Introduction to OSPF

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Routing Within a Hierarchical Structure

There are three types of routes that may be used by OSPF: intra-area, inter-area, and external routes. The sections that follow provide general descriptions of these route types.

Intra-Area Routing

Intra-area routing is the name used to describe routing within a logical area. These types of routes are described by Router LSAs (Type 1). In order for packets to be routed within a single area, intra-area routing is used. When displayed in the OSPF routing table, these types of links are designated with an “O.”

Inter-Area Routing

Inter-area routing is the name used to describe routing between two or more logical areas that still fall within the source Autonomous System. These types of routers are described by Summary LSAs (Types 3 and 4). When routing packets between two non-backbone areas, the backbone will be used. This means that inter-area routing has pieces of intra-area routing along its path, for example:

1.  An intra-area path is used from the source router to the area border router.
2.  The backbone is then used from the source area to the destination area.
3.  An intra-area path is used from the destination area’s area border router to the destination.

Put these three routes together and you will have an inter-area route. Of course, the SPF algorithm will calculate the lowest cost between these two points. When these types of routes are displayed in the OSPF routing table, these types of routes are indicated with an IA.

AS (Autonomous System) External Routes

External routing information can be gained by OSPF through a number of means as discussed. This information must then be made available throughout the Autonomous System in order for it to be of use. The ASBR routers will summarize the information and flood this information throughout the AS. Every router will receive this information with the exception of stub areas.

There are two specific types of external routes, which are as follows:

  E1 routes. E1 routes are the sum of internal and external OSPF metrics. They are identified by the E1 designation within the OSPF routing table. For example, if a packet is destined for another autonomous system, then E1 routes add all the metrics in both Autonomous Systems associated with reaching the destination.
  E2 routes. E2 routes are the default for OSPF. They do not add the internal OSPF metrics. For example, if a packet is destined for another autonomous system, then E2 routes only add the metrics from the destination AS associated with reaching the destination.

Multiple routes to the same destination will use the following order of preference in order to route: intra-area, inter-area, E1, and E2.

Chapter Summary

This chapter discussed in detail the background and evolution of the OSPF protocol by tracing the RFCs relating to OSPF. The “OSPF Overview” section of this chapter discussed the earliest appearance and creation of the shortest path first algorithm. From the established beginnings of OSPF, the evolution and modifications applied to it will be traced through the RFCs. Each RFC relating directly and indirectly to the protocol is summarized and discussed as needed.

The “OSPF Functional Environment” section of this chapter helped you understand the internetworking environment of the protocol so as to design a network in which it will operate properly. The functional environment is a key element in understanding OSPF. Within this section are several basics to the protocol that are discussed and explored: network types, router identification, adjacencies, Designated Routers, protocols within OSPF, and LSAs.

The “OSPF Routing Hierarchy” section demonstrated how the protocol allows for a variety of different configurations and implementations. This capability requires many different levels or areas. OSPF’s capability to perform as a hierarchical routing protocol allows it to be considered in networks of varying sizes. As a result of this, OSPF supports a variety of techniques and designations that make operation and design much smoother. Within this section, you learned about the types of OSPF routers and hierarchical design techniques. The latter of which included how OSPF separates the hierarchy through the use of areas and Autonomous Systems.

Case Study: Adding a New OSPF Router to a Network

This case study attempts to provide a scenario that will cover most of the information presented in this chapter. Suppose that a new OSPF router is added to a network. With this scenario in hand, follow along with the case study to understand the ramifications of how adding a new OSPF router would affect an operating network. Refer to Figures 4.19a-4.19d, which detail each step of the process as it occurs in the following sequence:.

1.  A new OSPF router is added to the network.
2.  This new router immediately transmits a multicast hello packet by using the all OSPF routers multicast address of At this point, the router doesn’t know who or if there is a designated router (see Figure 4-19a).

Figure 4-19a  Adding a new router.

If a Designated Router (DR) already exists on a network, a new router, even one with a better priority, will not take over as DR.
3.  The DR and BDR will respond back to the new router with a unicast hello packet specifically addressed to it. This begins the process of building an adjacency (see Figure 4-19b).

Figure 4-19b  DR and BDR respond.

4.  After the adjacency has been established with the DR, the new router will send a Router LSA to the DR describing the specific links it has available and their status. During this time, the BDR is also listening to see if the DR responds, thus proving it is up and operational (see Figure 4-19c).

Figure 4-19c  Router LSA is sent.

5.  The BDR continues to listen, thereby ensuring the DR is operating. The DR will send out a multicast ( network LSA to the entire network informing them of the new routes available, as a result of the new router. All routers must then respond with an acknowledgement so the DR knows they have received the new information (see Figure 4-19d).

Figure 4-19d  Network LSA is sent.

6.  At this point, the new router has been fully identified to all other routers within the network. This information also includes any networks it can reach.

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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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