)#area 0 range 169.254.64.0 255.255.224.0 RouterB(config-router)#exit RouterB(config)# 

Figure 5.4. Route summarization into multiple OSPF areas.

Although the area range command is used on ABRs to summarize internal routes, the summary-address router configuration command is typically used on an ASBR to summarize external routes that will be injected into the OSPF routing domain. This is not a hard and fast rule as this command can also be used on an ABR. You can configure the ASBR to summarize external routes prior to injecting the routes into the OSPF domain. Here is the syntax of the command. Notice that you will be in router configuration mode:

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 [View full width] 
 RouterA(config-network)#  summary-address   address  mask  [prefix  mask]   [not-advertise]  [tag   tag   ]  

This command is typically used on the ASBR that is injecting external routes into the OSPF domain (refer to Figure 5.3), but it may also be configured on an ABR. The address and mask keywords are the summary address specified and subnet mask for the summary route, respectively. The prefix keyword represents the IP route prefix for the destination, and mask is the subnet mask for the summary route. Not-advertise is an optional parameter that is implemented for route suppression, and the optional tag value is used with route maps. For example, the following snippet shows the summarization configuration of an external route on RouterA:

 RouterA>en RouterA#config t Enter configuration commands, one per line.  End with CNTL/Z. RouterA(config)#router ospf 10 RouterA(config-router)#summary-address 172.18.0.0 255.255.0.0 RouterA(config-router)#exit RouterA(config)# 

When you are planning your OSPF internetwork, consider the following:

• Plan your addressing scheme so that the range of subnets assigned within an area is contiguous.

• Your address space should enable you to divide areas easily as your network scales up. For instance, assign subnets according to simple octet boundaries. If you understand your entire address scheme, you can better implement future changes.

• Always allow for the addition of new routers to your OSPF environment and ensure that they are properly placed as internal, backbone, ABR, and/or ASBR routers. This action will generate a new topology and a recomputing of subsequent tables, so be deliberate .

One of the simplest ways to allocate addresses in OSPF is to assign a separate network number for each area, thereby creating a backbone area and multiple connected areas. Then you simply assign a separate IP network number to each area. Figure 5.5 illustrates this sort of design.

Figure 5.5. Separate network numbers for each area connected to a backbone area.

In the network illustrated in Figure 5.5, each area has its own unique NIC-assigned address. These can be Class A (Area 9), Class B (Area 7), or Class C (Area 8). The payback for this kind of design is a relatively easy scheme to configure and troubleshoot. Also, the internetwork operations are streamlined because each area has a unique major network number. In this example, the route summarization configuration at the ABRs is greatly simplified. Routes from Area 7 injecting into the backbone can be summarized as follows : "All routes starting with 172.16 are found in Area 7."

The chief disadvantage of this approach, however, is the tremendous waste of address space. With this simpler approach, you must make certain that ABRs are explicitly configured to do route summarization (enabled by default in OSPF). We look at more elaborate summarization techniques in the next section, which covers operation in a multiple-area NBMA environment.