Module 3 - Review of OSPF Fundamentals and Features potx

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BSCI Module 3 Lesson 1 - Review of OSPF

Fundamentals and Features

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BSCI

2

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Objective

 Link-State Routing Protocols

 Overview of OSPF Operations

 OSPF Data Structures

 OSPF Adjacency Database

 OSPF Area Structure

 OSPF Router Types

 OSPF Terminology

 Calculation the OSPF Metric

 Link-State Data Structure

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 Each router has a full picture of the network topology ,

including bandwidth information for links to remote networks Therefore, each router is able to

independently select a loop-free and efficient pathway , based on cost, to reach every network in the area

 OSPF is a classless routing protocol

 Support VLSM

 Support Discontigous

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 Routers with link-state routing protocols use a Hello protocol

to discover any neighbors on its links A neighbor is any

other router that is enabled with the same link-state routing

protocol.

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 Connected interfaces that are using

the same link state routing protocols

will exchange hello packets.

 Once routers learn it has neighbors

they form an adjacency

– 2 adjacent neighbors will exchange hello packets

– These packets will serve as a

keep alive function.

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 Each router builds its own Link State Packet (LSP)

 Contents of LSP:

– State of each directly connected link

– Includes information about neighbors such as neighbor ID, link type, & bandwidth.

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 Directly Connected Networks.

 Link: this is an interface on a router.

 Link state: this is the information about the state of the links.

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 Once LSP are created they are forwarded out to neighbors.

– After receiving the LSP the neighbor continues to forward it throughout routing area

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determined the shortest path

routes, these routes are

placed in the routing table

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BSCI

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Metrics and Routing Protocols

The Metric Parameters

 Different routing protocols use different metrics Two different routing

protocols might choose different paths to the same destination due to using different metrics.

 Metrics used in IP routing protocols include:Hop count, Bandwidth, Load,

Delay, Reliability, Cost

RIP OSPF

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the following characteristics:

–Respond quickly to network changes

–Send triggered updates when a

network change occurs

–Send periodic updates, known as

link-state refresh, at long intervals,

such as every 30 minutes

 LSAs are reliable (ACK)

 LSAs have a sequence number

 LSAs are propagated to all

neighboring devices using the

reserved class D multicast

address 224.0.0.5

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OSPF Data Structures

–Neighbor table:

•Also known as the adjacency database

•Contains list of recognized neighbors

•Commonly named a forwarding database

•Contains list of best paths to destinations

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–Routers discover neighbors by exchanging hello packets 224.0.0.5

–Routers declare neighbors to be up after checking certain parameters or options in the hello packet

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Forming OSPF Adjacencies

–Point-to-point WAN links:

•Both neighbors become fully adjacent

–LAN links:

•Neighbors form a full adjacency with the DR and BDR

•Routers maintain two-way state with the other routers (DROTHERs)

–Routing updates and topology information are passed only between adjacent routers

–Once an adjacency is formed, LSDBs are synchronized by exchanging LSAs

–LSAs are flooded reliably throughout the area (or network)

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protocols can partition networks into sub-domains called areas

 An area is a logical collection of OSPF networks,

routers, and links that have the same area identification

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• Requires a

hierarchical network design

OSPF Area Structure

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• Routers A and B are

backbone routers

• Backbone routers

make up area 0.

• Routers C, D, and E are

known as area border

routers (ABRs).

• ABRs attach all other

areas to area 0

Area Terminology

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BSCI

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OSPF Calculation

 Routers find the best paths to destinations by applying

Dijkstra’s SPF algorithm to the link-state database as follows:

–Every router in an area has the identical link-state database

–Each router in the area places itself into the root of the tree that is built

–The best path is calculated with respect to the lowest total cost of links to a specific destination

–Best routes are put into the forwarding database (routing table)

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Assume all links are Ethernet, with an OSPF cost of 10.

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BSCI

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Link-State Data Structures: LSA

Operation

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–Upon completion of this activity, the student will be able to compare routing issues between RIPv1 and OSPF.

 3.1.6 Drag and Drop: OSPF Router Types

–Upon completion of this activity, the student will be able to identify the four different types of OSPF routers

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2 Which multicast address LSA use

3 How many table are there in OSPF, name it

4 List two level hierarchy area in OSPF

5 List 4 types of OSPF router

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updates every 30 minutes.

–A two-tier hierarchical network structure is used by OSPF in which the network is divided into areas This area structure is used to separate the LSDB into more manageable sizes

–Adjacencies are built by OSPF routers using the Hello protocol Over these logical adjacencies, LSUs are sent to exchange database information between adjacent OSPF routers

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a table of best paths, known as the routing table

–After an LSA entry ages, the router that originated the entry sends an LSU about the network to verify that the link is still active The LSU can contain one or more LSAs

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Q and A

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 OSPF Design Guide

http://www.cisco.com/warp/public/104/1.html

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