Routing Routed

7/30/2019 Routing Routed

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Routing/Routed Protocols

Part I


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Routed Protocol Definition:

Routed Protocol used to transmit user

data (packets) through an internetwork.

Routed protocols are assigned to an

interface and determine the method of

packet delivery.

Examples: IP, IPX, AppleTalk, DECNet,

Banyan Vines


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Routing Protocol Definition:

Routing protocol any protocol that

defines algorithms to be used for updating

routing tables between routers. Basically,

a routing protocol determines the path of a

packet through an internetwork.

Examples: RIP, RIPv2, IGRP, EIGRP,

OSPF, IS-IS, BGP


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Remember:

A Routed Protocol defines logical

addressing. Most notable example on the

test IP

A Routing Protocol fills the routing table

with routing information. Examples on the

test RIP, IGRP, EIGRP, OSPF, IS-IS


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CCNA Exam Tips -- Routing

Since IP routing is basically what Ciscorouters do, this protocol is the backdropfor the whole CCNA exam. See prior

chapter notes on IP Addressing for moreinfo.

Next, some routing protocols that are

strongly rumored to be on the CCNA 801will be outlined.

But first, some generalities about routingprotocols


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Main Goals of Routing Protocols

To fill the routing table with current best,

loop-free routes

To notice when routes in the table are no

longer valid and remove them from the

routing table

To add new routes or replace lost routes

The time for finding a working route is called

convergence.


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Two Categories of Routing

Protocols

Exterior Routing Protocols used for use

between different organizations such as

ISPs or ISPs and their customers.

Ex: Border Gateway Protocol (BGP)

Interior Routing Protocols used to

distribute routing information inside a

single organization.

Ex: RIP, IGRP, EIGRP, OSPF, IS-IS


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Border Gateway Protocol (BGP)

The most popular exterior routing protocol& the only one on the CCNA 801 exam

ISPs use BGP to exchange routing info

between themselves and other ISPs andcustomers.

BGP advertises only routing info to

specifically defined peers using TCP. BGP does not use a metriclike internal

routing protocols


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Terminology of Interior

Routing Protocols

This is not as painful as it sounds.

There are only 6 basic concepts.


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TYPE of routing protocol

Each interior routing protocol can becharacterized based on the underlyinglogic used by the routing protocol.

The underlying logic is referred to as theTYPEof routing protocol.

The three types are:

1) Distance vector2) Link-state

3) Hybrid


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Full/partial Update

Full routing updates entire routing tables

are sent regularly

Partial routing updates only a subset of

the routing table is sent, typically just

information about changed routes.

Partial routing updates require less

overhead than full routing updates.


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Convergence

Convergence refers to the time required

for routers to react to changes in the

network.


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Metric

The metric refers to the numeric value that

describes how good a particular route is.

The lower the value, the better the route.


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Support for VLSM

Variable-length subnet masking (VLSM)means that, in a single Class A, B, or Cnetwork, multiple subnet masks can be

used. The advantage of VLSM is that it enables

you to vary the size of each subnet, basedon the needs of that subnet.

Some routing protocols support VLSM,and some do not


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Classless or Classful

Classless routing protocols transmit the

subnet mask along with each route in the

routing updates sent by that protocol.

Classful routing protocols do not transmit

mask information.

Only classless routing protocols support

VLSM. To say that a routing protocol is

classless is to say that it supports VLSM.


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Now we apply those terms to

some interior routing protocols.

.


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Distance Vector Protocols: RIP and

IGRP

Distance vector protocols advertiserouting information by sendingmessages, called routing updates, out

the interfaces on a router. These updates contain a series of

entries, with each entry representing asubnet and a metric.

Failure to receive updates from aneighbor in a timely manner results inthe removal of the routes previously

learned from that neighbor.


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Distance Vector Protocols: RIP and

IGRP

Routers send periodic full updates andexpect to receive periodic updates fromneighboring routers.

When possible, routers use broadcastsor multicasts to send routing updates.This way, all neighbors on a LAN canreceive the same routing information ina single update.

If a router learns multiple routes to thesame subnet, the router chooses the

best route based on the metric.


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Routing Information Protocol (RIP)

Been around 15+ years for use with IP

networks.

Easier to use than some newer routing

protocols, but severely limited in

comparison.


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Basic RIP Summary

Based on distance vector logic

Uses hop count for the metric

Hop count = number of routers between

two points Sends periodic full routing updates every 30

seconds

Converges slowly, often taking 3 to 5

minutes Does not support VLSM, also making it a

classful routing protocol


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Interior Gateway Routing Protocol

(IGRP)

IGRP is a Cisco-proprietary IP routingprotocol created to provide a betterdistance vector protocol.

The most obvious difference betweenRIP-1and IGRP is the metric.

IGRP advertises up to five parameters that

describe the metric for each route,although, by default only two are used bandwidth and delay.


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IGRP

Other three possible parameters usedto describe IGRP metric can include:reliability, load, and MTU (maximumtransmission unit).

IGRP calculates the metric based on amathematical formula that you do notreally need to know for the exam.

(Wendell Odom, CCNA INTRO, p.415)


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Distance Vector Protocols

Feature RIP (v1) IGRP

Update timer for fullrouting updates

30 seconds 90 seconds

Metric Hop Count Function ofbandwidth anddelay (default)

Supports VLSM No No

Infinite-metricvalue

16 4,294,967,295

Convergence Slow Slow


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Link-State Protocols: OSPF and

Integrated IS-IS

The goal of link-state protocols is to fill therouting tables with the current best routes.

Link-state advertises a large amount of

topological info about the network Discovers neighbor routers before

exchanging routing information.

A router running a link-state protocol usesmore memory and more processing cyclesthan do distance vector protocols.


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Link-State Protocols: OSPF and

Integrated IS-IS

To figure out the current best routes, arouter processes the link-state topologydatabase using an algorithm called the

Dijkstra Shortest Path First (SPF)algorithm.

This info helps link-state protocols avoidloops & converge quickly.

Quick convergence often less than 10seconds.


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Open Shortest Path First (OSPF)

OSPF is the most popular link-state IP

routing protocol today.

Because OSPF does not send full updates

on a regular short interval (like RIP), the

overall number of bytes sent for routing

information is typically smaller.


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Open Shortest Path First (OSPF)

OSPF uses a concept called costfor the

metric. Each link is considered to have a

cost; a routes cost is the sum of the cost

for each link.

By default, Cisco derives the cost value for

a link from the bandwidth.

OSPF supports VLSM.


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Integrated IS-IS

OSI defines a network layer protocol

called the Connectionless Network

Protocol (CLNP). It also defines a routing

protocol a routing protocol used toadvertise CLNP routes, called

Intermediate System-to-Intermediate

System (IS-IS). IS-IS advertises CLNProutes between intermediate systems,

which is what OSI calls routers.


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Integrated IS-IS

Integrated IS-IS has the capability to

advertise IP routes as well as CLNP

routes.

most installations could care less about

CLNP. (Wendell Odom, CCNA INTRO,

p.419)

Supports VLSM


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Balanced Hybrid Protocols:

Enhanced IGRP

EIGRP uses features similar to link-stateprotocols, and others similar to distancevector protocols, and yet others unlike

either of the two. The internal workings of EIGRP depend

on an algorithm called the DiffusingUpdate Algorithm (DUAL). Requires less processing than the Dijkstra

SPF algorithm.


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EIGRP Summary

A balanced hybrid protocol

Converges in less than 3 seconds

Discovers neighbors (via Hello packets)

before sending them information.

Requires little design effort

Supports VLSM

Cisco proprietary Metric based on bandwidth & delay, scaled

by multiples of 256.


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Thats IT

Next week, May 3, to be discussed:

-- Routing Protocol Configuration

Commands

-- Routing Protocol Logic

-- VLSM

Documents

Routing Routed