Foundation Summary


The "Foundation Summary" section of each chapter lists the most important facts from the chapter. Although this section does not list every fact from the chapter that will be on your exam, a well-prepared candidate should, at a minimum, know all the details in each "Foundation Summary" before going to take the exam.

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 can also be an internal router, an ASBR or an ABR.

  • 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.

  • 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 is also likely to leave the router's area, it makes sense to place the ASBR in a central location that all traffic leaving its area must traverse.

The five LSAs 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 DR and lists all the routers on the segment for which it is the DR 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.

Table 8-3 shows the codes used in the routing table.

Table 8-3. 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, and their cost. It is propagated within the area.

2 Network Link


This is generated by the DR 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 might 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)

O E1 or O E2

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 costit just reports the external cost to the remote destination.

Some restrictions govern creating a stub area or a totally stubby area. Because no external routes are allowed in these areas, the following restrictions are in place:

  • No external routes are allowed.

  • No virtual links are allowed.

  • No redistribution is allowed.

  • No ASBR routers are allowed.

  • The area is not the backbone area.

  • All the routers are configured to be stub routers.

In designing an OSPF network, it is important to consider the following:

  • Summarization:

    - Interarea : Performed at the ABR, creating type 3 and 4 LSAs

    - External : Performed at the ASBR creating type 5 LSAs

  • Capacity planning:

    - Router per area: 50

    - Neighbors per router: 60

    - Areas per router: 3

    - A router may not be a DR or BDR for more than one LAN

  • Virtual links: As a temporary solution during transition or after a break in the network

  • NBMA networks:

    - Creating the NBMA network as area 0 if it is a fully meshed network used to connect all other sites

    - In a hub and spoke network, defining the hub as area 0, with the spokes forming other areas

CCNP BSCI Exam Certification Guide
CCNP BSCI Exam Certification Guide (CCNP Self-Study, 642-801) (3rd Edition)
ISBN: 1587200856
EAN: 2147483647
Year: 2002
Pages: 194
Authors: Clare Gough © 2008-2017.
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