© 2007 Cisco Systems, Inc. All rights reserved.Cisco Public ITE PC v4.0 Chapter 1 1 Link-State Routing Protocols Routing Protocols and Concepts – Chapter 10

ITE PC v4.0 Chapter 1 2 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Objectives  Describe the basic features & concepts of link-state routing protocols.  List the benefits and requirements of link-state routing protocols.

ITE PC v4.0 Chapter 1 3 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Link-State Routing  The shortest path to a destination is not necessarily the path with the least number of hops

ITE PC v4.0 Chapter 1 4 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Link-State Routing -Definitions  Directly Connected Networks  Link: This is an interface on a router  Link state : This is the information about the state of the links  IP address Subnet mask Type of network Cost associated with link Neighboring routers on the link

ITE PC v4.0 Chapter 1 5 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Link-State Routing Sending Hello Packets to Neighbours  Link state routing protocols use a hello protocol Purpose of a hello protocol: -To discover neighbors (that use the same link state routing protocol) on its link

ITE PC v4.0 Chapter 1 6 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Hello packets Hello Packet contains Router ID of transmitting router  OSPF Hello Intervals –Usually multicast ( ) –Sent every 10 seconds on –Multiaccess networks and Point to point links  OSPF Dead Intervals –This is the time that must transpire before the neighbor is considered down –Default time is 4 times the hello interval

ITE PC v4.0 Chapter 1 7 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Link-State Routing Sending Hello Packets to Neighbors  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

ITE PC v4.0 Chapter 1 8 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Link-State Routing Routers also exchange Link State Packets (LSP) with neighbours which contain -State of each directly connected link -Includes information about neighbors such as neighbor ID, link type, & bandwidth.

ITE PC v4.0 Chapter 1 9 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Link-State Routing  LSPs (Link State Packets) are sent out under the following conditions -Initial router start up or routing process -When there is a change in topology

ITE PC v4.0 Chapter 1 10 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Link-State Routing Constructing a link state data base  Routers use a database to construct a topology map of the network

ITE PC v4.0 Chapter 1 11 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Link-State Routing

ITE PC v4.0 Chapter 1 12 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Link-State Routing  Determining the shortest path The shortest path to a destination determined by adding the costs & finding the lowest cost

ITE PC v4.0 Chapter 1 13 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Link-State Routing  Once the SPF algorithm has determined the shortest path routes, these routes are placed in the routing table.

ITE PC v4.0 Chapter 1 14 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Link-State Routing Protocols Routing protocol Builds Topological map Router can independently determine the shortest path to every network. Convergence A periodic/ event driven routing updates Use of LSP Distance vector No SlowGenerally NoNo Link StateYes FastGenerally YesYes Advantages of a Link-State Routing Protocol

ITE PC v4.0 Chapter 1 15 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Link-State Routing Protocols Requirements for using a link state routing protocol  Memory requirements Typically link state routing protocols use more memory  Processing Requirements More CPU processing is required of link state routing protocols  Bandwidth Requirements Initial startup of link state routing protocols can consume lots of bandwidth

ITE PC v4.0 Chapter 1 16 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Link-State Routing Protocols  2 link state routing protocols used for routing IP -Open Shortest Path First (OSPF) -Intermediate System-Intermediate System (IS-IS)

ITE PC v4.0 Chapter 1 17 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public OSPF Metric  OSPF uses cost as the metric for determining the best route -The best route will have the lowest cost -Cost is based on bandwidth of an interface  Cost is calculated using the formula 10 8 / bandwidth -Reference bandwidth  defaults to 100Mbps  can be modified using  auto-cost reference-bandwidth command

ITE PC v4.0 Chapter 1 18 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Basic OSPF Configuration Lab Topology  Topology used for this chapter Discontiguous IP addressing scheme Since OSPF is a classless routing protocol the subnet mask is configured in

ITE PC v4.0 Chapter 1 19 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Basic OSPF Configuration The router ospf command  To enable OSPF on a router use the following command R1(config)#router ospf process-id Process id  A locally significant number between 1 and this means it does not have to match other OSPF routers

ITE PC v4.0 Chapter 1 20 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Basic OSPF Configuration  OSPF network command -Requires entering: network address wildcard mask - the inverse of the subnet mask area-id - area-id refers to the OSPF area. OSPF area is a group of routers that share link state information -Example: Router(config-router)#network network-address wildcard-ask area area-id

ITE PC v4.0 Chapter 1 21 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Basic OSPF Configuration Verifying OSPF  Use the show ip ospf command to verify & trouble shoot OSPF networks Command will display the following:  Neighbor adjacency -No adjacency indicated by -  Neighboring router’s Router ID is not displayed  A state of full is not displayed -Consequence of no adjacency-  No link state information exchanged  Inaccurate SPF trees & routing tables

ITE PC v4.0 Chapter 1 22 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Basic OSPF Configuration CommandDescription Show ip protocols Displays OSPF process ID, router ID, networks router is advertising & administrative distance Show ip ospf Displays OSPF process ID, router ID, OSPF area information & the last time SPF algorithm calculated Show ip ospf interface Displays hello interval and dead interval Verifying OSPF - Additional Commands

ITE PC v4.0 Chapter 1 23 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Summary  Link State Routing protocols are also known as Shortest Path First protocols  Summarizing the link state process -Routers 1ST learn of directly connected networks -Routers then say “hello” to neighbors -Routers then build link state packets -Routers then flood LSPs to all neighbors -Routers use LSP database to build a network topology map & calculate the best path to each destination