routing&routing protocols

7/30/2019 Routing&Routing Protocols

1/10

Routing & Routing Protocols

Introduction When networking was first introduced, only a small number of

devices were interconnected As networks grew, broadcasts and collisions caused significant

issues In order to reduce broadcasts, Layer 3 devices, protocols, and

addresses are used to segment the network

Routing Basics Reasons to segment a network at Layer 3 include:

Creation of small troubleshooting areas Creation of small administrator management areas Interconnection of remote offices using WAN technologies Grouping clients together with similar network resources

A router is required to properly forward data betweenclients on different segments.

A routeris a device that forwards data based on alogical Layer 3 address Many routers support the use of different protocols

The routing process usually occurs between physicalnetwork interfaces but can also be accomplishedbetween logical network interfaces

When a single physical network interface has morethan one address assigned, it is said to have a logical

interface A router can be any network device with the proper

software to make routing decisions


2/10

Two key pieces of information are required for anydevice to route packets:

A route to one or more networks A destination Layer 3 address

The routes to a given network are stored in the memoryof the router and are referred to as a routing table

On a Router there is an entry for each network and itscorresponding network interface kept in memory

The table allows the router to properly forward framesout the correct interface

This occurs once the frame is received and thedestination address has been processed

A routing table only maintains the best possible route toa destination, not all possible routes

Routing table entries have the following functions: Network destination Netmask Gateway Interface Metric

The task of populating the routing table is accomplishedby using either dynamic routing or static routing

Dynamic Routing uses routing protocols to build routetables automatically

Static routing requires manual route table entries andupdates to different networks


3/10

Routing Protocols

Routing Protocol Classification

Dynamic routing using routing protocols

Routing Table & Routing protocol

The goal of a routing protocol is to build and maintain the routing table.This table contains the learned networks and associated ports for those

networks.Routers use routing protocols to manage information received from other

routers, information learned from the configuration of its owninterfaces, along with manually configured routes

Routing Protocols

Interior Exterior

DistVector LinkState Hybrid

RIPIGRP

OSPF EIGRP

EGPBGP


4/10

Purpose of routing protocols is to build a routing tablewith the best routes

Interior Routing protocols are categorized into twotypes:

Distance Vector Link State Hybrid

Distance Vector Routing Protocols

Distance vector routing protocols are simple Generally they are easy to configure They use simple logic to determine the best path to a

given destination The term metric refers to the method or measurement

used by the routing protocol logic to determine the bestpath to a given network

A distance vector routing protocol usually uses hopcount as its metric

A distance vector routing protocol is characterized byhow it communicates with other routing devices

Distance vector routing protocols use broadcasts toadvertise their entire routing table to directly connectedpeer routers

Convergence is the time it takes for a given set ofrouters to learn routes to all networks

Convergence describes the time it takes a set of routersto learn of a change in the network

Distance vector routing protocols generally take longer

to converge than link state protocols because they usea periodic route advertisement schedule

A routing loop occurs when routers get confused duringupdate operations, causing frames to bounce back andforth between a set of interfaces


5/10

Two easy methods to identify routing loops: Tracert or traceroute TCP/IP utilities

View the routing table and the metric associated with thenetwork

Prevent routing loops by using the following softwarebased methods:

Split horizon Hold-down timers Triggered updates Hop count limits Poisoning

Link State Routing Protocols Link state routing protocols are more intelligent than

distance vector protocols The metric used by most link state protocols is

bandwidth allowing more complex routingconfigurations

Routing protocols capable of making complex decisionsuse a mathematical formula or algorithm for deriving

the best path or route to a given network

Some link state protocols are capable of determiningthe best route to a destination network based on thefollowing:

Delay Load Reliability MTU

When more than one metric is used it is referred to as acomposite metric

Link state protocols only send updates when changesoccur, and they only send the changes, not the entireroute table


6/10

Link state protocols use multicast and unicast trafficinstead of broadcast traffic

Link state routers also develop an overall picture of thenetworks available by establishing neighbor

relationships

Administrative Classification Routing protocols are also separated by an

administrative classification based on where they areused in the networking environment:

Interior gateway protocols (IGPs) are used within acompanys network infrastructure to maintain routingtables and policies set by the network administrators

The two industry standard IGPs are: Routing Information Protocol

Interior Routing protocols operated within an Autonomous System.

Exterior Routing protocols operated between Autonomous Systems.

Interior Routing Vs Exterior Routing


7/10

Interior Gateway routing Protocol Open Shortest Path First

RIP is a distance vector protocol that uses hop count

for its metric when determining the best route to a givennetwork In most implementations, RIP uses split horizon, hop

count limit, and poisoning for routing loop prevention RIP is a classful routing protocol IGRP is a distance-vector routing protocol

developed by Cisco IGRP was developed specifically to address problems

associated with routing in large networks that werebeyond the range of protocols such as RIP

IGRP can select the fastest available path based ondelay, bandwidth, load, and reliability

IGRP also has a much higher maximum hop count limitthan RIP

IGRP has proven to be one of the most successfulrouting protocols.

A highly successful and widely deployed routing

protocol. Preserving many the effective features of RIP, while

greatly expanding its capabilities. Disadvantage of IGRP is the lack of supporting for

variable-length subnet masks (VLSM). EIGRP comes tosolve that problem.

The shortest path as measured by Open Shortest

Path First (OSPF) is actually the fastestpath based onbandwidth

Shortest refers to the shortest time OSPF is used in large networks and ones requiring

more intelligence than distance vector routing protocols


8/10

OSPF communicates using unicast and multicastpackets

It only transmits changes or updates to the routing tablewhen they occur

It uses hello packets to determine the current state of alink between itself and its neighbors

It utilizes a link state database to maintain a local viewof the entire routing environment

The configuration possibilities using OSPF Areas Autonomous system (AS) Backbone router Area border router (ABR) Autonomous system boundary router (ASBR)

Exterior Routing Protocols The decision making process of EGPs is far more

complex than that of internal protocols The power and routing flexibility associated with EGPs

requires knowledge and understanding of the complexnature of your network and its traffic

EGPs can let you influence and manage traffic only asit enters or leaves your AS

One member of EGPs is the Exterior Gateway Protocol(EGP) EGP was the first protocol developed that allowed

isolation of autonomous systems EPG is not used today and is replaced by the Border

Gateway Protocol


9/10

Border Gateway Protocol (BGP) version 4 is themost widely used exterior protocol in the world

BGP is a well established standard and commonly usedby ISPs and in very large companies

there are actually two different classifications of BGP internal external

iBGP is used for internal routing eBGP is used for external routing iBGP functions under different rules than eBGP If two routers running BGP are in the same AS, they are

running an iBGP connection

BGP communication starts by establishing peers Once the peers have been established, BGP routing

information is exchanged and updated as necessary BGP is an advanced distance vector protocol that uses

triggered updates for communicating changes in therouting environment

Routing loops in BGP are avoided by using the AS-path

attribute BGP uses active TCP sessions that are setup and

continuously maintained. Convergence in the routingenvironment is very fast

BGP has features that you can use to help speed theconvergence of the network routes under your control

Hierarchical Routing Hierarchical routing depends on hierarchical

addressing It is a routing technique originally designed to help

reduce the size of the routing tables on the Internet aswell as speed up the overall routing process


10/10

The concept uses an address block or blocks torepresent different sections of a network

Summarizing routes is often referred to as supernetting

networks The process of summarization is built around the binary

bit patterns just as in subnetting The difference is that rather than extending the subnet

mask by adding bits, we remove bits

By using summarization, you reduce the routing tableson each router

To accomplish the summarization, you need todetermine how many bits to unmask or un-subnet inorder to make the networks appear as one big addressblock

The routing protocol must transmit the network prefixalong with the network address during routeadvertisements

routing&routing protocols

Documents