© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public 1Version 4.0

Link-State Routing Protocols

Routing Protocols and Concepts – Chapter 10

2© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Introduction

Link-State Routing Introduction

Link-State Packets

Steps

3© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Link-State Routing Introduction Link state routing protocols

– Also known as shortest path first algorithms

– These protocols built around Dijkstra’s SPF

4© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Link-State Routing Introduction 2 link state routing protocols used for routing IP

– Open Shortest Path First (OSPF)– Intermediate System-Intermediate System (IS-IS)

5© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Link-State Routing Introduction Link

– An interface on the router Link State

– Information about an interface such as• IP address• Subnet mask• Type of network• Cost associated with link• Neighboring routers on the link

6© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Link-State Introduction

Routingprotocol

BuildsTopological

map

Router can independently determine the

shortest path to every network.

Convergenceevent driven

routing updatesUseof

LSP

Distance vector

No No Slow Generally No No

Link State Yes Yes Fast Generally Yes Yes

Advantages of a Link-State Routing Protocol

7© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Link-State Introduction 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

8© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Link-State Packets Sending Hello Packets to

Neighbors

– 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

9© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Link-State Packets LSPs are sent out under the following conditions:

– Initial router start up or routing process– When there is a change in topology

10© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Steps Dikjstra’s algorithm also known as the shortest path first

(SPF) algorithm

11© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Steps - Link-State Routing Link-State Routing Process

– How routers using Link State Routing Protocols reach convergence

• Step 1 - Learns of directly connected links• Step 2 – Says Hello• Step 3 – Creates LSP• Step 4 – Tells it’s neighbors• Step 5a – Created it’s database• Step 5b – Creates SPF Tree

12© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Step 1 – Discovering it’s links Directly Connected

Networks

Link– This is an interface on a

router

Link state– This is the information

about the state of the links

13© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Step 2 – Saying Hello Sending Hello Packets to Neighbors

– 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

14© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Step 3 – Creating the LSP

15© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Step 4 – Telling it’s Neighbors Flooding LSPs to All Neighbors

– Once LSP are created they are forwarded out to neighbors

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

16© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Step 5a – Creating a Link-State Database R1 now has the topology

17© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Step 5b – Creating a SPF Tree

18© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Step 5b SPF Tree

19© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Steps – Routing Table Once the SPF algorithm has determined the shortest

path routes, these routes are placed in the routing table

20© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Updates

Triggered updates can be smallThis requires less bandwidth

After convereged, LSP use less bandwidth

Routing loops are difficult since each router has the whole topology

Each router independently determines route to each network

21© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Areas

Similar to Process ID’s or Autonomous Systems

Splits routing topology into smaller areas

Saves# of LSP Packets

Bandwidth for convergence

Processing time

22© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public