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CCNA Study Notes
OSI Reference
1. Identify and describe the functions of each of the seven layers of
the
OSI reference model.
Physical Layer
The physical layer defines the electrical, mechanical, procedural, and
functional specifications for activating, maintaining, and deactivatin
g the
physical link between communicating network systems. Physical layer
specifications define such characteristics as voltage levels, timing o
f
voltage changes, physical data rates, maximum transmission distances,
and
the physical connectors to be used.
Data Link Layer
The data link layer provides reliable transit of data across a physica
l
network link. Different data link layer specifications define differen
t
network and protocol characteristics, including the following:
Physical addressing -- Physical addressing (as opposed to network
addressing) defines how devices are addressed at the data link layer.
Network topology -- Data link layer specifications often define how de
vices
are to be physically connected (such as in a bus or a ring topology).
Error notification -- Error notification involves alerting upper layer
protocols that a transmission error has occurred.
Sequencing of frames -- Sequencing of data frames involves the reorder
ing
of frames that are transmitted out of sequence.
Flow control -- Flow control involves moderating the transmission of d
ata
so that the receiving device is not overwhelmed with more traffic than
it
can handle at one time.
The Institute of Electrical and Electronics Engineers (IEEE) has subdi
vided
the data link layer into two sublayers: Logical Link Control (LLC) and
Media Access Control (MAC).
The LLC sublayer (defined in the IEEE 802.2 specification) manages
communications between devices over a single link of a network.
The MAC sublayer manages protocol access to the physical network mediu
m.
Network Layer
The network layer provides routing and related functions that allow
multiple data links to be combined into an internetwork. This is
accomplished by the logical addressing (as opposed to the physical
addressing) of devices. The network layer supports both connection-ori
ented
and connectionless service from higher-layer protocols.
Transport Layer
The transport layer implements reliable internetwork data transport
services that are transparent to upper layers. Transport layer functio
ns
typically include the following:
Flow control -- Flow control manages data transmission between devices
so
that the transmitting device does not send more data than the receivin
g
device can process.
Multiplexing -- Multiplexing allows data from several applications to
be
transmitted onto a single physical link.
Virtual circuit management -- Virtual circuits are established, mainta
ined,
and terminated by the transport layer.
Error checking and recovery -- Error checking involves various mechani
sms
for detecting transmission errors. Error recovery involves taking an a
ction
(such as requesting that data be retransmitted) to resolve any errors
that
occur.
Some examples of transport layer implementations follow:
Transmission Control Protocol (TCP), Name Binding Protocol (NBP), OSI
transport protocols
Session Layer
The session layer establishes, manages, and terminates communication
sessions between presentation layer entities. Communication sessions
consist of service requests and service responses that occur between
applications located in different network devices. These requests and
responses are coordinated by protocols implemented at the session laye
r.
Some examples of session layer implementations follow:
Apple ZIP, DEC SCP, NFS, SQL, RPC, X Windows, ASP
Presentation Layer
The presentation layer provides a variety of coding and conversion
functions that are applied to application layer data. These functions
ensure that information sent from the application layer of one system
will
be readable by the application layer of another system. Some examples
of
presentation layer coding and conversion schemes follow:
Common data representation formats -- The use of standard image, sound
, and
video formats allow the interchange of application data between differ
ent
types of computer systems. (JPEG, MPEG, GIF, ?)
Conversion of character representation formats -- Conversion schemes a
re
used to exchange information with systems using different text and dat
a
representations (such as EBCDIC and ASCII).
Common data compression schemes -- The use of standard data compressio
n
schemes allows data that is compressed at the source device to be prop
erly
decompressed at the destination.
Common data encryption schemes -- The use of standard data encryption
schemes allows data encrypted at the source device to be properly
unencrypted at the destination.
Presentation layer implementations are not typically associated with a
particular protocol stack. Some well known standards follow:
Data: ASCII, EBCDIC, Encryption
Visual Imaging: PICT, TIFF, GIF, JPEG
Video: MIDI, MPEG, QuickTime
Application Layer
The application layer interacts with software applications that implem
ent a
communicating component. Application layer functions typically include
the
following:
Identifying communication partners -- The application layer identifies
and
determines the availability of communication partners for an applicati
on
with data to transmit.
Determining resource availability -- The application layer must determ
ine
whether sufficient network resources for the requested communication a
re
available.
Synchronizing communication -- Communication between applications requ
ires
cooperation that is managed by the application layer.
The application layer is the OSI layer closest to the end user. That i
s,
both the OSI application layer and the user interact directly with the
software application. Some examples of application layer implementatio
ns:
TCP/IP applications -- TCP/IP applications are protocols in the Intern
et
Protocol suite, such as Telnet, File Transfer Protocol (FTP), and Simp
le
Mail Transfer Protocol (SMTP).
OSI applications -- OSI applications are protocols in the OSI suite su
ch as
File Transfer, Access, and Management (FTAM), Virtual Terminal Protoco
l
(VTP), and Common Management Information Protocol (CMIP).
2. Describe connection-oriented network service and connectionless net
work
service and identify the key differences between them.
Connection-Oriented Network Service
Connection-oriented service involves three phases:
Connection establishment -- During the connection establishment phase,
a
single path between the source and destination systems is determined.
Network resources are typically reserved at this time to ensure a
consistent grade of service (such as a guaranteed throughput rate).
Data transfer -- During the data transfer phase, data is transmitted
sequentially over the path that has been established. Data always arri
ves
at the destination system in the order in which it was sent.
Connection termination -- During the connection termination phase, an
established connection that is no longer needed is terminated. Further
communication between the source and destination systems requires that
a
new connection be established.
Connection-oriented service has two significant disadvantages as compa
red
to connectionless network service:
Static path selection -- Because all traffic must travel along the sam
e
static path, a failure anywhere along that path causes the connection
to
fail.
Static reservation of network resources -- A guaranteed rate of throug
hput
requires the commitment of resources that cannot be shared by other ne
twork
users. Unless full, uninterrupted throughput is required for the
communication, bandwidth is not used efficiently.
Connection-oriented services are useful for transmitting data from
applications that are intolerant of delays and packet re-sequencing. V
oice
and video applications are typically based on connection-oriented serv
ices.
Connectionless Network Service
Connectionless network service does not predetermine the path from the
source to the destination system, nor are packet sequencing, data
throughput, and other network resources guaranteed. Each packet must b
e
completely addressed because different paths through the network might
be
selected for different packets, based on a variety of influences. Each
packet is transmitted independently by the source system and is handle
d
independently by intermediate network devices. Connectionless service
offers two important advantages over connection-oriented service:
Dynamic path selection -- Because paths are selected on a packet-by-pa
cket
basis, traffic can be routed around network failures.
Dynamic bandwidth allocation -- Bandwidth is used more efficiently bec
ause
network resources are not allocated bandwidth that they are not going
to
use.
Connectionless services are useful for transmitting data from applicat
ions
that can tolerate some delay and re-sequencing. Data-based application
s are
typically based on connectionless service.
3. Describe data link addresses and network addresses and identify the
key
differences between them.
Data Link Layer Addresses
A data link layer address uniquely identifies each physical network
connection of a network device. Data link addresses are sometimes refe
rred
to as physical or hardware addresses. Data link addresses usually exis
t
within a flat address space and have a pre-established and typically f
ixed
relationship to a specific device. End systems typically have only one
physical network connection, and thus have only one data link address.
Routers and other internetworking devices typically have multiple phys
ical
network connections. They therefore have multiple data link addresses.
Network Layer Addresses
A network layer address identifies an entity at the network layer of t
he
OSI reference model. Network addresses usually exist within a hierarch
ical
address space. They are sometimes called virtual or logical addresses.
The
relationship of a network address with a device is logical and unfixed
. It
is typically based either on physical network characteristics (the dev
ice
is on a particular network segment) or on groupings that have no physi
cal
basis (the device is part of an AppleTalk zone). End systems require o
ne
network layer address for each network layer protocol they support. (T
his
assumes that the device has only one physical network connection.) Rou
ters
and other internetworking devices require one network layer address pe
r
physical network connection for each network layer protocol supported.
For
example, a router with three interfaces, each running AppleTalk, TCP/I
P,
and OSI, must have three network layer addresses for each interface. T
he
router therefore has nine network layer addresses.
4. Identify at least 3 reasons why the industry uses a layered model.
Reduce complexity Standardizes interfaces
Facilitates modular engineering Ensures interoperable technology
Accelerates evolution Simplifies teaching and learning
5. Define and explain the 5 conversion steps of data encapsulation
1. User information is converted to data
2. Data is converted to Segments
3. Segments are converted to Packets
4. Packets are converted to Frames
5. Frames are converted to Bits
6. Define flow control and describe the three basic methods used in
networking.
Flow control is a function that prevents network congestion by ensurin
g
that transmitting devices do not overwhelm receiving devices with data
.
There are a number of possible causes of network congestion. For examp
le, a
high-speed computer might generate traffic faster than the network can
transfer it, or faster than the destination device can receive and pro
cess
it. There are three commonly used methods for handling network congest
ion:
Buffering - Buffering is used by network devices to temporarily store
bursts of excess data in memory until they can be processed. Occasiona
l
data bursts are easily handled by buffering. However, excess data burs
ts
can exhaust memory, forcing the device to discard any additional datag
rams
that arrive.
Source quench messages - Source quench messages are used by receiving
devices to help prevent their buffers from overflowing. The receiving
device sends source quench messages to request that the source reduce
its
current rate of data transmission, as follows:
1. The receiving device begins discarding received data due to
overflowing buffers.
2. The receiving device begins sending source quench messages to the
transmitting device, at the rate of one message for each packet
dropped.
3. The source device receives the source quench messages and lowers
the
data rate until it stops receiving the messages.
4. The source device then gradually increases the data rate as long
as no
further source quench requests are received.
Windowing - Windowing is a flow-control scheme in which the source dev
ice
requires an acknowledgement from the destination after a certain numbe
r of
packets have been transmitted. With a window size of three, the source
requires an acknowledgment after sending three packets, as follows:
1. The source device sends three packets to the destination device.
2. After receiving the three packets, the destination device sends a
n
acknowledgment to the source.
3. The source receives the acknowledgment and sends three more packe
ts.
4. If the destination does not receive one or more of the packets fo
r
some reason (such as overflowing buffers), it does not receive en
ough
packets to send an acknowledgment. The source, not receiving an
acknowledgment, retransmits the packets at a reduced transmission
rate.
7. List the key internetworking functions of the OSI Network layer and
how
they are performed in a router.
Selects the best path through an internetwork, establishes network
addresses, & communicates paths.
Routers use a routing protocol between routers, use a routed protocol
to
carry user packets, set up and maintain routing tables, discover netwo
rks,
adapt to internetwork topology changes, use a two part address, and
contains broadcasts.
WAN Protocols
8. Differentiate between the following WAN services: Frame Relay, ISDN
/
LAPD, HDLC, & PPP.
Frame Relay - Industry-standard, switched data link layer protocol tha
t
handles multiple virtual circuits using HDLC encapsulation between
connected devices. Frame Relay is more efficient than X.25, the protoc
ol
for which it is generally considered a replacement.
ISDN - Integrated Services Digital Network. Communication protocol, of
fered
by telephone companies, that permits telephone networks to carry data,
voice, and other source traffic.
HDLC - High-Level Data Link Control. Bit-oriented synchronous data lin
k
layer protocol developed by ISO. Derived from SDLC, HDLC specifies a d
ata
encapsulation method on synchronous serial links using frame character
s and
checksums.
PPP - Point-to-Point Protocol. A successor to SLIP, PPP provides
router-to-router and host-to-network connections over synchronous and
asynchronous circuits.
9. Recognize key Frame Relay terms and features.
Frame Relay is a CCITT & ANSI standard for sending data over a public
data
network. It is a next-generation protocol to X.25 and is a
connection-oriented data-link technology. It relies on upper-layer
protocols for error correction and today's more dependable fiber and
digital networks.
Local access rate - clock speed of the connection to the Frame cloud.
Data-link connection identifier (DLCI) - a number that identifies the
logical circuit between the DTE and the
Frame Relay switch. The FR switch maps the DLCIs between each pair of
routers to create a PVC.
Local management interface (LMI) - a signaling standard between the DT
E
device and the FR switch that
Is responsible for managing the connection and maintaining status betw
een
the devices.
Committed information rate (CIR) - the average rate (bps) that the FR
switch agrees to transfer data.
Committed burst - the maximum number of bits that the switch agrees to
transfer during any Committed Rate
Measurement Interval.
Excess burst - the maximum number of uncommitted bits that the FR swit
ch
will attempt to transfer beyond
the CIR (typically limited to the port speed of the local access loop)
.
Backward explicit congestion notification (BECN) - when a FR switch
recognizes congestion in the network,
It sends a BECN packet to the source router instructing it to reduce i
ts
packet sending rate.
Forward explicit congestion notification (FECN) - when a FR switch
recognizes congestion in the network,
It sends a FECN packet to the destination device indicating that conge
stion
has occurred.
Discard eligibility (DE) indicator - when the router detects network
congestion, the FR switch will drop packets with the
DE bit set first. The DE bit is set on the oversubscribed traffic; tha
t is
the traffic that was received after the CIR was met.
10. List commands to configure Frame Relay LMIs, maps and subinterface
s
router(config-if)# encapsulation frame-relay [ cisco | ietf ] (cisco i
s the
default)
router(config-if)# frame-relay lmi-type [ ansi | cisco | q933i ]
(autosensed 11.2 and up)
router(config-if)# bandwidth kilobits (configur bandwidth for the link
,
default is T1)
router(config-if)# frame-relay inverse-arp [ protocol ] [ dlci ] (enab
led
by default)
router(config-if)# ip bandwidth-percent eigrp as-number percent (total
bandwidth EIGRP can use)
router(config-if)# keepalive number ( increase/decrease keepalive inte
rval,
default is 10 secs.)
router(config-if)# frame-relay local-dlci number (to specify DLCI for
local
interface)
router(config-if)# frame-relay map protocol protocol-address dlci [
broadcast ] [ ietf | cisco ]
payload-compress packet-by-packet (Cisco compression)
(broadcast - forward broadcasts to this address when multicast is not
enabled)
router(config-if)# interface serial number . subinterface-number [
multipoint | point-to-point ]
(multipoint - forwards broadcasts and routing updates, for routing IP
when
all routers are in same subnet)
(point-to-point - no broadcasts or updates, each router is in its own
subnet)
router(config-if)# ip unnumbered interface (point-to-point IP
sub-interface)
router(config-if)# frame-relay interface-dlci dlci-number (local DLCI
number being linked to sub-interface)
11. List commands to monitor Frame Relay operation on the router
Show interfaces serial - displays DLCI and LMI information
Show frame-relay pvc - displays PVC traffic statistics
Show frame-relay map - displays the route maps (static or dynamic)
Show frame-relay lmi - displays LMI informaion
12. Identify PPP operations to encapsulate WAN data on Cisco routers
Router(config)# username name password secret (name=host name of remot
e
router,Secret=identical on both routers)
Router(config-if)# encapsulation ppp
Router(config-if)# ppp authentication [chap | chap pap | pap chap | pa
p ]
(pap is clear text)
Router(config-if)# ppp pap sent-username username password password (f
or
router responding to pap request, 11.1 and up)
Router(config-if)# ppp chap hostname hostname (for same host name on
multiple routers)
Router(config-if)# ppp chap password secret (to send to hosts that wan
t to
authenticate the router)
13. State a relevant use and context for ISDN networking.
To support applications requiring high speed voice, video, and data
communications.
Digital service with fast connection setup and higher bandwidth than
traditional modems.
14. Identify ISDN protocols, function groups, reference points and
channels.
ISDN components include terminals, terminal adapters (TAs),
network-termination devices, line-termination equipment, and
exchange-termination equipment. ISDN terminals come in two types.
Specialized ISDN terminals are referred to as terminal equipment type
1
(TE1). Non-ISDN terminals such as DTE that predate the ISDN standards
are
referred to as terminal equipment type 2 (TE2). TE1s connect to the IS
DN
network through a four-wire, twisted-pair digital link. TE2s connect t
o the
ISDN network through a terminal adapter. The ISDN TA can either be a
stand-alone device or a board inside the TE2. If the TE2 is implemente
d as
a standalone device, it connects to the TA via a standard physical-lay
er
interface. Examples include EIA/TIA-232-C (formerly RS-232-C), V.24, a
nd
V.35.
Beyond the TE1 and TE2 devices, the next connection point in the ISDN
network is the network termination type 1 (NT1) or network termination
type
2 (NT2) device. These are network-termination devices that connect the
four-wire subscriber wiring to the conventional two-wire local loop. I
n
North America, the NT1 is a customer premises equipment (CPE) device.
In
most other parts of the world, the NT1 is part of the network provided
by
the carrier. The NT2 is a more complicated device, typically found in
digital private branch exchanges (PBXs), that performs Layer 2 and 3
protocol functions and concentration services. An NT1/2 device also ex
ists;
it is a single device that combines the functions of an NT1 and an NT2
.
A number of reference points are specified in ISDN. These reference po
ints
define logical interfaces between functional groupings such as TAs and
NT1s. ISDN reference points include the following:
R--The reference point between non-ISDN equipment and a TA.
S--The reference point between user terminals and the NT2.
T--The reference point between NT1 and NT2 devices.
U--The reference point between NT1 devices and line-termination equipm
ent
in the carrier network.
The U reference point is relevant only in North America, where the NT1
function is not provided by the carrier network.
The ISDN Basic Rate Interface (BRI) service offers two B channels and
one D
channel (2B+D). BRI B-channel service operates at 64 kbps and is meant
to
carry user data; BRI D-channel service operates at 16 kbps and is mean
t to
carry control and signaling information, although it can support user
data
transmission under certain circumstances. The D channel signaling prot
ocol
comprises Layers 1 through 3 of the OSI reference model. BRI also prov
ides
for framing control and other overhead, bringing its total bit rate to
192
kbps. The BRI physical layer specification is International
Telecommunication Union Telecommunication Standardization Sector (ITU-
T)
(formerly the Consultative Committee for International Telegraph and
Telephone [CCITT]) I.430.
ISDN Primary Rate Interface (PRI) service offers 23 B channels and one
D
channel in North America and Japan, yielding a total bit rate of 1.544
Mbps
(the PRI D channel runs at 64 kbps). ISDN PRI in Europe, Australia, an
d
other parts of the world provides 30 B plus one 64-kbps D channel and
a
total interface rate of 2.048 Mbps. The PRI physical-layer specificati
on is
ITU-T I.431.
ISDN physical-layer (Layer 1) frame formats differ depending on whethe
r the
frame is outbound (from terminal to network) or inbound (from network
to
terminal). The frames are 48 bits long, of which 36 bits represent dat
a.
Layer 2 of the ISDN signaling protocol is Link Access Procedure, D cha
nnel,
also known as LAPD. LAPD is similar to High-Level Data Link Control (H
DLC)
and Link Access Procedure, Balanced (LAPB). As the expansion of the LA
PD
acronym indicates, it is used across the D channel to ensure that cont
rol
and signaling information flows and is received properly. The LAPD fra
me
format is very similar to that of HDLC and, like HDLC, LAPD uses
supervisory, information, and unnumbered frames. The LAPD protocol is
formally specified in ITU-T Q.920 and ITU-TQ.921.
Two Layer 3 specifications are used for ISDN signaling: ITU-T (formerl
y
CCITT) I.450 (also known as ITU-T Q.930) and ITU-T I.451 (also known a
s
ITU-T Q.931). Together, these protocols support user-to-user,
circuit-switched, and packet-switched connections. A variety of call
establishment, call termination, information, and miscellaneous messag
es
are specified, including SETUP, CONNECT, RELEASE, USER INFORMATION, CA
NCEL,
STATUS, and DISCONNECT. These messages are functionally similar to tho
se
provided by the X.25 protocol.
ITU-T groups and organizes the ISDN protocols according to the followi
ng
gereral topic areas:
Protocols that begin with "E" recommend telephone network standards fo
r
ISDN.
Protocols that begin with "I" deal with concepts, terminology, and gen
eral
methods.
Protocols that begin with "Q" cover how switching and signaling should
operate.
15. Describe Cisco抯 implementation of ISDN BRI
Two 64 Kbps B channels and one 16 Kbps D channel.
IOS
16. Log into a router in both user and privileged modes.
User EXEC ? User mode entered by logging in. Prompt will be Router>. T
o
exit use the logout command.
Privileged EXEC ? From user EXEC mode, use the enable EXEC command. Pr
ompt
will be Router#.
To exit to user EXEC mode use the disable command.
17. Use the context-sensitive help facility.
Entering a question mark (?) at the system prompt displays a list of
commands available for each command mode. You can also get a list of a
ny
command抯 associated keyworkd and arguments with the context-sensitive
help
feature. To get help specific to a command mode, a command, a keyword,
or
arguments perform one of the following:
Task / Command
Obtain a brief description of the help system in command mode.
help
Configure a line or lines to receive help for the full set of user-lev
el
commands when a user types "?".
full-help
Configure a line to receive help for the full set of user-level comman
ds
for this exec session.
terminal full-help
Obtain a list of commands that begins with a particular character stri
ng.
abbreviated-command-entry? (no space between partial command and
question mark)
Complete a partial command name.
abbreviated-command-entry <Tab>
List all commands available for a particular command mode.
?
List a command抯 associated keywords.
command ? (space between command and question mark)
List a keyword抯 associated arguments.
Command keyword ? (space between work and question mark)
18. Use the command history and editing features.
With the current IOS release, the user interface provides a history or
record of commands that you have entered. This feature is particularly
useful for recalling long or complex command entries including access
lists. By default, the system records 10 command lines in its history
buffer. To set the number of command lines recorded during the current
terminal session use the following global command:
terminal history [size number-of-lines]
To configure the number of command lines the system records, complete
the
following command from line configuration mode:
history [size number-of-lines]
Useful editing commands:
Crtl-P (or the up arrow key) Recall commands in the history buffer sta
rting
with the most recent command.
Crtl-N (or the down arrow) Return to more recent commands in the histo
ry
buffer after recalling
commands with Crtl-P or the up arrow key.
Crtl-B (or left arrow key) Move the cursor back one character
Crtl-F (or right arrow key) Move the cursor forward one character
Crtl-A Move the cursor to the beginning of the command line
Crtl-E Move the cursor to the end of the command line
Esc B Move the cursor back one word
Esc F Move the cursor forward one word
Crtl-R or Crtl-L Redisplay the current command line
19. Examine router elements (RAM, ROM, CDP, show).
ROM - Read Only, Hard Wired, Boot Strap, IOS, ROM Monitor
RAM - IOS & Running Configuration (Main Memory)
NVRAM - Startup Config ? Saved via battery (10 yr Life Span)
Flash - IOS (PCMCIA Cards or SIMMs)
Shared RAM - Packet Buffering (Not all platforms)
The Cisco Discovery Protocol (CDP) is a media- and protocol-independen
t
protocol that runs on all Cisco-manufactured equipment including route
rs,
bridges, access servers and switches. CDP runs on all media that suppo
rts
Subnetwork Access Protocol (SNAP) including local area network, Frame
Relay
and ATM media. CDP runs over the data link layer only.
Specify the frequency of transmission of CDP updates.
cdp timer seconds
Specify the amount of time a receiving device should hold the informat
ion
sent by your device before discarding it.
cdp holdtime seconds
To disable CDP
no cdp run
To disable CDP on an interface
no cdp enable
delete the CDP table of information about neighbors
clear cdp table
display cdp updates received on the local router.
show cdp neighbors [type number] [detail]
display cdp entry for a specific neighbor router.
Show cdp entry {device name}
The show cdp neighbors command displays: Device ID, interface type and
number, hold-time settings, capabilities, platform and port ID informa
tion
about neighbors. Using the detail option displays the following additi
onal
neighbor details: network address, enabled protocols and
20. Manage configuration files from the privileged exec mode.
show startup-config to view the configuration in NVRAM (show config =
pre
10.3)
show running-config to view the current running configuration (write t
erm =
pre 10.3)
show version displays the configuration of the system hardware, the
software version, the names
and sources of configuration files, and the boot images.
show processes displays information about the active processes.
show protocols displays the configured protocols and status of any
configured Layer 3 protocol.
show mem shows statistics about the router's memory, including memory
free
pool statistics.
show ip route displays the entries in the routing table.
show flash shows information about the Flash memory device.
show interfaces displays statistics for all interfaces configured on t
he
router.
21. Control router passwords, identification and banner.
Cisco routers have two levels of passwords that can be applied; user a
nd
privileged EXEC. The user EXEC passwords are applied to the console,
auxiliary and virtual terminal lines of the Cisco router. Password
authentication can be either on the line, through a local username
definition or a TACACS, extended TACACS, TACACS+ or RADIUS server. To
enter
privileged EXEC mode, use the enable command. By default, the password
will
be compared against the password entered with the enable secret global
command.
To uniquely identify the router, use the hostname command as follows:
set the hostname hostname name
customize the prompt prompt string
remove the configuration prompt no service prompt config
Banners
banner exec
To display a banner on terminals with an interactive EXEC, use the ban
ner
exec global configuration command. This command specifies a message to
be
displayed when an EXEC process is created (a line is activated, or an
incoming connection is made to a VTY line). The no form of this comman
d
deletes the EXEC banner.
banner exec # message #
no banner exec
Syntax Description
Delimiting character of your choice--a pound sign (#) for example. You
cannot use the delimiting character in the banner message.
banner incoming
To specify a banner used when you have an incoming connection to a lin
e
from a host on the network, use the banner incoming global configurati
on
command. The no form of this command deletes the incoming connection
banner.
banner incoming # message #
no banner incoming
banner motd
banner motd # message #
no banner motd
An incoming connection is one initiated from the network side of the
router. Incoming connections are also called reverse Telnet sessions.
These
sessions can display MOTD banners and INCOMING banners, but they do no
t
display EXEC banners. Use the no motd-banner line configuration comman
d to
disable the MOTD banner for reverse Telnet sessions on asynchronous li
nes.
When a user connects to the router, the MOTD banner appears before the
login prompt. After the user successfully logs in to the router, the E
XEC
banner or INCOMING banner will be displayed, depending on the type of
connection. For a reverse Telnet login, the INCOMING banner will be
displayed. For all other connections, the router will display the EXEC
banner. Incoming banners cannot be suppressed. If you do not want the
incoming banner to appear, you must delete it with the no banner incom
ing
command.
22. Identify the main Cisco IOS commands for router startup.
Boot system flash (to boot from flash ROM, 1st try)
Boot host name (for TFTP boot)
Boot system rom (last resort - from ROM, limited IOS)
23. Enter an initial configuration using the setup command.
The command parser (Command Line Interface - CLI) allows you to make v
ery
detailed changes to your configurations. However, some major configura
tion
changes do not require the granularity provided by the command parser.
In
these cases, you can use the setup command facility to make major
enhancements to your configurations. For example, you might want to us
e
setup to add a protocol suite, to make major addressing scheme changes
, or
to configure a newly installed interface. Although you can use the com
mand
parser to make these major changes, the setup command facility provide
s you
with a high-level view of the configuration and guides you through the
configuration change process.
Additionally, if you are not familiar with Cisco products and the comm
and
parser, the setup command facility is a particularly valuable tool bec
ause
it asks you the questions required to make configuration changes.
Note: If you use setup to modify a configuration because you have adde
d or
modified the hardware, be sure to verify the physical connections usin
g the
show version command. Also, verify the logical port assignments using
the
show running-config command to ensure that you configure the proper po
rt.
To enter the setup command facility, enter 憇etup? in privileged EXEC
mode:
When you enter the setup command facility after first-time startup, an
interactive dialog called the System Configuration Dialog appears on t
he
system console screen. The System Configuration Dialog guides you thro
ugh
the configuration process. It prompts you first for global parameters
and
then for interface parameters. The values shown in brackets next to ea
ch
prompt are the default values last set using either the setup command
facility or the configure command. The prompts and the order in which
they
appear on the screen vary depending on the platform and the interfaces
installed in the device.
You must run through the entire System Configuration Dialog until you
come
to the item that you intend to change. To accept default settings for
items
that you do not want to change, press the Return key.
To return to the privileged EXEC prompt without making changes and wit
hout
running through the entire System Configuration Dialog, press Ctrl-C.
When you complete your changes, the setup command facility shows you t
he
configuration command script that was created during the setup session
. It
also asks you if you want to use this configuration. If you answer Yes
, the
configuration is saved to NVRAM. If you answer No, the configuration i
s not
saved and the process begins again. There is no default for this promp
t;
you must answer either Yes or No.
Router# setup
--- System Configuration Dialog ---
At any point you may enter a question mark '?' for help.
Use ctrl-c to abort configuration dialog at any prompt.
Default settings are in square brackets '[]'.
Continue with configuration dialog? [yes]:
First, would you like to see the current interface summary? [yes]:
Interface IP-Address OK? Method Status Protocol
Ethernet0 172.16.72.2 YES manual up up
Serial0 unassigned YES not set administratively down down
Serial1 172.16.72.2 YES not set up up
Configuring global parameters:
Enter host name [Router]:
The enable secret is a one-way cryptographic secret used
instead of the enable password when it exists.
Enter enable secret []:
The enable password is used when there is no enable secret
and when using older software and some boot images.
Enter enable password [ww]:
Enter virtual terminal password [ww]:
Configure SNMP Network Management? [yes]:
Community string [public]:
Configure IP? [yes]:
Configure IGRP routing? [yes]:
Your IGRP autonomous system number [15]:
Configuring interface Ethernet0:
Is this interface in use? [yes]:
Configure IP on this interface? [yes]:
IP address for this interface [172.16.72.2]:
Number of bits in subnet field [8]:
Class B network is 172.16.0.0, 8 subnet bits; mask is /24
24. Copy and manipulate configuration files
copy running-config startup-config save config variables to NVRAM (wri
te
memory = pre 10.3)
copy running-config tftp save config variables to a remote server on t
he
network. (write network = pre 10.3)
copy tftp running-config copies a file from a TFTP server to RAM (conf
ig
network = pre 10.3)
copy tftp startup-config loads a config file from a TFTP server direct
ly
into NVRAM (config overwrite = pre 10.3)
configure memory to re-execute the configuration commands located in N
VRAM
erase startup-config to erase the contents of NVRAM (write erase = pre
10.3)
25. List the commands to load Cisco IOS from: flash memory, a TFTP ser
ver
or ROM.
To configure a router to automatically boot an image in Flash memory,
perform the following tasks:
Task Command
Step 1 Enter configuration mode from the terminal configure terminal
Step 2 Enter the filename of an image stored in Flash memory boot syst
em
flash [filename]
boot system flash slot0:[filename]
boot system flash slot1:[filename]
boot system flash bootflash:[filename]
Step 3 Set the configuration register to enable loading image from Fla
sh
memory (generally 0x2102)
config-register value
Step 4 Save configuration file copy running-config startup-config
To configure a router to load a system image from a network server usi
ng
TFTP, rcp or MOP:
Task Command
Step 1 Enter configuration mode form the terminal configure terminal
Step 2 Specify the system image to be booted from a network server usi
ng
rcp, TFTP or MOP.
boot system [rcp | tftp] filename [ip address]
boot system mop filename [mac-address] [int]
Step 3 Set the configuration register to enable loading image from a
network server (generally 0x010F)
config-register value
Step 4 Save configuration file copy running-config startup-config
To specify the use of the ROM system image as a backup to other boot
instructions in the configuration file:
Task Command
Step 1 Enter configuration mode form the terminal configure terminal
Step 2 Enter the filename of an image stored in Flash memory boot syst
em
rom
Step 3 Set the configuration register to enable loading image from ROM
(generally 0x0101)
config-register value
Step 4 Save configuration file copy running-config startup-config
26. Prepare to backup, upgrade and load a backup Cisco IOS software im
age.
To prepare for backup check; access to the server, space available on
server, & naming conventions.
Router# copy flash tftp ( it will ask you for IP address &
source/destination file name )
To upgrade, back up current files first.
Router# show flash (verify available memory)
Router# copy tftp flash ( it will ask you for IP address of TFTP serve
r,
file name, & whether to erase flash )
! = 1 UDP serment has successfully transferred.
27. Prepare the initial configuration of your router and enable IP.
Network Protocols
28. Monitor Novell IPX operation on the router.
show ipx interface - IPX status and parameters
show ipx route - Routing table contents
show ipx servers - IPX server list
show ipx traffic - Number and type of packets
29. Describe the two parts of network addressing, then identify the pa
rts
in specific protocol address examples.
30. Create the different classes of IP addresses [and subnetting].
IP addressing supports five different address classes. The left-most
(high-order) bits indicate the network class. The following table prov
ides
reference information about the five IP address classes:
IP Format Purpose High-Order Address No. Bits
Max.
Address Bit(s) Range Network/Hos
t Hosts
Class
A N.H.H.H Large Org. 0 1 ? 126 7/24
2^24-2
B N.N.H.H Medium Org 10 128 ? 14/16
2^16-2
191
C N.N.N.H small Org. 110 192 ? 22/8
2^8-2
223
D N/A Multicast 1110 224 ? N/A
N/A
239
E N/A Experimental 1111 240 ? N/A
N/A
254
IP networks can be divided into smaller networks called subnetworks (o
r
subnets). Subnetting provides extra flexibility, makes more efficient
use
of network address utilization, and contains broadcast traffic because
a
broadcast will not cross a router. Subnets are under local administrat
ion.
As such, the outside world sees an organization as a single network, a
nd
has no detailed knowledge of the organization's internal structure. A
given
network address can be broken up into many subnetworks. For example,
172.16.1.0, 172.16.2.0, 172.16.3.0, and 172.16.4.0 are all subnets wit
hin
network 171.16.0.0. (All 0s in the host portion of an address specifie
s the
entire network.)
31. Configure IP addresses
Router(config-if)# ip address ip-address subnet-mask (assigns address
&
subnet mask, starts IP processing on an interface)
Router# term ip netmask-format { bitcount | decimal | hexadecimal }
(sets format of network mask for current session. Defaults back to bit
count.)
Router(config-if)# ip netmask-format { bitcount | decimal | hexadecima
l }
(sets format of network mask for a specific line)
32. Verify IP addresses
Telnet - verifies application-layer software between source and destin
ation
stations.
Ping - uses ICMP to verify hardware connection and logical address of
network layer.
Trace - uses TTL values to generate messages from each router used alo
ng
the path.
33. List the required IPX address and encapsulation type.
Interface Type Encapsulation Type IPX Frame Type
Ethernet novell-ether (default) Ethernet_802.3
arpa Ethernet_II
sap Ethernet_802.2
snap Ethernet_Snap
Token Ring sap (default) Token-Ring
snap Token-Ring_Sna
p
FDDI snap (default) Fddi_Snap
sap Fddi_802.2
34. Enable the Novell IPX protocol and configure interfaces.
Router(config)# ipx routing [node] (If no node is specified, MAC addre
ss of
interface is used.
If interface is serial, an address must be specified.)
Router(config)# ipx maximum-paths paths (configure round-robin load sh
aring
over multiple equal metric paths.)
Router(config-if)# interface type number.subinterface-numberipx networ
k
network [ encapsulation encapsulation type ]
(Specify a subinterface, then enable IPX routing with encapsulation ty
pe.)
Router(config-if)# ipx network network [ encapsulation encapsulation-t
ype ]
[ secondary ]
(Assign primary and secondary network number and encapsulation)
35. Identify the functions of the TCP/IP transport-layer protocols
TCP is a connection-oriented, reliable protocol. It is responsible for
breaking messages into segments, reassembling them at the destination
station, resending anything that is no received, and reassembling mess
ages
from the segments. TCP supplies a virtual circuit between end-user
applications.
UDP is a connectionless and unacknowledged. Although UDP is responsibl
e for
transmitting messages, no software checking for segment delivery is
provided at this layer.
36. Identify the functions of the TCP/IP network-layer protocols
IP provides connectionless, best-effort delivery routing of datagrams,
It
is not concerned with the content of the datagrams. Instead, it looks
for a
way to move the datagrams to their destination.
ICMP provides control and messaging capabilities.
ARP determines the data link layer address for known IP addressed.
RARP determines network addresses when data link layer addressed are k
nown.
37. Identify the functions performed by ICMP
The Internet Control Message Protocol (ICMP) is a network layer Intern
et
protocol that provides message packets to report errors and other
information relevant to IP packet processing back to the source. ICMP
is
documented in RFC 792. ICMP provides a number of helpful messages incl
uding
the following:
Destination Unreachable - The ICMP destination unreachable message is
sent
by a router if it is unable to deliver a packet to the ultimate
destination. The router discards the original packet. Destinations mig
ht be
unreachable for these reasons:
The source host specified a nonexistent address.
The router does not have a route to the destination (less frequent).
Destination unreachable messages include the following:
Network unreachable -- This message usually implies routing or address
ing
failures.
Host unreachable -- This message usually implies delivery failures suc
h as
a wrong subnet mask.
Protocol unreachable -- This message usually implies that the destinat
ion
does not support upper-layer protocol specified in the packet.
Port unreachable -- This message usually implies that the Transmission
Control Protocol
(TCP) port (socket) is not available.
Echo Request and Reply - The ICMP echo request message is sent by any
host
to test node reachability across an internetwork. It is generated by t
he
ping command. The ICMP echo reply message indicates that the node can
be
successfully reached.
Redirect - An ICMP redirect message is sent by the router to the sourc
e
host to stimulate more efficient routing. The router still forwards th
e
original packet to the destination. ICMP redirects allow host routing
tables to remain small because knowing the address of only one router
is
required (even if that router does not provide the best path). Even af
ter
receiving an ICMP redirect message, some devices might continue using
the
less efficient route.
Time Exceeded - An ICMP time-exceeded message is sent by the router if
an
IP packet's Time-to-Live field (expressed in hops or seconds) reaches
zero.
The Time-to-Live field prevents packets from continuously circulating
the
internetwork if the internetwork contains a routing loop. The router
discards the original packet.
Router Advertisement and Router Solicitation - The ICMP Router Discove
ry
Protocol (IDRP) uses router advertisement and router solicitation mess
ages
to discover the addresses of routers on directly attached subnets. IDR
P
works as follows:
1.Each router periodically multicasts router advertisement messages fr
om
each of its interfaces.
2.Hosts discover addresses of routers on directly attached subnets by
listening for these messages.
3.Hosts can use router solicitation messages to request immediate
advertisements, rather than waiting for unsolicited messages.
IRDP offers several advantages over other methods of discovering addre
sses
of neighboring routers. Primarily, it does not require hosts to recogn
ize
routing protocols, nor does it require manual configuration by an
administrator. Router advertisement messages allow hosts to discover t
he
existence of neighboring routers, but not which router is best to reac
h a
particular destination. If a host uses a poor first-hop router to reac
h a
particular destination, it receives a redirect message identifying a b
etter
choice.
Undeliverable ICMP messages (for whatever reason) do not generate a se
cond
ICMP message. Doing so could create an endless flood of ICMP messages.
38. Configure IPX access lists and SAP filters to control basic Novell
traffic
Routing
39. Add the RIP routing protocol to your configuration.
Router(config)# router rip (defines routing process)
Router(config-router)# network network number (selects participating
attached networks)
40. Add the IGRP routing protocol to your configuration.
Router(config)# router igrp autonomous-system (defines routing process
)
Router(config-router)# network network-number (selects participating
attached networks)
41. Explain the services of separate and integrated multiprotocol rout
ing.
Separate routing---The ships-in-the-night approach involves the use of
a
different routing protocol for each network protocol.
Integrated routing---Integrated routing involves the use of a single
routing protocol (for example, a link state protocol) that determines
the
least cost path for different routed protocols.
42. List problems that each routing type encounters when dealing with
topology changes and describe techniques to reduce these problems.
Distance Vector protocols, like RIP and IGRP, use the Bellman-Ford
algorithm They are slow to converge in a large LAN. This can lead to
inconsistent routing entries and cause routing loops.
Hop-Count Limit - RIP permits a maximum hop count of 15. Any destinati
on
greater than 15 hops away is tagged as unreachable. RIP's maximum hop
count
greatly restricts its use in large internetworks, but prevents a probl
em
called count to infinity from causing endless network routing loops.
Hold-Downs - Hold-downs are used to prevent regular update messages fr
om
inappropriately reinstating a route that has gone bad. When a route go
es
down, neighboring routers will detect this. These routers then calcula
te
new routes and send out routing update messages to inform their neighb
ors
of the route change. This activity begins a wave of routing updates th
at
filter through the network.
Triggered updates do not instantly arrive at every network device. It
is
therefore possible that a device that has yet to be informed of a netw
ork
failure may send a regular update message (indicating that a route tha
t has
just gone down is still good) to a device that has just been notified
of
the network failure. In this case, the latter device now contains (and
potentially advertises) incorrect routing information.
Hold-downs tell routers to hold down any changes that might affect rec
ently
removed routes for some period of time. The hold-down period is usuall
y
calculated to be just greater than the period of time necessary to upd
ate
the entire network with a routing change. Hold-down prevents the
count-to-infinity problem.
Split Horizons - Split horizons derive from the fact that it is never
useful to send information about a route back in the direction from wh
ich
it came. The split-horizon rule helps prevent two-node routing loops.
Poison Reverse Updates - Whereas split horizons should prevent routing
loops between adjacent routers, poison reverse updates are intended to
defeat larger routing loops. The idea is that increases in routing met
rics
generally indicate routing loops. Poison reverse updates are then sent
to
remove the route and place it in hold-down. Poison Reverse update are
updates sent to other routers with an unreachable metric.
Link State
Link State routing uses the Dijkstra algorithm to compute the shortest
path
first to another network.
Link State routing protocols, like OSPF & NLSP, notify other routers o
f
topology changes with link-state updates. The router receiving these L
SP's
recalculate their routing table. The 2 link-state concerns are:
Processing and memory required for link-state routing.
Bandwidth consumed for initial link-state "flood".
Link state updates can arrive at different times based on bandwidth be
tween
routers. To solve this problem:
Dampen the periodic update (longer intervals)
Use time stamps
Use targeted mulitcast (not flood), define router hierarchies (i.e.
partition network)
43. Describe the benefits of network segmentation with routers.
Manageability - There are explicit protocols operating among routers,
giving the network administrator greater control over path selection;
and
network routing behavior is more visible.
Functionality - Because routers are visible to the end stations, you c
an
implement mechanisms to provide flow control, error and congestion con
trol,
fragmentation and reassembly services, and explicit packet lifetime
control.
Multiple active paths - With the implementation of a router, you can u
se a
network topology using more than one path between stations. Operating
at
the network layer, routers can examine protocol, destination service a
ssess
point (DSAP), source service access point (SSAP), and path metric
information before making forwarding or filtering decisions.
Network Security
44. Configure standard and extended access lists to filter IP traffic.
Standard access lists - check the source address of a packets that cou
ld be
routed. The result permits or denies output for an entire protocol sui
te,
based on the network/subnet/host address.
Place standard access lists close to the destination.
Router(config)# access-list access-list-number {permit | deny } source
[
source-mask ]
*IP standard access lists use access-list-numbers 1 to 99
Router(config-if)# ip access-group access-list-number { in | out }
Extended access lists - check for both source and destination packet
addresses. They also can check for specific protocols, port numbers, a
nd
other parameters, which allows administrators more flexibility to desc
ribe
what checking the access list will do. Packets can be permitted or den
ied
output based on where the packet originated and on its destination.
Place extended lists close to the source.
Router(config)# access-list access-list-number { permit | deny } proto
col
source source-mask destination destination-mask
[ operator operand ] [ established ]
*IP extended access lists use access-list-numbers 100-199
Router(config-if)# ip access-group access-list-number { in | out }
45. Monitor and verify selected access list operations on the router.
Show ip interface - displays IP interface information and indicates wh
ether
any access lists are set.
Show access-lists [ name/number ] - displays the contents of all or
specific access list(s)
LAN Switching
46. Describe the advantages of LAN segmentation
Dividing the network into smaller segments reduces the number of users
per
segment, thereby increasing the bandwidth available to each user in th
e
segment.
47. Describe LAN segmentation using bridges
A bridge is a data link layer device used to connect two segments. It
is
protocol independent and transparent to the end user. Bridges "learn"
which
end stations can be reached through which port from the source address
of a
packet. If the destination is on the same segment as the source, the p
acket
is not forwarded. Bridges introduce a latency penalty due to processin
g
overhead ( 20-30 % in loss of throughput for acknowledgment-oriented
protocols, and 10-20 % for sliding window protocols). Bridges forward
multicast and broadcast packets to other attached segments (these
destinations do no appear in the address tables).
48. Describe LAN segmentation using routers
Routers operate at the network layer and are used to extend a network
across multiple data links, finding routes between the source and
destination stations on a internetwork. They typically perform functio
ns
associated with bridging, such as making forwarding decisions based on
table look-up. Unlike a bridge, the router is known to the stations us
ing
its services, and a well-defined protocol must be used among the stati
ons
and the router. Routers introduce a latency penalty (associated with
examining more fields than a bridge) of 30-40 % loss of throughput for
acknowledgment-oriented protocols, and 20-30 % for sliding window
protocols.
49. Describe LAN segmentation using switches
A switched Ethernet connection operates like a network with only two n
odes.
In a switched Ethernet network, the utilization can reach closer to th
e 100
% rate. A switch segments a LAN collision domain into smaller collisio
n
domains thus reducing or eliminating station contention for media acce
ss.
LAN switches use the data-link layer information to create a direct
point-to-point path across the switch or across several switches betwe
en
the source and destination. Use of the MAC layer information for
transmitting packets enables a LAN switch to be protocol independent.
50. Name and describe two switching methods
Port configuration - allows a port to be assigned to a physical networ
k
segment under software control.
Frame (packet) - increases available bandwidth by allowing multiple
transmissions to occur in parallel.
51. Describe full- and half-duplex Ethernet operation
Half-duplex Ethernet has each circuit used for a specific purpose. Whe
n a
node is transmitting, other nodes are receiving.
Efficiency is typically 50-60 percent of the 10 Mbps bandwidth.
Full-duplex Ethernet allows simultaneous transmission and reception. I
t
requires a switched connection between two nodes. A transmit circuit
connection is wired directly to the receiver circuit at the other end
of
the connection. Since just two stations are connected in this arrangem
ent,
a collision-free environment exists here. Full-duplex Ethernet offers
100 %
efficiency in both directions. (10 Mbps transmit, and 10 Mbps receive.
)
This produces a theoretical 20 Mbps of throughput.
52. Describe network congestion problems in ethernet networks
Transmission of graphic files, images, full-motion video, and multimed
ia
applications exceed the 10 Mbps bandwidth of traditional Ethernet. Als
o,
use of the internet has increased network utilization.
53. Describe the benefits of network segmentation with bridges.
Provides more bandwidth per user due to fewer users per segment.
Packets with the destination and source addresses on the same segment
are
not forwarded.
Network installation is simple because it "learns" its connected netwo
rk
topology.
54. Describe the benefits of network segmentation with switches.
More effective utilization of the available medium bandwidth.
Greater flexibility in the network infrastructure.
Use of existing hardware, such as NICs and cabling, lowers cost.
Advanced switching features, such as VLANs.
Improves performance without impacting addressing structure within the
network.
55. Describe the features and benefits of Fast Ethernet
The IEEE 802.3u 100BaseT Fast Ethernet standard is based on Ethernet's
CSMA/CD protocol but is 10 times faster. Fast Ethernet is well suited
for
bursty communication such as client/server applications, centralized s
erver
farms or power workgroups, and backbone implementations.
The benefits of Fast Ethernet are:
High performance (10 times that of 10BaseT network).
Allows the use of existing cabling and network equipment, thus reducin
g the
overall cost of
implementation and allowing easy integration into the existing 10BaseT
neetworks.
Uses the same MAC and shares common circuitry. Dual speed adapters and
switch can be used for easy migration from 10 Mbps to 100 Mbps
Based on the proven CSMA/CD technology which is well specified and
exhaustively tested & verified
56. Describe the guidelines and distance limitations of Fast Ethernet
100BaseTX : uses Cat 5 UTP, RJ-45 connectors, and has a distance limit
of
100 meters.
100BaseFX: uses multimode fiber, SC/ST/MIC connectors, & has a distanc
e
limit of 412 meters (half-duplex) or 2 kilometers (full-duplex)
100BaseT4: uses 4-pair Cat 3, 4, or 5 UTP, RJ45 connectors, & can use
voice
grade wire.
Total length of network between end stations for Ethernet.
1 Class I repeater (UTP Medium) 200 meters (UTP & Fiber) 261 meters
1 Class II repeater (UTP Medium) 200 meters (UTP & Fiber) 308 meters
2 Class II repeaters (UTP Medium) 205 meters (UTP & Fiber) 216 meters
UTP/Fiber configuration assumes a UTP distance of 105 meters.
57. Distinguish between cut-through and store-and-forward switching.
Cut through switching will forward the packet as soon as the destinati
on
MAC is known. Store and forward will forward after the packet has been
received and declared to be valid. Cut through is faster, but you may
pass
"bad" packets.
58. Describe the operation of the Spanning Tree Protocol and its benef
it
Spanning-Tree Protocol is a link management protocol that provides pat
h
redundancy while preventing undesirable loops in the network. For an
Ethernet network to function properly, only one active path can exist
between two stations. Multiple active paths between stations cause loo
ps in
the network. If a loop exists in the network topology, the potential e
xists
for duplication of messages. When loops occur, some switches see stati
ons
appear on both sides of the switch. This condition confuses the forwar
ding
algorithm and allows duplicate frames to be forwarded.
To provide path redundancy, Spanning-Tree Protocol defines a tree that
spans all switches in an extended network. Spanning-Tree Protocol forc
es
certain redundant data paths into a standby (blocked) state. If one ne
twork
segment in the Spanning-Tree Protocol becomes unreachable, or if
Spanning-Tree Protocol costs change, the spanning-tree algorithm
reconfigures the spanning-tree topology and reestablishes the link by
activating the standby path.
Spanning-Tree Protocol operation is transparent to end stations, which
are
unaware whether they are connected to a single LAN segment or a switch
ed
LAN of multiple segments.
Election of the Root Switch
All switches in an extended LAN participating in Spanning-Tree Protoco
l
gather information on other switches in the network through an exchang
e of
data messages. These messages are bridge protocol data units (BPDUs).
This
exchange of messages results in the following:
The election of a unique root switch for the stable spanning-tree netw
ork
topology.
The election of a designated switch for every switched LAN segment.
The removal of loops in the switched network by placing redundant swit
ch
ports in a backup state.
The Spanning-Tree Protocol root switch is the logical center of the
spanning-tree topology in a switched network. All paths that are not n
eeded
to reach the root switch from anywhere in the switched network are pla
ced
in Spanning-Tree Protocol backup mode.
BPDUs contain information about the transmitting switch and its ports,
including switch and port Media Access Control (MAC) addresses, switch
priority, port priority, and port cost. The Spanning-Tree Protocol use
s
this information to elect the root switch and root port for the switch
ed
network, as well as the root port and designated port for each switche
d
segment.
A BPDU exchange results in the following:
One switch is elected as the root switch.
The shortest distance to the root switch is calculated for each switch
.
A designated switch is selected. This is the switch closest to the roo
t
switch through which frames will be forwarded to the root.
A port for each switch is selected. This is the port providing the bes
t
path from switch to the root switch.
Ports included in the Spanning-Tree Protocol are selected.
If all switches are enabled with default settings, the switch with the
lowest MAC address in the network becomes the root swit |
|