ca ex s2m10 link-state routing protocol

8/3/2019 CA Ex S2M10 Link-State Routing Protocol

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Please purchase a personal

license.

CCNA Ex loration version 4.0


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Objectives

Describe the basic features & concepts of link-staterouting protocols.

List the benefits and requirements of link-state routingprotocols.

Hc vin mng Bach Khoa - Website: www.bkacad.com 2


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Link-State Routing



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Link-State Routing Protocols

Link state routing protocolsAlso known as shortest path first algorithms

Built around Dijkstras SPF



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Introduction to the SPF Algorithm

Dikjstras algorithm also known as the shortest path first (SPF)algorithm.

This algorithm accumulates costs along each path, from source to

destination. Each router determines its own cost to each destination in the

topology.



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Introduction to the SPF Algorithm

The shortest path to a destination is not necessarilythe path with the least number of hops



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Link-State Routing Process

How routers using Link State Routing Protocols reach convergence


. .

2. Link state routers exchange hello packet to meet other directlyconnected link state routers.

3. Each router builds its own Link State Packet (LSP) whichincludes information about neighbors such as neighbor ID, linktype, & bandwidth.

4. After the LSP is created the router floods it to all neighbors whothen store the information and then forward it until all routershave the same information.

5. Once all the routers have received all the LSPs, the routers thenconstruct a topological map of the network which is used to

determine the best routes to a destination.


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Link: this is an interface on arouter

Information about the state of

those links is known as link-states.This informationincludes:

The interface's IP addressand subnet mask.

1.Learning about Directly Connected Network


The type of network, suchas Ethernet (broadcast) orSerial point-to-point link.

The cost of that link.

Any neighbor routers onthat link.


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2.Sending Hello Packets to Neighbors


Routers with link-state routing protocols use a Helloprotocol to discover any neighbors on its links. Aneighbor is any other router that is enabled with thesame link-state routing protocol.


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When 2 link-state routers learn thatthey are neighbors, they form an

adjacency. These small Hello packetscontinue to be exchanged betweentwo adjacent neighbors whichserve as a "kee alive" function to

2.Sending Hello Packets to Neighbors


monitor the state of the neighbor. If a router stops receiving Hello

packets from a neighbor, thatneighbor is consideredunreachable and the adjacency is

broken.


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Each router builds its ownLink State Packet (LSP).

Contents of LSP:1. State of each directly

connected link

2. Includes informationabout neighbors

3.Building the Link-State Packet


,

link type, &bandwidth.


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4.Flooding Link-State Packets to Neighbors


Each router floods the LSP to all neighbors, who then store all LSPsreceived in a database.

Each router floods its link-state information to all other link-state routersin the routing area.

Whenever a router receives an LSP from a neighboring router, itimmediately sends that LSP out all other interfaces except the interfacethat received the LSP.


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LSPs are sent out under the following conditions During initial startup of the router or of the routing protocol process on

that router

Whenever there is a change in the topology, including a link going downor coming up, or a neighbor adjacency being established or broken



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Unlike distance vector routing protocols that must firstrun the Bellman-Ford algorithm to process routingupdates beforesending them to other routers, link-staterouting protocols calculate the SPF algorithm aftertheflooding is complete.

As a result, link-state routin rotocols reach


convergencemuch fasterthan distance vector routingprotocols.


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5.Constructing a link state database

Each router uses the database to construct a complete map of thetopology and computes the best path to each destination network.



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Each router in the topologydetermines the shortest path from itsown perspective.

5.Constructing a link state database



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Shortest Path First (SPF) Tree

ignore


Building a portion of the SPF

tree:Process begins by examiningR2s LSP informationR1 ignores1st LSP: R1 alreadyknows its connected to R2


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Building a portion of the SPFtree:

R1 uses 2nd LSPReason: R1 can create a linkfrom R2 to R5. This informationis added to R1s SPF tree


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Building a portion of the SPF

tree:R1 uses 3rd LSP

Reason: R1 learns that R2is connected to 10.5.0.0/16.

This link is added to R1s

SPF tree.


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Determining the shortestpath

The shortest path to a


est nat on eterm neby adding the costs &finding the lowest cost


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ImplementingLink-State Routing Protocols



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Routingprotocol

BuildsTopological

map

Router canindependentlydetermine the

shortest path toConvergence

A periodic/event driven

routing updates

Useof

LSP

Advantages of a Link-State Routing Protocol


Distancevector

No No Slow Yes/Yes No

Link State Yes Yes Fast No/Yes Yes


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Advantages of a Link-State Routing Protocol



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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


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Requirements for using a link state routing protocol

2 link state routing protocols used for routing IPOpen Shortest Path First (OSPF) Intermediate System-Intermediate System (IS-IS)


Multi-areas OSPF and IS-IS are discussed in CCNP


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Summary



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ca ex s2m10 link-state routing protocol

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