Distance Vector Routing

gascitytankNetworking and Communications

Oct 28, 2013 (4 years and 9 days ago)

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ITE PC v 4.0

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Distance Vector Routing
Protocols

Routing Protocols and Concepts


Chapter 4

Sandra Coleman, CCNA, CCAI


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Objectives


Identify the characteristics of distance vector routing
protocols.


Describe the network discovery process of distance
vector routing protocols using Routing Information
Protocol (RIP).


Describe the processes to maintain accurate routing
tables used by distance vector routing protocols.


Identify the conditions leading to a routing loop and
explain the implications for router performance.


Recognize that distance vector routing protocols are
in use today


Light Red is the test
-
worthy information

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Static routing? I don’t think so..

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Distance Vector Routing Protocols


Examples of Distance Vector routing protocols
:


Routing Information Protocol (RIP)


Version 1 & 2


Interior Gateway Routing Protocol (IGRP)


Enhanced Interior Gateway Routing Protocol
(EIGRP)



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Distance vector comparisons

Protocol

Metric

Routing
update sent

Rip

Hop count

30 seconds

IGRP

Bandwidth,
delay,

load,
reliability

90 seconds

EIGRP

Hybrid,

proprietary
protocol

Topology
change
triggers

Make sure you can determine a path packets will take
BASED on the routing protocol used!

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Distance Vector Routing Protocols


Distance Vector Technology


The Meaning of Distance Vector
:


A router using distance vector routing protocols
knows 2 things:


Distance
to final destination


Vector, or direction,
traffic should be
directed

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Distance Vector Routing Protocols

Characteristics of Distance Vector routing protocols
:



Periodic updates


30 or 90 seconds



Neighbors







Broadcast updates


255.255.255.255



Entire routing table is included with routing update

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Distance Vector Routing Protocols


Routing Protocol Algorithm
:


-
Defined as
a procedure for accomplishing a certain task

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Distance Vector Routing Protocols

Routing Protocol Characteristics


Criteria used to compare routing protocols

includes


Time to convergence


how quickly all routers
share consistent information


Scalability


can the network continue to
grow?


Resource usage
-

memory, CPU, bandwidth


Implementation & maintenance


intelligence
level required of network admin.


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Distance Vector Routing Protocols

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Network Discovery


Router initial start up

(Cold Starts)

-
Initial network discovery


Directly connected networks are initially placed in
routing table These will be sent to their directly
connected neighbors! Also true if the router loses
power for any reason!

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Network Discovery


Initial Exchange

of Routing Information


If
a routing protocol

is
configured

then




-
Routers will exchange routing information


Routing updates received from other routers
(play the
animation in 4.2.2.1 & 4.2.3.1)



-
Router checks update for new information


If

there is
new information
:




-
Metric is updated




-
New information is




stored in routing table

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Network Discovery


Exchange of Routing Information


Router convergence

is reached when




-
All routing tables in the network contain the same


network information


Routers continue to exchange routing information



-
If no new information is found then Convergence is

reached

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Network Discovery


Convergence must be reached

before a network is
considered completely operable
(animation 4.2.4.1
-

count # rounds)


Speed of achieving convergence consists of 2
interdependent categories



-
Speed of broadcasting routing information



-
Speed of calculating routes

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Routing Table Maintenance


Periodic Updates
: RIPv1 & RIPv2



These are
time intervals

in which a router sends

out its entire routing table.


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RIPv2 Updates


Changes may occur for several reasons,
including:

Failure of a link

Introduction of a new link

Failure of a router

Change of link parameters

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Routing Table Maintenance


RIP uses 4 timers

-
Update timer (30 seconds)


time since last update
shows up in the routing table

-
Invalid timer (180 seconds)
-

route is marked as
invalid if not updated in this amt. of time

-
Holddown

timer (180 seconds)


amt of time a route
must stay marked as unreachable so that all routers
learn about it

-
Flush timer (240 seconds)


the amount of time that
passes before a route is actually removed from the
routing table (60 seconds longer than the invalid
timer).

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Update timer


time
SINCE

last update

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Routing Table Maintenance


Bounded Updates: EIGRP


EIRPG routing updates are sent
only

to the routers that
need it and
only

about the changes in the links…not
the entire routing table



-
Partial updates



-
Triggered by topology changes



-
Bounded



-
Non periodic

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Routing Table Maintenance


Triggered Updates


Conditions in which triggered updates are sent



-
Interface changes state



-
Route becomes unreachable



-
Route is placed in routing table

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Routing Table Maintenance


Random Jitter

Synchronized updates

A condition where multiple routers on multi access LAN
segments transmit routing updates at the same time.


Problems

with synchronized updates

-
Bandwidth consumption

-
Packet collisions


Solution

to problems with

synchronized updates



-

Used of random variable



called RIP_JITTER






subtracts a variable amount of time to the update interval for
each router in the network. This random jitter, or variable amount
of time, ranges from 0% to 15% of the specified update interval.

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Networks NOT converged?


Can cause the following problems:

Routing loops

Inconsistent traffic forwarding

Inconsistent routing table entries

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Routing Loops


Routing loops are


A condition in which
a packet is
continuously
transmitted within a
series of routers
without ever
reaching its
destination.

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Routing Loops


Routing loops

may be
caused by
:



-
Incorrectly configured static routes



-
Slow convergence



-
Incorrectly configured discard routes


Routing loops

can
create the following issues



-
Excess use of bandwidth



-
CPU resources may be strained



-
Network convergence is degraded



-
Routing updates may be lost or not processed in a timely

manner

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Routing Loops


Count to Infinity



This is
a routing loop whereby packets bounce

infinitely around a network.

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Routing Loops


Setting a maximum


for RIP it is 16, once it reaches the
16
th

hop, it will declare the network UNREACHABLE!


Distance Vector routing protocols

set a specified metric
value to indicate infinity


Once a router “counts to infinity” it marks the route as
unreachable

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Routing Loops


Preventing loops with
holddown

timers

-
Holddown

timers allow a router to not accept any changes to a
route for a specified period of time. (IGNORE updates!)

-
Point of using
holddown

timers


Allows routing updates to propagate through network with
the most current information.

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Routing Loops


The
Split Horizon Rule

is
used to prevent routing
loops


Split Horizon rule
:



A router should not advertise a network through the

interface from which the update came.

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Routing Loops


Split horizon with poison
reverse



The rule states that

once a router learns of

an unreachable route

through an interface,

advertise it as

unreachable back

through the same

interface



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Route Poisoning


Route poisoning is yet another method employed by
distance vector routing protocols to prevent routing
loops.


Route poisoning is used to mark the route as
unreachable (metric of 16) in a routing update that is
sent to other routers.


Unreachable is interpreted as a metric that is set to the
maximum. For RIP, a poisoned route has a metric of
16.


this speeds up the convergence process


quicker than
waiting for ‘count to infinity’


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TTL


Time to Live


IP & TTL


prevent undeliverable packets from
circulating on the network endlessly


Purpose of the TTL field




The TTL field is found in an IP header and is


used to prevent packets from endlessly



traveling on a network


How the TTL field works



-
TTL field contains a numeric value




The
numeric value is decreased by one

by


every router on the route to the destination.





If
numeric value reaches

0

then Packet



is discarded.

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Routing Protocols Today


Factors used to determine whether to use RIP or EIGRP
include



-
Network size



-
Compatibility between models of routers



-
Administrative knowledge

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Routing Protocols Today


RIP


Features of RIP:




-
Supports
split horizon & split horizon with


poison reverse




-
Capable of
load balancing





-
Easy to configure




-
Works in a multi vendor router environment

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Routing Protocols Today


EIGRP


Features of EIGRP
:




-
Triggered updates




-
EIGRP hello protocol used to establish



neighbor adjacencies




-
Supports VLSM & route summarization




-
Use of topology table to maintain all routes




-
Classless distance vector routing protocol




-
Cisco proprietary protocol

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You’ve survived yet another chapter in your life!... Ch. 4

1.
NOW


open up the cisco curriculum and go to section 4.1.4.2. I
want you to do this
during

this class meeting until you get it correct
(Really think about it based on what you’ve learned today!)


2.
Study Guide


due on test day!

1.
Routing table maintenance


pg. 172


174

2.
Routing loops


pg. 174


176

3.
Comparing distance vector routing protocols


pg. 176


3.
Online Test


ch. 4 will be turned on until Sunday, Feb. 17, 2013
midnight!


4.
Labs


4
-
1

we will do it in Packet tracer. Your test will be similar!


5.
Test on Ch. 4


Monday, Feb. 18,, 2013


You will be given routing
tables of 4 different routers and you will have to build the topology in
packet tracer!