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CCNP Routing Studies: Basic Open Shortest Path First

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Understanding basic Open Shortest Path First (OSPF) routing principles not only applies to the CCNP certification but to all Cisco-based certifications. A concrete understanding of how OSPF routing works is fundamental for any small or large network. This chapter focuses on a number of objectives falling under the CCNP routing principles.
This chapter is from the book

This chapter is from the book

This chapter focuses on a number of objectives falling under the CCNP routing principles. Understanding basic Open Shortest Path First (OSPF) routing principles not only applies to the CCNP certification but to all Cisco-based certifications. A concrete understanding of how OSPF routing works is fundamental for any small or large network. OSPF is commonly used in large service provider networks or large financial institutions. This chapter assumes knowledge of the previous chapter, which deals conceptually with IP routing principles and in particular link-state routing protocols.

The chapter starts by covering the basic OSPF concepts. It then briefly explains why OSPF is considered an improved routing protocol over Routing Information Protocol (RIP) by covering how OSPF discovers, chooses, and maintains routing tables.

Nonbroadcast multiaccess (NBMA) is a particular challenge in any network environment. This chapter covers how OSPF overcomes any limitations imposed by NBMA networks.

Five practical scenarios, included in the chapter, help you complete your understanding and ensure you have all the basic OSPF routing skills to complement your understanding of how to configure and maintain OSPF on Cisco Internet Operating System (IOS) routers.

Basic OSPF

OSPF is a link-state routing protocol. Link-state protocols use the shortest path first (SPF) algorithm to populate the routing table. OSPF shares information with every router in the network.

OSPF is considered a difficult protocol to configure and requires a thorough understanding of terms that are commonly used. Table 3-1 explains briefly the common OSPF terminology used throughout this chapter.

Table 3-1 Common OSPF Terms

Term

Description

Link state

Information is shared between directly connected routers. This information propagates throughout the network unchanged and is also used to create a shortest path first (SPF) tree.

Area

A group of routers that share the same area ID. All OSPF routers require area assignments.

Autonomous system (AS)

A network under a common network administration.

Cost

The routing metric used by OSPF. Lower costs are always preferred. You can manually configure the cost with the ip ospf cost command. By default, the cost is calculated by using the formula cost = 108 / bandwidth.

Router ID

Each OSPF router requires a unique router ID, which is the highest IP address configured on a Cisco router or the highest numbered loopback address. You can manually assign the router ID.

Adjacency

When two OSPF routers have exchanged information between each other and have the same topology table. An adjacency can have the following different states or exchange states:

 

1. Init state—When Hello packets have been sent and are awaiting a reply to establish two-way communication.

 

2. Establish bi-directional (two-way) communication—Accomplished by the discovery of the Hello protocol routers and the election of a DR.

 

3. Exstart—Two neighbor routers form a master/slave relationship and agree upon a starting sequence to be incremented to ensure LSAs are acknowledged.

 

4. Exchange state—Database Description (DD) packets continue to flow as the slave router acknowledges the master's packets. OSPF is operational because the routers can send and receive LSAs between each other. DD packets contain information, such as the router ID, area ID, checksum, if authentication is used, link-state type, and the advertising router. LSA packets contain information, such as router ID also but in addition include MTU sizes, DD sequence numbering, and any options.

 

5. Loading state—Link-state requests are sent to neighbors asking for recent advertisements that have not yet been discovered.

 

6. Full state—Neighbor routers are fully adjacent because their link-state databases are fully synchronized. Routing tables begin to be populated.

Topology table

Also called the link-state table. This table contains every link in the whole network.

Designated router (DR)

This router is responsible for ensuring adjacencies between all neighbors on a multiaccess network (such as Ethernet). This ensures all routers do not need to maintain full adjacencies with each other.

 

The DR is selected based on the router priority. In a tie, the router with the highest router ID is selected.

Backup DR

A backup router designed to perform the same functions in case the DR fails.

Link-state advertisement (LSA)

A packet that contains all relevant information regarding a router's links and the state of those links.

Priority

Sets the router's priority so a DR or BDR can be correctly elected.

Router links

Describe the state and cost of the router's interfaces to the area. Router links use LSA type 1.

Summary links

Originated by area border routers (ABRs) and describe networks in the AS. Summary links use LSA types 3 and 4.

Network links

Originated by DRs. Network links use LSA type 2.

External links

Originated by autonomous system boundary routers (ASBRs) and describe external or default routes to the outside (that is, non- OSPF) devices for use with redistribution. External Links use the LSA type 5.

Area border router (ABR)

Router located on the border of one or more OSPF areas that connects those areas to the backbone network.

Autonomous system

 

boundary router (ASBR)

ABR located between an OSPF autonomous system and a non-OSPF network.


OSPF has so many features that the most efficient way to appreciate them is to enable OSPF on routers and observe how the routers dynamically discover IP networks.

Before covering various OSPF scenarios, this chapter covers how OSPF is configured in single and multiple OSPF areas.

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