OSPF ICCNOSP001

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1OSPF (OPEN SHORTEST PATH FIRST)

OSPF(OPEN SHORTEST PATH

FIRST)

MODULE ID: ICCNOSP001

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Topics Covered

1.OSPF Features2.Comparison of OSPF and RIP3.Hierarchical Design4.OSPF Terminology5.DR and BDR6.OSPF Areas7.Types of Networks8.OSPF Metric9.Configuring OSPF 10.OSPF and Loopback Interfaces

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OSPF Features

An open standard routing protocol -allows multi-vendor deployment

Uses Dijkstra algorithm. Faster convergence Supports multiple, equal-cost routes to the same

destination. Consists of areas and autonomous systems Minimizes routing update traffic Scalable Supports VLSM/CIDR Unlimited hop count Supports only IP routing

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Comparison of OSPF & RIP

Characteristic OSPF RIPv2 RIPv1

Type of Protocol Link-state Distance Vector Distance Vector

Classless support Yes Yes No

VLSM support Yes Yes No

Auto symmetrization No Yes Yes

Manual symmetrization Yes No No

Discontiguous support Yes Yes No

Route propagation

Multicast on change Periodic Multicast Periodic Broadcast

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Comparison of OSPF & RIP

Characteristic OSPF RIPv2 RIPv1

Path metric Bandwidth Hops Hops

Hop count limit None 15 15

Convergence Fast Slow Slow

Peer Authentication Yes Yes No

Hierarchical network

Yes (Using Areas) No (Flat only) No (Flat only)

Updates Event triggered Route Table updatesRoute Table

updatesRoute

computation Dijkstra Bellman-Ford Bellman-Ford

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Hierarchical Design

OSPF is a fast, scalable, and robust protocol that can be actively deployed in thousands of production networks.

OSPF must be designed in a hierarchical fashion, The larger internetwork can be separated into smaller

internetworks called areas. The reasons for creating OSPF in a hierarchical

design include: To decrease routing overhead To speed up convergence To confine network instability to single areas of the

network

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OSPF design example

Backbone Router

Area Border Router (ABR)

Autonomous System Border Router (ASBR)

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Hierarchical Design

Each router connects to the backbonecalled area 0, or the backbone area.

OSPF must have an area 0, and all routers should connect to this area if at all possible,

Routers that connect other areas to the backbone within an AS are called Area Border Routers (ABRs).

At least one interface must be in area 0.

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Hierarchical Design

OSPF runs inside an autonomous system, but can also connect multiple autonomous systems together.

The router that connects these ASes together is called an Autonomous System Boundary Router (ASBR).

Create other areas of networks to help keep route updates to a minimum, and to keep problems from propagating throughout the network.

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OSPF Terminology

Link: A link is a network or router interface assigned to any given network.

This link, or interface, will have state information associated with it (up or down) as well as one or more IP addresses.

Router ID: The Router ID (RID) is an IP address used to identify the router.

Cisco chooses the Router ID by using the highest IP address of all configured loopback interfaces.

If no loopback interfaces are configured with addresses, OSPF will choose the highest IP address of all active physical interfaces.

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OSPF Terminology

Neighbors: Neighbors are two or more routers that have an interface on a common network,such as two routers connected on a point-to-point serial link.

Adjacency: An adjacency is a relationship between two OSPF routers that permits the direct exchange of route updates.

OSPF directly shares routes only with neighbors that have also established adjacencies.

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OSPF Terminology

Hello protocol: The OSPF Hello protocol provides dynamic neighbor discovery and maintains neighbor relationships.

Hello packets and Link State Advertisements (LSAs) build and maintain the topological database.

Hello packets are addressed to 224.0.0.5. Neighborship database: The neighborship database

is a list of all OSPF routers for which Hello packets have been seen.

Various details, including the Router ID and state, are maintained on each router in the neighborship database.

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OSPF Terminology

Topology database: The topology database contains information from all of the Link State Advertisement packets that have been received for an area.

The router uses the information from the topology database as input into the Dijkstra algorithm that computes the shortest path to every network.

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OSPF Terminology

Link State Advertisement: A Link State Advertisement (LSA) is an OSPF data packet containing link-state and routing information thats shared among OSPF routers.

An OSPF router will exchange LSA packets only with routers to which it has established adjacencies.

Designated router: A designated router (DR) is elected whenever OSPF routers are connected to the same multi-access network.

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DR and BDR

To minimize the number of adjacencies formed, a DR is chosen (elected) to disseminate/receive routing information to/from the remaining routers on the broadcast network or link.

This ensures that their topology tables are synchronized.

All routers on the shared network will establish adjacencies with the DR and backup designated router (BDR)

The election is won by the router with the highest priority, and the Router ID is used as a tiebreaker if the priority of more than one router is the same.

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OSPF Terminology

Backup designated router: A backup designated router (BDR) is a hot standby for the DR on multi-access links (broadcast networks).

The BDR receives all routing updates from OSPF adjacent routers, but doesnt flood LSA updates.

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OSPF Terminology

OSPF areas: An OSPF area is a grouping of contiguous networks and routers.

All routers in the same area share a common Area ID.

The Area ID is associated with specific interfaces on the router.

This would allow some interfaces to belong to area 1 while the remaining interfaces can belong to area 0.

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OSPF Areas

All of the routers within the same area have the same topology table.

When configuring OSPF, there must be an area 0, and this is configured on the backbone routers

Hierarchical network organization enhances the scalability of OSPF

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Types of Networks

Broadcast (multi-access): Broadcast (multi-access) networks such as Ethernet allow multiple devices to connect to (or access) the same network, as well as provide a broadcast ability in which a single packet is delivered to all nodes on the network.

In OSPF, a DR and a BDR must be elected for each broadcast multi-access network.

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Types of Networks

Non-broadcast multi-access: Non-Broadcast Multi-Access (NBMA) networks are types such as Frame Relay, X.25, and Asynchronous Transfer Mode (ATM).

These networks allow for multi-access, but have no broadcast ability like Ethernet.

NBMA networks require special OSPF configuration to function properly and neighbor relationships must be defined.

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Types of Networks

DR and BDR are elected on broadcast and nonbroadcast multi-access networks.

Point-to-point: Point-to-point refers to a type of network topology consisting of a direct connection between two routers that provides a single communication path.

The point-to-point connection can be physical, as in a serial cable directly connecting two routers, or it can be logical, as in two routers thousands of miles apart and connected by a circuit in a Frame Relay network.

This type of configuration eliminates the need for DRs or BDRs.

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Types of Networks

Point-to-multipoint: Point-to-multipoint refers to a type of network topology consisting of a series of connections between a single interface on one router and multiple destination routers.

All of the interfaces on all of the routers sharing the point-to-multipoint connection belong to the same network.

As with point-to-point, no DRs or BDRs are needed.

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SPF Tree Calculation

Within an area, each router calculates the best/shortest path to every network in that same area.

This calculation is based upon the information collected in the topology database and an algorithm called shortest path first (SPF).

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OSPF Metric OSPF uses a metric referred to as cost. A cost is associated with every outgoing interface included

in an SPF tree. The cost of the entire path is the sum of costs of the

outgoing interfaces along the path. Cost is an arbitrary value as defined in RFC 2338, Cisco uses a simple equation of Cost = 10^8 / bandwidth. The bandwidth is the configured bandwidth for the

interface. A 100Mbps Fast Ethernet interface would have a default

OSPF cost of 1 and a 10Mbps Ethernet interface would have a cost of 10.

An interface set with a bandwidth of 64,000 would have a default cost of 1563.

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OSPF Metric

This value may be overridden by using the command. ip ospf cost

The cost is manipulated by changing the value to a number within the range of 1 to 65,535.

When connecting links between routers from different vendors, the cost must be adjusted to match another vendors router.

Both routers must assign the same cost to the link for OSPF to work.

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Configuring OSPF

Basic elements of OSPF configuration: Enabling OSPF Configuring OSPF areas

router ospf

network area

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OSPF and Loopback Interfaces Configuring loopback interfaces when using the OSPF

routing protocol is important, Loopback interfaces are logical interfaces, which are

virtual, software-only interfaces; they are not real router interfaces.

Using loopback interfaces with OSPF configuration ensures that an interface is always active for OSPF processes.

They can be used for diagnostic purposes as well as OSPF configuration.

The reason to configure a loopback interface on a router is because the highest IP address on a router will become that routers RID in the absence of a loopback interface.

The RID is used to advertise the routes as well as elect the DR and BDR.

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Configuring Loopback Interfaces

Router(config)#int loopback 0Router(config-if)#ip address 172.16.10.1 255.255.255.255Router(config-if)#no shutRouter(config-if)#^ZRouter#

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Summary

1. OSPF Features2. Hierarchical Structure3. Type of Networks4. DR & BDR5. Metric6. Configuring OSPF7. Loopback Interfaces

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References

Books Hand book on Internetworking by CISCO Building Scalable Cisco Internetworks by CISCO CCNA Study guide by Todd Lamle

URLs http://www.cisco.com http://en.wikipedia.org

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Documents

OSPF ICCNOSP001