Semester 3 Chapter 5 Study Guide

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Oct 28, 2013 (3 years and 9 months ago)

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INTERNETWORKING III

CHAPTER 5 STUDY GUIDE



1.

Routing refers to the process of choosing the
best path

over which to send packets and
how to cross multiple physical networks. This is the basis of all Internet communication.


2.

In IP networks, the router forward
s packets from the source network to the destination
network based on the IP
routing table.
After the router determines which path to use, it can
proceed with
switching

the packet. This means it accepts the packet on one interface and
forwards it to anothe
r interface that is the next hop on the best path to the packet's
destination.


3.

Entries in the routing tables contain an IP address of the next
hop

along the route to the
destination. Each entry specifies
1

hop and points to a router that is directly conne
cted,
which means that it can be reached across a single network.


4.

A variety of metrics can be used to define the best path. Some routing protocols, such as
Routing Information Protocol (RIP), use only
one
metric and that is
hop count
. And some
routing pro
tocols, such as IGRP, use a combination of metrics. The metrics most commonly
used by routers are:



Hop count


The number of routers that a packet must go through to reach its
destination.



Bandwidth



The data capacity of a link.



Delay



The length of time

to move the packet from the source to destination.



Load


The amount of activity on a network resource.



Reliability


The error rate of each network link.



Ticks


The delay on a data link using IBM PC clock ticks.



Cost


An arbitrary value assigned by an
administrator.


5.

After examining a packet's destination protocol address, the router determines that it either
knows or does not know how to forward the packet to the next hop. If the router does not
know how to forward the packet and there is no default ro
ute, it typically
drops
the packet.


6.

Below is a list of protocols. Label the protocols as routed or routing and if they are routing
protocols, label those as IGP (interior gateway protocols) or EGP (exterior gateway
protocols)


PROTCOL

ROUTED/ROUTING

Appl
eTalk

Routed

APPN

Routing
-
IGP

BGP

Routing
-
EGP

EGP

Routing
-
EGP

EIGRP

Routing
-
IGP

IGRP

Routing
-
IGP

IPX

Routed

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IS
-
IS

Routing
-
EGP

OSPF

Routing
-
IGP

RIP

Routing
-
IGP

TCP/IP

Routed


7.

Routers are capable of
multiprotocol

routing, which means they support
multiple
independent routing protocols, such as IGRP and RIP. This capability allows a router to
deliver packets from several routed protocols, such as TCP/IP and IPX, over the same data
links.


8.

Exterior routing protocols require the following information
before routing can begin:



A list of
neighbor

(also called peer) routers with which to exchange routing information



A list of
networks
to advertise as directly reachable


9.

The

optimal route
-

Refers to the ability of the routing protocol to select the best r
oute. The
best route depends on the metrics and metric weightings used to make the calculation. For
example, one routing protocol might use the number of hops and the delay, but might weigh
the delay more heavily in the calculation.


10.

Simplicity and efficie
ncy
-

This is particularly important when the software implementing
the routing protocol must run on a computer with limited physical resources.


11.

Robustness
-

Routing protocols should perform correctly in the face of unusual or
unforeseen circumstances, su
ch as hardware failures, high load conditions, and incorrect
implementations. Because routers are located at network junction points, they can cause
considerable problems when they fail. The best routing protocols are often those that have
withstood the te
st of time and proven stable under a variety of network conditions.


12.

Rapid convergence
-

This is the speed and ability of a group of networking devices running
a specific routing protocol to agree on the topology of a network after a change in that
topolog
y. When a network event, such as a change in a network's topology, causes routes
to either go down or become available, routers distribute routing update messages. Routing
update messages are sent to networks, thereby causing the recalculation of optimal r
outes
and eventually causing all routers to agree on these routes. Routing protocols that converge
slowly can cause routing loops or network outages.


13.

Flexibility
-
Routing protocols should quickly and accurately adapt to a variety of network
circumstances.

For example, assume that a network segment has gone down. Many routing
protocols quickly select the next
-
best path for all routes that normally use a given segment.
Routing protocols can be programmed to adapt to changes in network bandwidth, router
queue

size, network delay, and other variables.


14.

IGRP has a maximum hop count of
255
, it defaults to
100
, and it is usually set to 50 or less.


15.

Dynamic routing protocols can be supplemented with
static
routes where appropriate. For
example, a gateway of last re
sort (that is, a router to which all unroutable packets are sent)
may be designated. This router acts as a central storing place for all unroutable packets,
ensuring that all messages are at least handled in some way.



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16.

Below is a list of protocols. Label
the protocols as distance vector, link state, or hybrid.

EIGRP

Hybrid

IGRP

Distance
-
vector

IS
-
IS

Link state

NLSP

Link state

OSPF

Link state

RIP

Distance
-
vector


17.

Each routing protocol must be configured separately. With any routing protocol, you must
follow two basic steps:

1.

Create the routing process with one of the router commands.

2.

Configure the protocol specifics.

IGRP has the additional requirement of an
autonomous system
number.


18.

IGRP uses a combination of user configurable metrics. Name the metric
s used.



network delay



bandwidth



reliability



load


19.

IGRP advertises three types of routes:



Interior

routes are routes between subnets in the network attached to a router interface.
If the network attached to a router is not subnetted, IGRP does not advertise

these
routes. Additionally, subnet information is not included in IGRP updates, which poses a
problem for noncontiguous IP subnets.



System

routes are routes to other major networks within the AS. The router derives
system routes from directly connected ne
twork interfaces and system route information
provided by other routers that use IGRP. These routes do not include subnetting
information.



Exterior

routes are routes to networks outside the AS that are considered when
identifying a gateway of last resort.
The router chooses a gateway of last resort from the
list of exterior routes that IGRP provides. The router uses the gateway of last resort if it
does not have a better route for a packet and the destination is not a connected network.
If the AS has more t
han one connection to an external network, different routers can
choose different exterior routers as the gateway of last resort.


20.

IGRP provides a number of features that are designed to enhance its stability, including the
following:



Holddowns
-

When a ro
uter learns that a network is further away than was previously
known, or it learns that the network is down, the route to that network is placed into
holddown. During the holddown period, the route is advertised, but incoming
advertisements about that netw
ork from any router other than the one that originally
advertised the network's new metric are ignored. This mechanism is often used to help
avoid routing loops in the network, but has the effect of increasing the topology
convergence time.

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Split Horizons
-

This occurs when a router tries to send information about a route back
in the direction from which it came and helps prevent routing loops.



Poison Reverse Updates
-

Whereas split horizons should prevent routing loops
between adjacent routers, these are i
ntended to defeat larger routing loops. Increases in
routing metrics generally indicate routing loops. These are then sent to remove the route
and place it in holddown. A router poisons the route by sending an update with a metric
of infinity to a router t
hat originally advertised a route to a network. Poisoning the route
can help speed convergence.


21.

For each path through an AS, IGRP records the segment with the lowest bandwidth, the
accumulated delay, the smallest maximum transmission unit (MTU), and the r
eliability and
load. A router running IGRP sends an IGRP update broadcast every
90
seconds. It declares
a route inaccessible if it does not receive an update from the first router in the route within
three update periods (
270
seconds). After five update pe
riods (
450
seconds), the router
removes the route from the routing table. IGRP uses flash update and poison reverse to
speed up the convergence of the routing protocol. A flash update is the sending of an
update sooner than the standard periodic update int
erval of notifying other routers of a
metric change.