Distance Vector vs. Link State docx

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Distance Vector

Link State

Hybrid

Distance Vector vs. Link State

Route table

Topology Incremental Update

Routing by rumor Periodic Update

A B C D

X

E

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Distance Vector vs. Link State

Distance Vector

• Updates frequently

• Each router is

"aware" only of its

immediate neighbors

• Slow convergence

• Prone to routing loops

• Easy to configure

Link State

• Updates are event

triggered

• Each router is

"aware" of all other

routers in the "area"

• Fast convergence

• Less subject to

routing loops

• More difficult to

configure

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Comparison Continued

Distance Vector

• Fewer router resources

required

• Updates require more

bandwidth

• Does not "understand"

the topology of the

network

Link State

• More router resource

intensive

• Updates require less

bandwidth

• Has detailed knowledge

of distant networks and

routers

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Link State

Example

OSPF

IS-IS

OSPF is used for corporate networks

IS-IS is used for ISP’s

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Open Shortest Path First (OSPF)

 OSPF is an open standards routing protocol

 This works by using the Dijkstra algorithm

 OSPF provides the following features:

Minimizes routing update traffic

Allows scalability (e.g. RIP is limited to 15 hops)

Has unlimited hop count

Supports VLSM/CIDR

Allows multi-vendor deployment (open standard)

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Link State

There are two types of Packets

Hello

LSA’s

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OSPF Hello

• When router A starts it send Hello packet – uses 224.0.0.5

• Hello packets are received by all neighbors

• B will write A’s name in its neighbor table

• C also process the same way

A

B C

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"Hello" Packets

• Small frequently issued packets

• Discover neighbours and negotiate "adjacencies"

• Verify continued availability of adjacent neighbours

• Hello packets and Link State Advertisements (LSAs)

build and maintain the topological database

• Hello packets are addressed to 224.0.0.5.

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Link State Advertisement

(LSA)

An OSPF data packet containing link state and routing

information that is shared among OSPF routers

LSAs are shared only with routers with whom it has

formed adjacencies

LSA packets are used to update and maintain the

topology database.

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Link State

There are three type of tables

Neighbor

Topology

Routing

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Tables

 Neighbor

Contain information about the neighbors

Neighbor is a router which shares a link on same

network

Another relationship is adjacency

Not necessarily all neighbors

LSA updates are only when adjacency is established