CCNA 2 Module 7

gascitytankNetworking and Communications

Oct 28, 2013 (3 years and 7 months ago)

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CCNA 2 Module 7

Distance Vector Routing
Protocols

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Objectives


Distance vector routing


RIP


IGRP

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Overview


Dynamic routing makes it possible to avoid the
configuration of static routes.


Dynamic routing makes it possible to avoid the time
-
consuming and exacting process of configuring static
routes.


Dynamic routing also makes it possible for routers to
react to changes in the network and to adjust their
routing tables accordingly.


However, dynamic routing can cause problems.

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Distance Vector Updates

Routing table updates occur
periodically or when the topology
in a distance vector protocol
network changes.

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Problem: Routing Loops


Routing loops can occur when
inconsistent routing tables are
not updated due to slow
convergence in a changing
network.

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Problem: Counting to Infinity

This condition, which is called
count to infinity
, loops packets around the
network in spite of the fact that the destination network, which is Network 1,
is down.


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Solution: Defining a Maximum for Infinity

The invalid updates of Network 1 will continue to loop until some other process
stops the looping.


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Solution: Split Horizon

Some routing loops occur when incorrect information that is sent back to a
router contradicts the correct information that the router originally distributed.

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Solution: Route Poisoning

Route poisoning is used by various distance vector protocols to overcome large
routing loops and offer detailed information when a subnet or network is not
accessible. To accomplish this, the hop count is usually set to one more than
the maximum.


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Solution: Triggered Updates


A
triggered update

is sent immediately in response to some
change in the routing table.


The router that detects a topology change immediately sends an
update message to adjacent routers.



Triggered updates, used in conjunction with route poisoning,
ensure that all routers know of failed routes before any holddown
timers can expire.

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Solution: Holddown Timers

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Solution: Holddown Timers

Continued


When router receives an update indicating that network is
inaccessible, the router marks the route as
inaccessible

and starts a holddown timer.



If at any time before the holddown timer expires an
update is received from the same neighbor indicating that
the network is again accessible, the router marks the
network as accessible and removes the holddown timer.


If an update arrives from a different neighboring router
with a better metric than originally recorded for the
network, the router marks the network as accessible and
removes the holddown timer.


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Key Characteristics of RIP


To prevent indefinite routing loops, RIP implements a limit on the
number of hops allowed in a path from a source to a destination.


The maximum number of hops in a path is 15.


RIP implements
split horizon

and
holddown

mechanisms to
prevent the propagation of incorrect routing information.

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RIP V1 and RIP V2


RIP has evolved over the years from a Classful Routing
Protocol,
RIP Version 1

(RIP v1), to a Classless Routing
Protocol,
RIP Version 2

(RIP v2).


RIP v2 enhancements include:


Ability to carry additional packet routing information.


Authentication mechanism to secure table updates.


Supports variable length subnet masking (VLSM).

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Configuring RIP


Using router rip and network commands to Enable RIP


Enabling RIP on an IP
-
addressed network


Monitoring IP packet flow using the show ip protocol
command


The
show ip route

command

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Configuring RIP

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Configuring RIP (triggered update)


A router running RIP can be configured to send a
triggered update when the network topology changes
using the
ip rip triggered

command.


This command is issued only on
serial interfaces

at the
router(config
-
if)# ip rip triggered

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Using the
ip classless

Command


Sometimes a router receives packets destined for an
unknown subnet of a network that has directly connected
subnets.


In order for the Cisco IOS software to forward these packets
to the best supernet route possible, use the
ip classless

global configuration command.


A supernet route is a route that covers a greater range of
subnets with a single entry. For example, an enterprise uses
the entire subnet 10.10.0.0 /16, then a supernet route for
10.10.10.0 /24 would be 10.10.0.0 /16.


When this feature is disabled, any packets received that are
destined for a subnet that numerically falls within the
router’s subnetwork addressing scheme will be discarded.

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Using the
ip classless

Command

Continued

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RIP Configuration Issues


To reduce routing loops and counting to
infinity, RIP uses

the following
:



Defining

infinity



Split horizon



Route poisoning & poison reverse



Triggered updates



Holddown timers


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RIP Configuration Issues (Continued)


The split horizon rule is based on the theory that it
is not useful to send information about a route back
in the direction from which it came.


The following command is used to disable
split
horizon
:

GAD(config
-
if)#no ip split
-
horizon


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RIP Configuration Issues (Continued)


Holddown

timers help prevent counting to infinity but also
increase convergence time.


The default holddown for RIP is
180
seconds. This will prevent
any inferior route from being updated but may also prevent a
valid alternative route from being installed.


The holddown timer can be decreased to speed up convergence
but should be done with caution.


To change the holddown timer:

Router(config
-
router)#timers basic update invalid
holddown flush

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RIP Configuration Issues (Continued)


Ideally, the timer should be set just longer than the
longest possible update time for the internetwork.

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RIP Configuration Issues (Continued)


One additional item that affects convergence time, and is
configurable, is the update interval.


The default RIP update interval in Cisco IOS is 30
seconds.


This can be configured for longer intervals to conserve
bandwidth, or for shorter intervals to decrease
convergence time.


To change the update internal:

GAD(config
-
router)#update
-
timer seconds

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RIP Configuration Issues (Continued)


Another issue with routing protocols is the unwanted
advertisement of routing updates out a particular interface.


To control the set of interfaces that will exchange routing
updates, the network administrator can disable the sending
of routing updates on specified interfaces by configuring
the
passive
-
interface

command.

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The
show ip protocols

Command

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The
show ip route

Command

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Troubleshooting RIP Update Issues


debug ip rip


show ip rip database



show ip protocols {summary}



show ip route



debug ip rip {events}



show ip interface brief

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Preventing Routing Updates through
an Interface

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Load Balancing with RIP


Load balancing is a concept
that allows a router to take
advantage of multiple best
paths to a given destination.


These paths are either
statically defined by a
network administrator or
calculated by a dynamic
routing protocol such as RIP.



RIP is capable of load balancing over as many as
six

equal
-
cost paths, with
four
paths being default.


RIP performs what is referred to as “round robin” load
balancing. This means that RIP takes turns forwarding
packets over the parallel paths.

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Administrative Distance

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Integrating Static Route with RIP


A router running RIP can receive a default route via
an update from another router running RIP.


Another option is for the router to generate the
default route itself.


The administrator can override a static route with
dynamic routing information by adjusting the
administrative distance values.

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IGRP Features


IGRP is a distance vector routing protocol developed by
Cisco. IGRP sends routing updates at 90 second
intervals, advertising networks for a particular
autonomous system.


By default, the IGRP routing protocol uses bandwidth and
delay as metrics.


Additionally, IGRP can be configured to use a
combination of variables to determine a composite
metric. Those variables include:


Bandwidth, Delay, Load and Reliability


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IGRP Commands


Using
router igrp

and
network

commands to enable
IGRP


Enabling IGRP on an IP
-
addressed network


Monitoring IP packet flow using the
show ip
protocol

command


The
show ip interfaces

command


The
show ip route

command


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IGRP Metrics


Bandwidth


The lowest bandwidth value in the path



Delay


The cumulative interface delay along the path



Reliability


The reliability on the link towards the
destination as determined by the exchange of keepalives



Load


The load on a link towards the destination based
on bits per second


By default, only bandwidth and delay are considered.


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IGRP Routes:

Interior, System, & Exterior

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IGRP Stability Features


Holddowns, Split horizons & Poison
-
reverse updates

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IGRP Timer


The
update timer

specifies how frequently routing update messages
should be sent. The IGRP default for this variable is 90 seconds.


The
invalid timer

specifies how long a router should wait before
declaring that route invalid. The IGRP default for this variable is three
times the update period.


The
holddown timer

specifies the amount of time for which
information about poorer routes is ignored. The IGRP default for this
variable is three times the update timer period plus 10 seconds.


Finally, the
flush timer

indicates how much time before a route is
flushed from the routing table. The IGRP default is seven times the
routing update timer.


IGRP lacks support for variable length subnet masks (VLSM). Rather
than develop an IGRP version 2 to correct this problem, Cisco has
built upon IGRP's legacy of success with Enhanced IGRP.

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Configuring IGRP

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Verifying IGRP Configuration


Some commands for checking IGRP configuration are as
follows:

show interface
interface

show running
-
config

show running
-
config interface
interface

show running
-
config | begin interface
interface

show running
-
config | begin igrp

show ip protocols

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Troubleshooting IGRP


The following commands are useful when
troubleshooting IGRP:


show ip protocols {summary}

show ip route

debug ip igrp events IGRP protocol events

debug ip igrp transactions IGRP protocol
transactions

ping

traceroute