DistanceVectorRouting

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28 Οκτ 2013 (πριν από 4 χρόνια και 15 μέρες)

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S Ward Abingdon and Witney College

Distance Vector
Routing

CCNA Exploration Semester 2

Chapter 4

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Topics


Characteristics of distance vector routing
protocols


Distance vector routing protocols in use today


How they discover routes


How they maintain routing tables


Routing loops

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RIP v1

RIP v2

IGRP

EIGRP

Routing protocols

Interior

Exterior

Distance vector

Link state

OSPF

IS
-
IS

EGP

BGP

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Distance vector knowledge


A distance vector protocol learns:


The distance to a network, measured in hops or in
some other way


The direction of the network: which port should be
used to reach it


It puts the routes in the routing table


It does not know any more details of the route
or the other routers along the way

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Distance vector

Network 192.168.48.0
is 3 hops away using
port fa0/0

Network 192.168.22.0
is 2 hops away using
port fa0/1


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Link state knowledge


A link state routing protocol finds out about all
the routers in the system and the networks
they link to.


It builds up a complete picture of the topology


It can then work out the best path to any
network


It puts these best paths in the routing table

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Link state

I know all the routers and paths in
this system of networks.

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Metrics


RIP v1 and 2


hop count, maximum 15



IGRP and EIGRP

bandwidth, delay,





load, reliability


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Distance vector


Exchange complete routing tables with
immediate neighbours


Do this at regular intervals


Adjust the metric, e.g. add 1 to the hop count,
or add number based on bandwidth and
delay of link.


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Adjust the metric

192.168.13.0
is 2 hops
away

192.168.13.0
is 3 hops
away

Send update

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Sending updates


RIP v1

Whole routing table



Broadcast every 30 sec


RIP v2

Whole routing table



Multicast every 30 sec


IGRP

Whole routing table



Broadcast every 90 sec


EIGRP

Initial learning process then small



updates when topology changes


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RIP v1

RIP v2

IGRP

Routing protocols

Distance vector

Link state

OSPF

IS
-
IS

EIGRP

Slow to converge

Easy to configure

Small networks

Little use of resources

Fast to converge

Harder to configure

Large networks

Much use of resources

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Distance vector updates


Routers start up.


R1 adds directly
connected networks
to table.

Network

Interface

Hop

10.1.0.0

Fa0/0

0

10.2.0.0

S0/0/0

0

10.1.0.0

10.2.0.0

10.3.0.0

10.4.0.0

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Distance vector updates


Exchange of routing
table information.

10.1.0.0

10.2.0.0

10.3.0.0

10.4.0.0

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Distance vector updates


R1 has learned
about 10.3.0.0 from
R2.


It does not know
about 10.4.0.0

Network

Interface

Hop

10.1.0.0

Fa0/0

0

10.2.0.0

S0/0/0

0

10.3.0.0

S0/0/0

1

10.1.0.0

10.2.0.0

10.3.0.0

10.4.0.0

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Distance vector updates


Exchange of routing
table information.

10.1.0.0

10.2.0.0

10.3.0.0

10.4.0.0

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Distance vector updates


R1 has learned
about 10.4.0.0 from
R2.


R2 previously
learned about it
from R3.

Network

Interface

Hop

10.1.0.0

Fa0/0

0

10.2.0.0

S0/0/0

0

10.3.0.0

S0/0/0

1

10.4.0.0

S0/0/0

2

10.1.0.0

10.2.0.0

10.3.0.0

10.4.0.0

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Update timer

R 10.3.0.0 [120/1] via 10.2.0.2, 00:00:04, Serial0/0


Show ip route gives number of seconds since
last update.

Routing Protocol is “rip”

Sending updates every 30 seconds, next due in 3
seconds


Show ip protocols says when next update is
due.


Update timer default is 30 seconds


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RIP timers


Routing Protocol is “rip”

Sending updates every 30 seconds, next due in 26 seconds

Invalid after 180 seconds, hold down 180, flushed after 240



R 10.3.0.0 [120/1] via 10.2.0.2, 00:00:04, Serial0/0

R 10.4.0.0 [120/2] via 10.2.0.2, 00:00:04, Serial0/0

Routing table contains two RIP routes

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RIP timers


Routing Protocol is “rip”

Sending updates every 30 seconds, next due in 30 seconds

Invalid after 180 seconds, hold down 180, flushed after 240



R 10.3.0.0 [120/1] via 10.2.0.2, 00:00:00, Serial0/0

R 10.4.0.0 [120/2] via 10.2.0.2, 00:00:30, Serial0/0

30 seconds


update

Route to 10.3.0.0 refreshed

Route to 10.4.0.0 not included

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RIP timers


Routing Protocol is “rip”

Sending updates every 30 seconds, next due in 30 seconds

Invalid after 180 seconds, hold down 180, flushed after 240



R 10.3.0.0 [120/1] via 10.2.0.2, 00:00:00, Serial0/0

R 10.4.0.0 [120/2] via 10.2.0.2, 00:01:00, Serial0/0

60 seconds


update

Route to 10.3.0.0 refreshed

Route to 10.4.0.0 not included

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RIP timers


Routing Protocol is “rip”

Sending updates every 30 seconds, next due in 30 seconds

Invalid after 180 seconds, hold down 180, flushed after 240



R 10.3.0.0 [120/1] via 10.2.0.2, 00:00:00, Serial0/0

R 10.4.0.0 [120/2] via 10.2.0.2, 00:01:30, Serial0/0

90 seconds


update

Route to 10.3.0.0 refreshed

Route to 10.4.0.0 not included

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RIP timers


Routing Protocol is “rip”

Sending updates every 30 seconds, next due in 30 seconds

Invalid after 180 seconds, hold down 180, flushed after 240



R 10.3.0.0 [120/1] via 10.2.0.2, 00:00:00, Serial0/0

R 10.4.0.0 [120/2] via 10.2.0.2, 00:02:00, Serial0/0

120 seconds


update

Route to 10.3.0.0 refreshed

Route to 10.4.0.0 not included

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RIP timers


Routing Protocol is “rip”

Sending updates every 30 seconds, next due in 30 seconds

Invalid after 180 seconds, hold down 180, flushed after 240



R 10.3.0.0 [120/1] via 10.2.0.2, 00:00:00, Serial0/0

R 10.4.0.0 [120/2] via 10.2.0.2, 00:02:30, Serial0/0

150 seconds


update

Route to 10.3.0.0 refreshed

Route to 10.4.0.0 not included

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RIP timers


Routing Protocol is “rip”

Sending updates every 30 seconds, next due in 30 seconds

Invalid after 180 seconds, hold down 180, flushed after 240



R 10.3.0.0 [120/1] via 10.2.0.2, 00:00:00, Serial0/0

R 10.4.0.0 [120/16] via 10.2.0.2, 00:03:00, Serial0/0

180 seconds


update

Route to 10.3.0.0 refreshed

Route to 10.4.0.0 not included

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RIP timers


Routing Protocol is “rip”

Sending updates every 30 seconds, next due in 30 seconds

Invalid after 180 seconds, hold down 180, flushed after 240



R 10.3.0.0 [120/1] via 10.2.0.2, 00:00:00, Serial0/0

R 10.4.0.0 [120/16] via 10.2.0.2, 00:03:30, Serial0/0

210 seconds


update

Route to 10.3.0.0 refreshed

Route to 10.4.0.0 not included

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RIP timers


Routing Protocol is “rip”

Sending updates every 30 seconds, next due in 30 seconds

Invalid after 180 seconds, hold down 180, flushed after 240



R 10.3.0.0 [120/1] via 10.2.0.2, 00:00:00, Serial0/0


240 seconds


update

Route to 10.3.0.0 refreshed

Route to 10.4.0.0 not included

Route has been removed.

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RIP_JITTER


RIP updates can become synchronised


This is a problem if routers are linked by hubs
because the updates will collide


RIP_JITTER is a random variable that makes
updates vary a little from the default 30
seconds

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Triggered updates


These are to speed up convergence


Interface goes up/down, route
added/removed


Router detects change, sends update to
neighbour at once without waiting for timer


Neighbour passes on update at once.

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EIGRP


Does not send regular updates


Does not send its whole routing table


Sends only information about changes


Sends only to routers that need the
information


Non
-
periodic, partial, bounded.

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Routing loop


A packet is sent from router to router in a
loop until it is eventually dropped when its
TTL field drops to 0


Caused by incorrect or out of date
information in routing tables


Very bad for network


uses up bandwidth
and processing power in routers

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Avoiding routing loops


Defining a maximum metric to prevent count
to infinity


Holddown timers


Split horizon


Route poisoning or poison reverse


Triggered updates

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Maximum metric


Routers exchanging wrong information can
report higher and higher values of the metric.


RIP sets a maximum metric.


The hop count can go up to 15.


If it reaches 16 then the route is regarded as
unreachable.

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Holddown timers


Router receives update saying that a network is
down.


Router marks the network as possibly down and
starts holddown timer.


Update with a better metric for that network arrives:
network is reinstated and holddown timer removed.


Update with the same or worse metric for that
network arrives: update is ignored.


Timer runs out : network removed from table.


Packets still forwarded to network while timer runs.

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Split horizon


Router receives information about a route
through an interface.


It will not send out information about the
same route through that interface.

Route to
10.1.1.0 in
3 hops

Route to
10.1.1.0 in
4 hops

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Route poisoning


A router detects that a route has gone down.


It marks that route as unreachable in its
routing table. (16 hops for RIP)


It sends out updates that show the route as
unreachable.


Neighbour routers pass on these “poison”
updates.

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Poison reverse


This is an exception to split horizon.


If a router receives an update marking a route
as unreachable then it will send this
information back to the router that sent it.

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RIP v1 and RIP v2


RIP v1


Classful, does not send
subnet mask in updates
so does not support
VLSM


Sends updates as
broadcasts


No authentication


No manual route
summarisation




RIP v2


Classless, i
ncludes the
subnet mask in routing
updates, so supports
VLSM.


Sends updates as
multicasts


Authentication for
security


Supports manual route
summarization.

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RIP v2 or EIGRP?


RIP runs on any make of router, EIGRP only
on Cisco routers.


EIGRP is suitable for large networks


EIGRP uses a more efficient metric and may
choose faster routes.


EIGRP converges faster than RIP


EIGRP uses less bandwidth but it needs
more processing power and RAM


RIP is simpler to configure

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The End