CCNA Exp2 - Chapter03 - Introduction to Dynamic Routing Protocols

8/3/2019 CCNA Exp2 - Chapter03 - Introduction to Dynamic Routing Protocols

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Objectives

Describe the role of dynamic routing protocols and placethese protocols in the context of modern network design.

Identify several ways to classify routing protocols. Describe how metrics are used by routing protocols and

identify the metric types used by dynamic routing protocols.

Determine the administrative distance of a route anddescribe its importance in the routing process.

Identify the different elements of the routing table.


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The Evolution of Dynamic Routing Protocols


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Dynamic Routing Protocols

Dynamic routing protocols help the network administrator overcomethe time-consuming and exacting process of configuring andmaintaining static routes.

28 routers shown in the figure? What happens when a link goes down?How do you ensure that redundant paths are available? Dynamic

routing is the most common choice for large networks like the oneshown.


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The Role of Dynamic Routing Protocol

Routing protocols are used to facilitate the exchange of routinginformation between routers.

Dynamic routing protocols allow routers to dynamically shareinformation about remote networks and automatically add thisinformation to their own routing tables.

Routing protocols determine the best path to each network which isthen added to the routing table.

Consuming system resources(CPU, Memory, bandwidth).


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Network discovery and routing table maintenance

The purpose of a routing protocol includes: Discovery of remote networks.

Maintaining up-to-date routing information.

Choosing the best path to destination networks.

Ability to find a new best path if the current path is no longer

available. Components of a routing protocol:

Data structures.

Algorithm.

Routing protocol messages.


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Dynamic routing operation

The router sends and receives routing messages on its interfaces.

The router shares routing messages and routing information with otherrouters that are using the same routing protocol.

Routers exchange routing information to learn about remote networks. When a router detects a topology change the routing protocol can

advertise this change to other routers.




Advantages



Dynamic Routing Protocols Classification



IGP and EGP

An autonomous system (AS) - otherwise known as a routing domain -

is a collection of routers under a common administration. Two types of routing protocols are: interior and exterior routing

protocols.

Interior Gateway Protocols (IGP) are used for intra-autonomoussystem routing - routing inside an autonomous system.

Exterior Gateway Protocols (EGP) are used for inter-autonomoussystem routing - routing between autonomous systems.



Distance Vector and Link State

Interior Gateway Protocols (IGPs) can be classified as two types:

Distance vector routing protocols

Link-state routing protocols

Distance Vector Routing Protocol Operation

Distance vector means that routes are advertised as vectors of

distance and direction. Distance is defined in terms of a metric such as hop count .

Direction is simply the next-hop router or exit interface.

Algorithm is Bellman-Ford.




Distance Vector Routing Protocol Features:

The network is simple and flat and does not require a specialhierarchical design.

The administrators do not have enough knowledge to configure andtroubleshoot link-state protocols.

Specific types of networks, such as hub-and-spoke networks, arebeing implemented.

Worst-case convergence times in a network are not a concern.




Link-state Protocol Operation

A link-state router uses the link-state information to create a topologymap and to select the best path to all destination networks in thetopology.

Link-state protocols Features: The network design is hierarchical, usually occurring in large

networks.

The administrators have a good knowledge of the implementedlink-state routing protocol.

Fast convergence of the network is crucial.



Classfull and Classless

Classful Routing Protocols

Classful routing protocols do not send subnet mask information inrouting updates.

Classful routing protocols cannot be used when a network is subnettedusing more than one subnet mask, in other words classful routingprotocols do not support variable length subnet masks (VLSM) and

inability to support discontiguous networks.



Classfull and Classless

Classless Routing Protocols

Classless routing protocols include the subnet mask with the networkaddress in routing updates.

Classless routing protocols are required in most networks todaybecause of their support for VLSM, discontiguous networks and otherfeatures which will be discussed in later chapters.

Classless routing protocols are RIPv2, EIGRP, OSPF, IS-IS, BGP.



Convergence

Convergence is when all routers' routing tables are at a state of

consistency. The network has converged when all routers have complete and

accurate information about the network.

Convergence time is the time it takes routers to share information,calculate best paths, and update their routing tables.

Convergence properties include the speed of propagation of routinginformation and the calculation of optimal paths.



Purpose of a Metric

A metric is a value used by routing protocols to assign costs to reach

remote networks. The metric is used to determine which path is most preferable when

there are multiple paths to the same remote network.



Metrics and Routing Protocols

The Metric Parameters

Different routing protocols use different metrics. Two different routingprotocols might choose different paths to the same destination due tousing different metrics.

Metrics used in IP routing protocols include: Hop count, Bandwidth,Load, Delay, Reliability, Cost.

RIP

OSPF



Metrics and Routing Protocols

The Metric Field in the Routing Table

The metric for each routing protocol is: RIP: Hop count

IGRP and EIGRP: Bandwidth, Delay, Reliability, and Load

IS-IS and OSPF: Cost



Load Balancing

But what happens when two or more routes to the same destination

have identical metric values? How will the router decide which path touse for packet forwarding?

In this case, the router does not choose only one route. Instead, therouter "load balances" between these equal cost paths. The packetsare forwarded using all equal-cost paths.



Purpose of Administrative Distance

Multiple Routing Sources

How does a router determine which route to install in the routing tablewhen it has learned about the same network from more than onerouting source?

The Purpose of Administrative Distance

Administrative distance (AD) defines the preference of a routingsource. Administrative distance is an integer value from 0 to 255.

Each routing source - including specific routing protocols, static routes,and even directly connected networks - is prioritized in order of most-to least-preferable using an administrative distance value.


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Purpose of Administrative Distance


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Dynamic Routing Protocols

show ip route show ip protocols


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Static Routes

Static routes are entered by an administrator who wants to manually

configure the best path to the destination.

Static routes have a default AD value of 1.

Directly connected networks, which have a default AD value of 0.


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Identifying Elements of the Routing Table


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Take advantage of AD to create backup route

R1 has a default route with AD = 0(default) R1(config)#ip route 0.0.0.0 0.0.0.0 s0/0

R3 has a second default used as backup route whenDefault route 1 becomes fail. Hence, Default route 1 andDefault route 2 must be propagated to all routers but the

AD of Default route 2 must be change.


Default route 2

Default route 1R1R2R3

S0/0

S0/1

RIP OSPF


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Take advantage of AD to create backup route

R2 learns Default route 1 from R1 through OSPF with AD = 110.

R3 learns Default route 1 from R2 through RIP with AD = 120. Use Default route 2 as backup route ADs Default route 2

must be greater than 120.

R3(config)#ip route 0.0.0.0 0.0.0.0 s0/1 180


Default route 2

Default route 1R1R2R3

S0/0

Modifiy AD to 180

RIP OSPF


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Summary


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Documents

CCNA Exp2 - Chapter03 - Introduction to Dynamic Routing Protocols