SEMESTER 2 Chapter 4 Distance Vector Routing Protocols V 4.0 4.1.1 What are the three distance vector routing protocols? RIP, IGRP, EIGRP

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SEMESTER 2 Chapter 4

Distance Vector Routing Protocols

V 4.0


What are the three distance
vector routing protocols?


What are the key
characteristics of RIP?

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.

What are the key
characteristics of IGRP?

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

Routing updates are broadcast every 90 seconds, by

IGRP is the predecessor of EIGRP and is now

What are the key
characteristics of EIGRP?

It can perform unequal cost load balancing.

It uses Diffusing Update Algorith
m (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.


How are distance vector routes

As vectors of distance and direction

s a router in a distance
vector network have knowledge
of the entire path to a
destination network?


What are the 2 things a
distance vector router knows?

The direction or interface in which packets should be
forwarded and

The distance or how far it is

to the destination network

What are the two reasons
sending an entire routing table
in an update inefficient?

consume bandwidth but also consume router CPU
resources to process the updates

What is the periodic update
time for RIP?

30 Sec


is the periodic update
time for IGRP?

90 Sec

What is the address for
broadcast updates?


What is the algorithm used for?

to calculate the best paths and then send that
information to the neighbors

What processes does the
protocol define?

Mechanism for sending and receiving routing

Mechanism for calculating the best paths and installing
routes in the routing table.

Mechanism for detecting and reacting to topology


Define Time to Convergence.

e to convergence defines how quickly the routers in
the network topology share routing information and
reach a state of consistent knowledge. The faster the
convergence, the more preferable the protocol. Routing
loops can occur when inconsistent routing ta
bles are
not updated due to slow convergence in a changing

Define Scalability.

Scalability defines how large a network can become
based on the routing protocol that is deployed. The
larger the network is, the more scalable the routing
protocol n
eeds to be.

Classless (Use of
VLSM) or Classful

Classless routing protocols include the subnet mask in
the updates. This feature supports the use of Variable
Length Subnet Masking (VLSM) and better route
summarization. Classful routing protocols
do not
include the subnet mask and cannot support VLSM.

Define Resource Usage.

Resource usage includes the requirements of a routing
protocol such as memory space, CPU utilization, and
link bandwidth utilization. Higher resource requirements
more powerful hardware to support the
routing protocol operation in addition to the packet
forwarding processes.

Implementation and

Implementation and maintenance describes the level of
knowledge that is required for a network administ
to implement and maintain the network based on the
routing protocol deployed.

List the advantages for
distance vector protocols.

Simple implementation and maintenance

Low resource requirements

List the disadvantages for
distance vector protocols.

Slow convergence

Limited scalability

Routing loops

Definitely practice the exercise
at the right. It will help you
remember each protocol.


What are the routes that router
will initially discover after

The directly connected netw

After completing the initial
discovery what does the router
start to do?

Exchange information

What does the initial exchange
of routing information contain?

Information about their directly connected networks

What does the router do with
the information that is not
contained in its routing table?

It is added to the routing table

After the first exchange of
routing updates what has been
added to each routing table?

Neighbors directly connected routes

What is convergence?

When all

routers have consistent knowledge of the

What is slit horizon?

Prevents information from being sent out the interface
that it was received


The amount of time it takes for
a network to converge is
directly proportional to


the size of that network

The speed of achieving
convergence consists of what
two things?

w quickly the routers propagate

a change in the
topology in a routing update to its neighbors.

The speed of calculating best path routes using the
new routing

information collected.


What are the two reasons
routers exchange routing

o exchange routing information with their neighbors
and to maintain up
date routing information in the
routing table

What is the period update time
for RIP?


What are the four reasons
listed for topology changes?

Failure of a link

Introduction of a new link

Failure of a router

Change of link parameters

What are the three additional




If an update has not been
ceived to refresh an existing
route after 180 seconds
, what
happens to that route in the
routing table?

It is marked invalid

What happens to the route after
240 seconds have elapsed?

It is removed from the routing table. Flushed

What does a holddown ti

Keeps a route in the routing table, marked as
unreachable, for at least 180 seconds so all other
routers receive that information

What are the two commands
that will show the timers?

Show ip route

Show ip protocols


Define bounded update.

pdates that are sent only to the routers that need the
information instead of sending information to all routers

What are characteristic of
EIGRP updates?

periodic because they are not sent out on a
regular basis.

Partial updates sent only when there

is a change in
topology that influences routing information.

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


What is a triggered update?

a routing tabl
e update that is sent immediately in
response to a routing change

What are the three reasons to
send a triggered update?

An interface changes state (up or down)

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

A route is installed in the routing t

What are the two problems with
Packets containing the update message can be
triggered updates?

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 recei
ved 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


What is it called when all the
routers send updates at the
same time on

a network with a
hub at the center?

Synchronized updates

It this a problem on a switched



What is a routing loop?

a packet is continuously transmitted within a series of
routers without ever reaching its intended destination

What are some reasons for
routing loops?

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

level courses)

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

Incorrectly configured or installed discard routes

What mechanism is built into IP
to overcome routing loops?

TTL time

What cond
itions can be created
because of routing loops?

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

A router's CPU will be strained due to looping

A router's CPU will be burdened with useless
packet forwar
ding 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


What are some of the
mechanisms used to avoid
routing loops?

Defining a maximum metric to prevent count to infinity

Holddown timers

Split horizon

Route poisoning or poison reverse

Triggered updates


What is count to infinity?

a condition t
hat exists when inaccurate routing updates
increase the metric value to "infinity" for a network that
is no longer reachable


What is infinity defined by?

Maximum metric value

What is infinity for RIP?



What is it called when a route

up, then down, then up


What are holddown timers used

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

Describe the process of how a
holddown timer works.

1. A router receives an up
date 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 fro
m 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

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.


What is the split horizon rule?

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


What is route poisoning?

to mark the route as unreachable in a routing update
that is sent to other routers

What is split horizon with
poison reverse?

when sending updates out a specific interface,
signate any networks that were learned on that
interface as unreachable


What is time
live (TTL)?

an 8
bit field in the IP header that limits the number of
hops a packet can traverse through the network before
it is discarded

What happens when a

TTL reaches 0?

The packet is discarded


What are the factors that affect
the distance vector protocol
you choose?

Size of the network

Compatibility between models of routers

Administrative knowledge required

What are the features of RIP?

Supports split horizon and split horizon with poison
reverse to prevents loops.

Is capable of load balancing up to six equal cost paths .
The default is four equal cost paths.

What features were introduced
with RIPv2?

Includes the subnet mask in the rou
ting updates,
making it a classless routing protocol.

Has authentication mechanism to secure routing table

Supports variable length subnet mask (VLSM).

Uses multicast addresses instead of broadcast.

Supports manual route summarization.

What are
the features of

Triggered updates (EIGRP has no periodic updates).

Use of a topology table to maintain all the routes
received from neighbors (not only the best paths).

Establishment of adjacencies with neighboring routers
using the EIGRP hello prot

Support for VLSM and manual route summarization.
These allow EIGRP to create hierarchically structured
large networks.

What are the advan
ges of

Although routes are propagated in a distance vector
manner, the metric is based on minimum band
and cumulative delay of the path rather than hop count.

Fast convergence due to Diffusing Update Algorithm
(DUAL) route calculation. DUAL allows the insertion of
backup routes into the EIGRP topology table, which are
used in case the primary route fa
ils. Because it is a
local procedure, the switchover to the backup route is
immediate and does not involve the action in any other

Bounded updates mean that EIGRP uses less
bandwidth, especially in large networks with many

EIGRP supports
multiple network layer protocols
through Protocol Dependent Modules, which include
support for IP, IPX, and AppleTalk.