The Fundamentals of OSPF Routing Design

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Inter-Area Route Summarization

Inter-area route summarization is done on ABRs, and it applies to routes from within the AS. It does not apply to external routes injected into OSPF via redistribution. To take advantage of summarization, network numbers in areas should be assigned in a contiguous way so that you can lump these addresses into one range when summarizing them.

To specify an address range, perform the following task in router configuration mode:

    area area-id range address mask 

The area-id is the area containing networks to be summarized. The address and mask will specify the range of addresses to be summarized in one range. Figure 5-14 illustrates an example of summarization.

Figure 5-14  An example of inter-area route summarization.

In Figure 5-14, Router B is summarizing the range of subnets found within area 1 from to into one range: with a mask of into the backbone. This is achieved by masking the first three left-most bits of 64, using a mask of

In the same way, Router C is generating the summary address into the backbone. Note that this summarization was successful because you have two distinct ranges of subnets, 64-95 and 96-127 in areas 1 and 2 respectively.

It would be hard to summarize if the subnets between area 1 and area 2 were overlapping. The backbone area would receive summary ranges that overlap and routers in the middle would not know where to send the traffic based on the summary address. The following is the relative configuration of Router B, and you can extrapolate Router C’s configuration as well:

    Router B#    router ospf 100    area 1 range 

External Route Summarization

External route summarization is specific to external routes that are injected into OSPF via redistribution done by ASBRs. Also, make sure that external ranges being summarized are contiguous. Summarization that overlaps ranges from two different routers could cause packets to be sent to the wrong destination. Summarization is done via the following router ospf subcommand:

    summary-address ip-address mask 

This command is effective only on ASBRs doing redistribution into OSPF.

In Figure 5-15, Router A and Router D (both ASBRs) are injecting external routes into OSPF by redistribution. Router A is injecting subnets in the range 128.213.64-95 and Router D is injecting subnets in the range 128.213.96-127.

Figure 5-15  An example of external route summarization.

To properly summarize the subnets into one range on each router, you can configure the routers as follows:

    Router A#    router ospf 100    summary-address    redistribute bgp 50 metric 1000 subnets    Router D#    router ospf 100    summary-address    redistribute bgp 20 metric 1000 subnets 

This will cause Router A to generate one external route with a mask of and will cause Router D to generate one external route with a mask of Note that the summary-address command has no effect if used on Router B because Router B is not doing the redistribution into OSPF, nor is it an ASBR.

Route Summarization and Route Distribution

Route summarization addresses two important questions of route information distribution:

  What information does the backbone need to know about each area? The answer to this question focuses attention on area-to-backbone routing information.
  What information does each area need to know about the backbone and other areas? The answer to this question focuses attention on backbone-to-area routing information.

If you know the answers to these questions, you will be able to effectively design how you need to summarize routes within your OSPF network.

Area-to-Backbone Route Advertisements

There are several key considerations when setting up your OSPF areas for proper summarization. OSPF route summarization occurs in the ABRs. OSPF supports variable length subnet masks (VLSM), so it is possible to summarize on any bit boundary in a network or subnet address. OSPF requires manual summarization. As you design the areas, you need to determine summarization at each ABR.

Backbone-to-Area Route Advertisements

Four potential types of routing information exist in an area and are listed in Table 5-2, which shows the different types of areas according to the routing information that they use.

Table 5-2 OSPF area route types
Area Type Default Intra-Area Inter-Area External

Non-stub Yes Yes Yes Yes
Stub Yes Yes Yes No
TSA Yes Yes No No
NSSA Yes Yes Yes Yes

The types of routes defined in Table 5-2 for OSPF areas are as follows:

  Default Routes. If an explicit route cannot be found for a given IP network or subnetwork, the router will forward the packet to the destination specified in the default route.
  Intra-Area Routes. Explicit network or subnet routes must be carried for all networks or subnets inside an area.
  Inter-Area Routes. Areas may carry explicit network or subnet routes for networks or subnets that are in this AS but not in this area.
  External Routes. When different ASs exchange routing information, the routes they exchange are referred to as external routes.

OSPF Addressing and Summarization Scenarios

The following sections discuss OSPF route summarization and the three most commonly encountered IP addressing scenarios:

  Scenario 1: Assigning Unique Network Numbers to Each OSPF Area
  Scenario 2: Complex Address Assignment with Only a Single NIC Address
  Scenario 3: Using Private IP Addresses

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OSPF Network Design Solutions
OSPF Network Design Solutions
ISBN: 1578700469
EAN: 2147483647
Year: 1998
Pages: 200
Authors: Tom Thomas

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