Distance Vector Routing

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Distance Vector Routing
Protocols

Routing Protocols and Concepts


Chapter 4


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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

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Routing Protocols


Routing Information Protocol

(RIP)


Interior Gateway Routing Protocol

(IGRP)


Enhanced Interior Gateway Routing Protocol

(EIGRP)


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RIP

Routing Information Protocol (RIP) was originally specified in RFC
1058. It has the following key characteristics:


Hop count is used as the metric for path selection.


If the hop count for a network is greater than 15, RIP cannot supply a
route to that network.


Routing updates are broadcast or multicast every 30 seconds, by default.

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IGRP

Interior Gateway Routing Protocol (IGRP) is a proprietary protocol
developed by Cisco. IGRP has the following key design
characteristics:


Bandwidth, delay, load and reliability are used to create a composite
metric.


Routing updates are broadcast every 90 seconds, by default.


IGRP is the predecessor of EIGRP and is now obsolete.

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EIGRP

Enhanced IGRP (EIGRP) is a Cisco proprietary distance vector
routing protocol. EIGRP has these key characteristics:


It can perform unequal cost load balancing.


It uses Diffusing Update Algorithm (DUAL) to calculate the shortest path.


There are no periodic updates as with RIP and IGRP. Routing updates are
sent only when there is a change in the topology.

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Distance Vector

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Characteristics of Distance Vector routing protocols

:



Periodic updates



Neighbors







Broadcast updates



Entire routing table is included


with routing update

Routers using distance vector routing are not aware of the network topology.

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Routing Protocol Algorithm

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Routing Protocols Characteristics


Time to Convergence


How quickly the routers in the network topology share routing information and reach
a state of consistent knowledge

Scalability


How large a network can become based on the routing protocol that is deployed

Classless (Use of VLSM) or Classful



Classless routing protocols include the subnet mask in the updates. Classful
routing protocols do not

Resource Usage


Memory space, CPU utilization, and link bandwidth utilization
.

Implementation and Maintenance


The level of knowledge that is required for a network administrator to implement
and maintain the network based on the routing protocol deployed
.

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Router initial start up (Cold Starts)

-
Initial network discovery

Directly connected networks are initially placed in the routing table

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Initial Exchange of Routing Information


If a routing protocol is configured then



Routers will exchange routing information



Updates is received from other routers



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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Router Convergence

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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Routing Table Maintenance

RIP uses 4 timers

Update timer

Invalid timer

Holddown timer

Flush timer

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Invalid Timer


If an update has not been received to refresh an
existing route after 180 seconds (the default), the route
is marked as invalid by setting the metric to 16


The route is retained in the routing table until the flush
timer expires.

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Holddown Timer


Stabilizes routing information


Helps prevent routing loops during periods when the topology is
converging on new information


Once a route is marked as unreachable, it must stay in holddown
long enough for all routers in the topology to learn about the
unreachable network


By default, the holddown timer is set for 180 seconds
.

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Flush Timer



By default, the flush timer is set for 240 seconds, which is 60
seconds longer than the invalid timer


When the flush timer expires, the route is removed from the routing
table

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Checking Timers

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Eigrp


bounded updates

EIGRP uses updates that are



Non
-
periodic



Partial updates



Bounded, meaning the propagation of partial updates are automatically bounded so
that only those routers that need the information are updated

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Triggered updates

Triggered updates can speed up convergence
but…………..

Packets containing the update message can be dropped or corrupted by some link in the
network


The triggered updates do not happen instantaneously

It is possible that a router that has not yet received the triggered update will issue a regular
update at just the wrong time, causing the bad route to be reinserted in a neighbor that had
already received the triggered update.

Triggered updates are sent when one of the following occurs:


An interface changes state (up or down)


A route has entered (or exited) the "unreachable" state


A route is installed in the routing table

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Synchronized Updates

RIP_JITTER

subtracts a variable amount of
time to the update interval for
each router in the network.


From 0% to 15% of the specified
update interval


Update interval varies randomly
in a range from 25 to 30 seconds
for the default 30
-
second
interval.

Only a problem with Hubs

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Routing Loops


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


The loop may be a result of:


Incorrectly configured static routes


Incorrectly configured route redistribution (redistribution is a process of
handing the routing information from one routing protocol to another
routing protocol and is discussed in CCNP
-
level courses)


Inconsistent routing tables not being updated due to slow convergence in
a changing network


Incorrectly configured or installed discard routes

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Routing Loops


A routing loop can create the following conditions:


Link bandwidth will be used for traffic looping back and forth between the
routers in a loop.


A router's CPU will be burdened with useless packet forwarding that will
negatively impact the convergence of the network.


Routing updates may get lost or not be processed in a timely manner.
These conditions would introduce additional routing loops, making the
situation even worse.


Packets may get lost in "black holes."

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Routing Loops

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Preventing 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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Count to Infinity

A routing loop whereby packets bounce infinitely around a network
.

R1

From

To

Metric

to

10.4.0.0

R2

R3

2

R3
(M2)

R2

3

R2
(M3)

R1

4

R2
(M3)

R3

4

R3
(M4)

R2

5

R2
(M5)

R1

6

R2
(M5)

R3

6

R3
(M6)

R2

7

R2

R3

Solution:

Defining a maximum metric


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

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Hold Down Timer

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Holddown timers


Used to prevent regular update messages from inappropriately reinstating
a route that may have gone bad

1
. A router receives an update from a neighbor indicating that a network that
previously was accessible is now no longer accessible

2. The router marks the network as possibly down and starts the holddown timer

3. If an update with a better metric for that network is received from any
neighboring router during the holddown period, the network is reinstated
and the holddown timer is removed

4. If an update from any other neighbor is received during the holddown period with
the same or worse metric for that network, that update is ignored. Thus, more time
is allowed for the information about the change to be propagated

5. Routers still forward packets to destination networks that are marked as
possibly down. This allows the router to overcome any issues associated
with intermittent connectivity. If the destination network truly is unavailable
and the packets are forwarded, black hole routing is created and lasts until
the holddown timer expires
.


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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.

-
Point of using holddown timers


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

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Split Horizon Rule

A router should not advertise a network through the interface from which the
update came




R2 only advertises networks 10.3.0.0 and 10.4.0.0 to R1



R2 only advertises networks 10.1.0.0 and 10.2.0.0 to R3


R1 only advertises network 10.1.0.0 to R2






R3 only advertises network 10.4.0.0 to R2

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Route Poisening

Network 10.4.0.0 becomes unavailable due to a link failure.



R3 poisons the metric with a value of 16 and then sends out a triggered update


R2 processes that update and invalidates the routing entry in its routing table



R2 then sends the poison update to R1 setting the metric value to 16



R1 processes the update and invalidates the routing entry for 10.4.0.0 in its routing table
.


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Split horizon
with

poison reverse


Once a router learns of an
unreachable route through an
interface,advertise it as unreachable
back through the same interface


Poison reverse is a specific
circumstance that overrides split
horizon. It occurs to ensure that R3
is not susceptible to incorrect
updates about network 10.4.0.0


Split horizon is enabled by default.
However split horizon with poison
reverse may not be the default on
all IOS implementations

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Routing Loops and TTL value

Even in the case of a routing loop packets will not loop endlessly in the network


Eventually the TTL value will be decreased to 0 and the packet will be discarded by
the router

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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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