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This chapter is from the book

Foundation Summary

The "Foundation Summary" is a collection of quick reference information that provides a convenient review of many key concepts in this chapter. For those of you who already feel comfortable with the topics in this chapter, this summary will help you recall a few details. For those of you who just read this chapter, this review should help solidify some key facts. For any of you doing your final preparations before the exam, these tables and figures will be a convenient way to review the day before the exam.

OSPF Routers

The following list identifies the different OSPF routers:

  • Internal router—Within an area, the functionality of the router is straightforward. It is responsible for maintaining a current and accurate database of every subnet within the area. It is also responsible for forwarding data to other networks by the shortest path. Flooding of routing updates is confined to the area.

  • Backbone router—The design rules for OSPF require that all the areas be connected through a single area known as the backbone area or Area 0. A router within this area is referred to as a backbone router. It may also be an internal router or an Area Border Router.

  • Area Border Router (ABR)—This router is responsible for connecting two or more areas. It holds a full topological database for each area to which it is connected and sends LSA updates between the areas. These LSA updates are summary updates of the subnets within an area. It is at the area border that summarization should be configured for OSPF because this is where the LSAs make use of the reduced routing updates to minimize the routing overhead on both the network and the routers.

  • Autonomous System Boundary Router (ASBR)—To connect to the outside world, or to any other routing protocol, you need to leave the OSPF domain. OSPF is an interior routing protocol or Interior Gateway Protocol (IGP); gateway is an older term for a router. The router configured for this duty is the ASBR. Although you can place this router anywhere in the OSPF hierarchical design, it should reside in the backbone area. Because any traffic leaving the OSPF domain will likely also leave the router's area, it makes sense to place the ASBR in a central location that all traffic leaving its area must traverse.

Link-State Advertisements

The five link-state advertisements are as follows:

  • The router link—This LSA states all the links to the router sending out the LSA. The list is of all the neighbors attached to the router. The LSA is flooded to the area.

  • The network link—This LSA is sent out by the designated router and lists all the routers on the segment for which it is the designated router and has a neighbor relationship. The LSA is flooded to the whole area.

  • The network summary link—This LSA is sent between areas and summarizes the IP networks from one area to another. It is generated by an ABR.

  • The AS external (ASBR) summary link—This LSA is sent to a router that connects to the outside world (ASBR). It is sent from the ABR to the ASBR. The LSA contains the metric cost from the ABR to the ASBR.

  • The external link—This LSA is originated by AS boundary routers and flooded throughout the AS. Each external advertisement describes a route to a destination in another autonomous system. Default routes for the AS can also be described by AS external advertisements.

Routing Table Codes

Table 6-5 shows the codes used in the routing table.

Table 6-5 OSPF Routing Table Codes And Associated LSAs

LSA Type

Routing Table Entry


1 Router Link


This is generated by the router, listing all the links to which it is connected, their status, their and cost. It is propagated within the area.

2 Network Link


This is generated by the designated router on a multiaccess LAN to the area.

3 or 4 Summary Link (between areas)


LSA Type 3 includes the networks or subnets within an area that may have been summarized and that are sent into the backbone and between ABRs. LSA Type 4 is information sent to the ASBR from the ABR. These routes are not sent into totally stubby areas.

5 Summary Link/External Link (between autonomous systems)

0E1 or 0E2

The routes in this LSA are external to the autonomous system. They can be configured to have one of two values. E1 will include the internal cost to the ASBR added to the external cost reported by the ASBR. E2 does not compute the internal cost—it just reports the external cost to the remote destination.

Command Summaries

This section contains a list of the commands explained in this chapter. This list is not intended to teach the use of the commands, but to remind you of the options available.

Table 6-6 OSPF Command Summary



network network-number wildcard-mask area area-id

This command identifies the interfaces that are running OSPF and places them in the appropriate area.

summary-address address mask [not-advertise][tag tag]

This command consolidates routes into a summary route before injecting them into the external world. Remember that the mask is the subnet, not the wildcard mask. The options to not advertise will suppress routes that match the mask. The tag option is for use with redistribution.

area area-id range address mask

This command is used to summarize the routes at the ABR before injecting them into another area.

no area area-id range address mask

This command disables the configured summarization at the ABR.

area area-id stub

This command turns the area into a stub area. This command must be configured on every router in the area.

area area-id stub no-summary

This command turns an area into a totally stubby area.

area area-id default-cost cost

This configures the cost for the default summary route used for a stub or totally stub area. The default is 1.

area area-id virtual-link router-id

This command creates a virtual link.

show ip ospf border-routers

This command lists the ABR routers in the AS.

show ip ospf virtual-links

This command shows the virtual links and the current parameters.

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