>enable
#config terminal
#hostname Cisco
#enable secret Cisco
#interface ethernet0
#ip address 192.168.17.1 255.255.255.0
#no shutdown
#exit
#interface ethernet1
#ip address 192.168.19.1 255.255.255.0
#no shutdown
#exit
#interface serial0
#ip address 192.168.21.1 255.255.255.0
#clock rate 64000
#no shutdown
#exit
#network 192.168.17.0
#network 192.168.19.0
#network 192.168.21.0
#Ctrl+z
#copy running-config startup-config
>enable
#config terminal
#hostname Cisco
#enable secret cisco1
#line con0
#login
#password test
#line vty 04
#login
#password king
#Ctrl+z
#copy running-config startup-config
1
>enable
#config t
#hostname ciscoNAT1
#enable secret Cisco
#access list-5 permit 10.100.5.0 0.0.0.255
#ip nat pool lan 192.168.15.1 192.168.15.1 netmask 255.255.255.0
#ip nat inside source list-5 pool lan overload
#ip nat inside source static 10.100.15.5 192.168.15.5
#int s0
#ip nat outside
#int e0
#ip nat inside
#exit
#exit
#copy run start
Important Router Parts
1. ROM - Read Only Memory.
This is a form of permanent memory used by the Router to store:
The "Power-On Self Test" that checks the Router on boot up.
The "Bootstrap Startup Program" that gets the Router going.
a very basic form of the Cisco IOS software.
(to change the ROM you have to remove and replace chips)
2. Flash Memory
An Electronically Erasable and Re-Programmable memory chip.
The "Flash" contains the full Operating System, or "Image".
This allows you to upgrade the OS without removing chips.
3. NVRAM - Non-Volatile RAM
This stores your Router's "Startup Configuration File".
Similar to Flash memory, this retains data even when power is lost.
4. RAM - Random Access Memory
This is regular computer memory chips.
These are the working memory of the Router,
and provide Caching, Packet Buffering, and hold Routing Tables.
The RAM is also where the Running Operating System
lives when the Router is on.
2
RAM loses all its data when reset or powered off.
5. Interfaces - Where the Router meets the Outside World
Basically your Router will have Serial interfaces,
Which are mostly used to connect long-distance as in a WAN (Wide-Area Network).
You will also have LAN (Local-Area Network) Interfaces,
such as Ethernet, Token Ring, and FDDI (Fiber Distributed Data Interface)
What Happens As Your Router Boots Up
1. The ROM "Power-On Self-Test" checks the Router Hardware.
This includes the CPU (Central Processor Unit), memory, and interfaces.
2. The ROM "Bootstrap Program", which is stored in ROM, runs itself
3. The "Bootfield" is read to find out the proper Operating System source.
4. The RAM "Operating System Image" is loaded into RAM. (Random Access
Memory)
5. The NVRAM "Configuration File" saved in NVRAM is loaded into the RAM.
The Configuration File is then executed one line at a time.
If no "Configuration File" is found in NVRAM,
the Cisco IOS will offer you the chance to use the
"Initial Configuration Dialog".
Set of Questions to answer for basic configuration.
User Exec Mode
>
Privileged Exec Mode
#
To change from Privilege to User mode just type Disable
To ends your session type: Control-Z ^Z
Type of commands: Global, Major, and Sub-command
Global “Configure” Router(config)
Major “Line” Router(Config-line)
Sub-command “Login” “Password”
There are 5 separate Passwords you need to protect your Router.
1.
2.
3.
4.
5.
6.
Console - protects the Console Port
Auxilary - protects the AUX Port (for your modem)
TTY - Protects against un-authorized Telenet Port logons
Enable - Guards the use of the Enable Mode Super-user status.
Enable Secret - an Encrypted Secret form of the Above (better!)
VTY 04 Virtual ports
3
Simple Operating Instructions






CTRL-A
CTRL-E
CTRL-B
CTRL-F
ESCAPE-B
ESCAPE-F
goes to the "Beginning" of the Line.
goes to the "End" of the Line.
go "Back One Character". (can also use LEFT Arrow key)
go "Forward One Character". (can also use RIGHT Arrow key)
go "Backward to the Beginning of the Next Word".
go "Forward to the Beginning of the Next Word".
You can increase the size of your HISTORY buffer by using the command:
Terminal History Size
Router# Terminal History Size 99
Privileged Mode
enable - get to privileged mode
disable - get to user mode
enable password <password_here> - sets privileged mode password
enable secret <password_here> - sets encrypted privileged mode password
Setting Passwords
enable secret <password_here> - set encrypted password for privileged access
enable password <password_here> - set password for privileged access (used when there is
no enable secret and when using older software)
Set password for console access:
(config)#line console 0
(config-line)#login
(config-line)#password <password_here>
Set password for virtual terminal (telnet) access (password must be set to access router
through telnet):
(config)#line vty 0 4
(config-line)#login
(config-line)#password <password_here>
Set password for auxiliary (modem) access:
(config)#line aux 0
4
(config-line)#login
(config-line)#password <password_here>
Configuring the Router
sh running-config - details the running configuration file (RAM)
sh startup-config - displays the configuration stored in NVRAM
setup - Will start the the automatic setup; the same as when you first boot the
router
config t - use to execute configuration commands from the terminal
config mem - executes configuration commands stored in NVRAM; copies
startup-config to running-config
config net - used to retrieve configuration info from a TFTP server
copy running-config startup-config - copies saved config in running config (RAM) to
NVRAM or "write memory" for IOS under ver.11
copy startup-config running-config - copies from non-volatile (NVRAM) to
current running config (RAM)
boot system flash <filename_here> - tells router which IOS file in flash to boot
from
boot system tftp - tells router which IOS file on the tftp server to boot from
boot system rom - tell router to boot from ROM at next boot
copy flash tftp - Copies flash to tftp server
copy tftp flash - Restores flash from tftp server
copy run tftp - Copies the current running-config to tftp server
copy tftp run - Restores the running-config from tftp server
General Commands
no shutdown - (enables the interface)
reload - restarts the router
sh ver - Cisco IOS version, uptime of router, how the router started, where
system was loaded from, the interfaces the POST found, and the configuration
register
sh clock - shows date and time on router
sh history - shows the history of your commands
sh debug - shows all debugging that is currently enabled
no debug all - turns off all debugging
sh users - shows users connected to router
sh protocols - shows which protocols are configured
banner motd # Your_message # - Set/change banner
hostname <router_name_here> - use to configure the hostname of the router
clear counters - clear interface counters
5
Processes & Statistics
sh processes - shows active processes running on router
sh process cpu - shows cpu statistics
sh mem - shows memory statistics
sh flash - describes the flash memory and displays the size of files and the
amount of free flash memory
sh buffers - displays statistics for router buffer pools; shows the size of the
Small, Middle, Big, Very Big, Large and Huge Buffers
sh stacks - shows reason for last reboot, monitors the stack use of processes
and interrupts routines
CDP Commands
(Cisco Discovery Protocol uses layer 2 multicast over a SNAP-capable link to
send data):
sh cdp neighbor - shows directly connected neighbors
sh cdp int - shows which interfaces are running CDP
sh cdp int eth 0/0 - show CDP info for specific interface
sh cdp entry <cdp_neighbor_here> - shows CDP neighbor detail
cdp timer 120 - change how often CDP info is sent (default cdp timer is 60)
cp holdtime 240 - how long to wait before removing a CDP neighbor (default
CDP holdtime is 180)
sh cdp run - shows if CDP turned on
no cdp run - turns off CDP for entire router (global config)
no cdp enable - turns off CDP on specific interface
Miscellaneous Commands
sh controller t1 - shows status of T1 lines
sh controller serial 1 - use to determine if DCE or DTE device
(config-if)#clock rate 6400 - set clock on DCE (bits per second)
(config-if)#bandwidth 64 - set bandwidth (kilobits)
IP Commands
Configure IP on an interface:
int serial 0
ip address 157.89.1.3 255.255.0.0
int eth 0
ip address 2008.1.1.4 255.255.255.0
6
Other IP Commands:
sh ip route - view ip routing table
ip route <remote_network> <mask> <default_gateway>
[administrative_distance] - configure a static IP route
ip route 0.0.0.0 0.0.0.0 <gateway_of_last_resort> - sets default gateway
ip classless - use with static routing to allow packets destined for
unrecognized subnets to use the best possible route
sh arp - view arp cache; shows MAC address of connected routers
ip address 2.2.2.2 255.255.255.0 secondary - configure a 2nd ip address on
an interface
sh ip protocol
IPX Commands
Enable IPX on router:
ipx routing
Configure IPX + IPX-RIP on an int:
int ser 0
ipx network 4A
Other Commands:
sh ipx route - shows IPX routing table
sh ipx int e0 - shows ipx address on int
sh ipx servers - shows SAP table
sh ipx traffic - view traffic statistics
debug ipx routing activity - debugs IPS RIP packets
debug ipx sap - debugs SAP packets
Routing Protocols
Configure RIP:
router rip
network 157.89.0.0
network 208.1.1.0
7
Other RIP Commands:
debug ip rip - view RIP debugging info
Configure IGRP:
router IGRP 200
network 157.89.0.0
network 208.1.1.0
Other IGRP Commands:
debug ip igrp events - view IGRP debugging info
debug ip igrp transactions - view IGRP debugging info
Access Lists
sh ip int ser 0 - use to view which IP access lists are applies to which int
sh ipx int ser 0 - use to view which IPX access lists are applies to which int
sh appletalk int ser 0 - use to view which AppleTalk access lists are applies to
which int
View access lists:
sh access-lists
sh ip access-lists
sh ipx access-lists
sh appletalk access-lists
Apply standard IP access list to int eth 0:
access-list 1 deny 200.1.1.0 0.0.0.255
access-list 1 permit any
int eth 0
ip access-group 1 in
Apply Extended IP access list to int eth 0:
access-list 100 deny tcp host 1.1.1.1 host 2.2.2.2 eq 23
access-list 100 deny tcp 3.3.3.0 0.0.0.255 any eq 80
int eth 0
ip access-group 100 out
8
Apply Standard IPX access list to int eth 0:
access-list 800 deny 7a 8000
access-list 800 permit -1
int eth 0
ipx access-group 800 out
Apply Standard IPX access list to int eth 0:
access-list 900 deny sap any 3378 -1
access-list 900 permit sap any all -1
int eth 0
ipx access-group 900 out
Wan Configurations
PPP Configuration
encapsulation ppp
ppp authentication <chap_or_pap_here>
ppp chap hostname <routername_here>
ppp pap sent-username <username_here>
sh int ser 0 - use to view encapsulation on the interface
Frame-Relay Configuration
encapsulation frame-relay ietf - use IETF when setting up a frame-relay
network between a Cisco router and a non-Cisco router
frame-relay lmi-type ansi - LMI types are Cisco, ANSI, Q933A; Cisco is the
default; LMI type is auto-sensed in IOS v11.2 and up
frame-relay map ip 3.3.3.3 100 broadcast - if inverse ARP won't work, map
Other IP to Your DLCI # (local)
keepalive 10 - use to set keepalive
sh int ser 0 - use to show DLCI, LMI, and encapsulation info
sh frame-relay pvc - shows the configured DLCI's; shows PVC traffic stats
sh frame-relay map - shows route maps
sh frame-relay lmi - shows LMI info
Keyboard Shortcuts
CTRL-P - show previous command
CTRL-N - show next command
SHIFT-CTRL-6 - Break
9
Notes
Static and Dynamic Routing
Static Routing - manually assigned by the Admin user entering the routes
(Routed Protocols - IP, IPX and AppleTalk)
Dynamic Routing - generated/determined by a Routing Protocol (Routing
Protocols - RIP I, RIP II, IGRP, EIGRP, OSPF, NLSP, RTMP)
Dynamic
1) With Dynamic Routing, routers pass information between each other so that
routing tables are regularly maintained.
2) The routers then determine the correct paths packets should take to reach
their destinations.
3) Information is passed only between routers.
4) A routing domain is called an Autonomous System, as it is a portion of the
Internetwork under common admin authority.
5) Consists of routers that share information over the same protocol. Can be split
into routing areas.
Distance Vector and Link-State Routing
Routing Protocols
I) Interior (within an autonomous system - AS - group of routers under the
same administrative authority)
a) Distance Vector - understands the direction and distance to any
network connection on the internetwork. Knows how
many hops (the metric) to get there. All routers w/in the internetwork listen for
messages from other routers, which are sent
every 30 to 90 seconds. They pass their entire routing tables. Uses hop count
for measurement. 1) Used in smaller networks
that are have fewer than 100 routers. 2) Easy to configure and use. 3) As
routers increase in number, you need to consider
CPU utilization, convergence time, and bandwidth utilization. 4) Convergence
is due to routing updates at set intervals. 5) When
a router recognizes a change it updates the routing table and sends the whole
table to all of its neighbors.
1) RIP - 15 hop count max
2) IGRP - 255 hop count max, uses reliability factor (255 optimal),
and bandwidth
3) RTMP
b) Link State - understands the entire network, and does not use
10
secondhand information. Routers exchange LSP?s (hello
packets). Each router builds a topographical view of the network, then uses
SPF (shortest path first) algorithm to determine the
best route. Changes in topology can be sent out immediately, so convergence
can be quicker. Uses Bandwidth, congestion for measurement; Dijkstra's
algorithm;
1) Maintains Topology Database.
2) Routers have formal neighbor relationship.
3) Exchanges LSA (Link State Advertisement) or
hello packets with directly connected interfaces.
4) These are exchanged at short intervals (typically 10 sec).
5) Only new info is
exchanged.
6) Scales well, however link?state protocols are more complex. 7) Requires
more processing power, memory, and bandwidth.
1) OSPF - decisions based on cost of route (metric limit of 65,535)
2) EIGRP - hybrid protocol (both Distance-Vector and Link State), Cisco
proprietary
3) NLSP
4) IS-IS
II) Exterior
1) EGP (Exterior Gateway Protocol)
2) BGP (Border Gateway Protocol)
Routing Protocols used for each Routed Protocol
IP - RIP, IGRP, OSPF, IS-IS, EIGRP
IPX - IPX RIP, NLSP, EIGRP
AppleTalk - RTMP, AURP, EIGRP
11
Problems with Routing Protocols
1) Routing Loops - occur when routing tables are not updated fast enough
when one of the networks becomes unreachable. Due to the slow convergence
(updates of routing table between all routers), some routers will end up with
incorrect routing table and will broadcast that routing table to other routers. This
incorrect routing tables will cause packets to travel repeatedly in circles.
2) Counting to infinity - occurs when packets end up in a routing loop; hop
count increases with every pass through a router on the network
Solutions to Problems with Routing Protocols
1) Define the maximum number of hops - When the number of hops
reaches this predefined value, the distance is considered infinite, thus the
network is considered unreachable. This does stop routing loops, but only limit
the time that packet can travel inside the loop.
2) Split horizon - The packets can not be sent back to the same interface that
they originally came from. During the updates, one router does not send updates
to the router that it received the information from.
3) Route poisoning - The router sets the cost/distance of routes that are
unreachable to infinity. Used with hold-down timers
4) Triggered updates - The router sends updates of the routing table as
soon as it detects changes in the network. Does not wait for the prescribed time
to expire.
5) Hold-Downs - After the router detects unreachable network, the routers
waits for a specified time before announcing that a network is unreachable. The
router will also wait for a period of time before it updates its routing table after it
detects that another router came online (Router keeps an entry for the network
possibly down state, allowing time for other routers to re-compute for this
topology change). Hold-downs can only partially prevent counting to infinity
problem. Prevents routes from changing too rapidly in order to determine if a link
has really failed, or is back up
Encapsulation Types
Encapsulation
802.2
802.3
Ethernet
II
Snap
sap
novell-ether
arpa (Internet
Standard)
snap
12
Wan Service Providers
1) Customer premises equipment (CPE) - Devices physically located at
subscriber?s location; examples: CSU/DSU, modem, wiring on the customer's
location
2) Demarcation (or demarc) - The place where the CPE ends and the local
loop portion of the service begins. (Usually in the "phone closet").
3) Local loop - Cabling from the demarc into the WAN service provider?s
central office; wiring from customer's location to the nearest CO
4) Central Office switch (CO) - Switching facility that provides the nearest
point of presence for the provider?s WAN service; location of telephone
company's equipment where the phone line connects to the high speed line
(trunk); Regional Telco Office where the local loop terminates (the Telco location
nearest you)
5) Toll network - The switches and facilities, (trunks), inside the WAN
provider?s "cloud."
DTE - the router side and receive clocking
DCE - the CSU/DSU side and provide clocking
WAN Devices
Routers - Offer both internetwork and WAN interface controls
ATM Switches - High-speed cell switching between both LANs and WANs
X.25 and Frame-Relay Switches - Connect private data over public circuits
using digital signals
Modems - Connect private data over public telephone circuits using analog
signals
CSU/DSU (Channel Service Units/Data Service Units) - Customer Premises
Equipment (CPE) which is used to terminate a digital circuit at the customer site
Communication Servers - Dial in/out servers that allow dialing in from remote
locations and attach to the LAN
Multiplexors - Device that allows more than one signal to be sent out
simultaneously over one physical circuit
ISDN
ISDN BRI (Basic Rate Interface) - 2 64K B channels, plus 1 16K D channel
ISDN PRI (Primary Rate Interface) - 23 64K B channels, plus 1 64K D channel
(North America & Japan), 30 64K B channels, plus 1 64K D channel (Europe &
Australia)
Classful and Classless Protocols
Classful - summarizes routing info by major network numbers; ex. RIP, IGRP
Classless - BGP, OSPF
13
Administrative Distances for IP Routes
Administrative Distances are configured using ip route command:
Example:
ip route 154.4.55.0 255.255.255.0 195.23.55.1 85 (where 85 is the
administrative distance)
IP Route
Administrative Distance
Directly connected interface
0
Static route using connected
interface
0
Static route using IP address
1
EIGRP summary route
5
External BGP route
20
Internal EIGRP route
90
IGRP route
100
OSPF route
110
IS-IS route
115
RIP route
120
EGP route
140
External EIGRP route
170
Internal BGP route
200
Route of unknown origin
255
Switching Terminology
Store-and-Forward ? copies entire frame into buffer, checks for CRC errors
before forwarding. Higher latency.
Cut-Through ? reads only the destination address into buffer, and forwards
immediately; Low latency; "wire-speed"
Fragment free ? modified form of cut-through; switch will read into the first 64
14
bytes before forwarding the frame. Collisions will usually occur within the first 64
bytes. (default for 1900 series).
Access Lists
1-99
IP Standard Access List
100-199
IP Extended Access List
200-299
Protocol Type-code Access List
300-399
DECnet Access List
600-699
Appletalk Access List
700-799
48-bit MAC Address Access List
800-899
IPX Standard Access List
900-999
IPX Extended Access List
1000-1099 IPX SAP Access List
1100-1199
Extended 48-bit MAC Address Access
List
1200-1299 IPX Summary Address Access List
15
Access
List
Standard
IP
Filters
Wildcard
Masks
Additional Notes
Source IP
address
field in the
packet's IP
header
To put
simply,
when the IP
is broken
down to
binary, the
1's allow
everything
and the 0's
must match
exactly.
Wildcard mask
examples:
0.0.0.0=entire address
must match.
0.255.255.255=only
the first octet must
match, the rest will
allow everything.
255.255.255.255=allow
everything
Source IP
or
Destination
IP, or TCP
Extended
Same as
or UDP
IP
standard
Source or
Destination
Ports, or
Protocol
Standard
IPX
Packets
sent by
clients and
servers,
and SAP
updates
sent by
servers
and
routers
Source
Network or
Node, or
Destination
Network or
Extended
Node, or
IPX
IPX
Protocol,
or IPX
Socket, or
SAP
The key word ANY
implies any IP value is
allowed, the keyword
HOST implies the IP
exactly has to match
Configured
as a
-1 means any and all
hexadecimal
network numbers (
number
works like ANY)
instead of
binary
Match
multiple
networks
The most practical use
with one
of the protocol type is
statement,
for NetBIOS
again in
hexadecimal
16
SAP
Sent and
received
SAP traffic
Updates its own SAP
tables. Again uses -1
to mean "ANY"
N/A
Troubleshooting Tools:
Ping Results
!
success
,
timeout
U destination unreachable
? unknown packet type
& TTL exceeded
Traceroute Results
!H router rec'd, but didn't forward because of access-list
P protocol unreachable
N network unreachable
U port unreachable
, timeout
Accessing Router with Terminal Emulation
Using HyperTerminal on a Windows machine adjust the following settings:
VT100 Emulation
Connection Speed: 9600 Baud
Data Bits: 8
Parity: None
Stop Bits: 1
Flow Control: None
On a Linux machine you may use Seyon or Minicom (at least one should come
with your distribution).
17
Router Startup Sequence
POST
Bootstrap program loaded from ROM
IOS is loaded from either flash (default), TFTP, or ROM
IOS image loaded into low-addressed memory; hardware and software is
determined
Config file is load from NVRAM; if no configuration exists in NVRAM, the initial
configuration dialog will begin
Miscellaneous Notes
Multiple Loop Problems ? complex topology can cause multiple loops to
occur. Layer 2 has no mechanism to stop the loop. This is the main reason for
Spanning ? Tree Protocol.
Spanning-Tree Protocol (STP) IEEE 802.1d. ? developed to prevent
routing loops; uses STA (Spanning-Tree Algorithm) to calculate a loop-free
network topology; allows redundant paths without suffering the effects of loops in
the network
Virtual LAN?s (VLAN's) ? sets different ports on a switch to be part of
different sub-networks. Some benefits: simplify moves, adds, changes; reduce
administrative costs; have better control of broadcasts; tighten security; and
distribute load. Relocate the server into a secured location.
HDLC (High-Level Data Link Control) - Link layer protocol for Serial links.
Cisco Default. Supports the following modes: Normal Response Mode ? as per
Secondary under SDLC; Asynchronous Response Mode allows secondary to
communicate without permission; Asynchronous Balanced mode combines the
two stations. Has lower overhead than LAPB but less error checking.
Modular Switch/VIP Syntax
type slot/port (example: e 2/1)
type slot/port-adapter/port (example: e 2/0/1)
18
Class A, B, C, D, & E Networks
------------------------------------------------------------------------------------------------------------
A Network 1-126
Class A Private Network
Class
10.0.0.0 to 10.255.255.255
Subnet Mask 255.0.0.0
------------------------------------------------------------------------------------------------------------
B Network 128-191
Class B Private Network
Class
172.16.0.0 to 172.31.255.255 Subnet Mask 255.255.0.0
------------------------------------------------------------------------------------------------------------
C Network 192-223
Class C Private Network
Class
192.168.0.0 to
192.168.255.255 Subnet Mask 255.255.255.0
------------------------------------------------------------------------------------------------------------
D Multicast 224-239
Class D Multicast Private Network
Class
239.0.0.0 to 239.255.255.0
------------------------------------------------------------------------------------------------------------
Class
E Reserved 240-255
------------------------------------------------------------------------------------------------------------
19
APIPA 169.254.0.0 to 169.254.255.254 Subnet Mask 255.255.0.0
------------------------------------------------------------------------------------------------------------
OSI Layers
1.
2.
3.
4.
5.
6.
7.
Physical
Data-link
Network
Transport
Session
Presentation
Application
7. Application
6. Presentation
5. Session
4. Transport
3. Network
2. Data-Link
1. Physical
Open System Interconnection
1.
Physical
2.
Data Link
- REPEATERS & NIC operate in Physical Layer, SLIP, PPP,
TWISTD PAIR, THINNET, COAX, AUI, protocols operate in Physical Layer.
– BRIDGES, SWITCHES and PPP,ETHERNET, TOKEN RING
protocol operate in Data Link Layer.
Data Link Layer assembles and disassembles frames for transmission and
reception.
Media Access Control (MAC) Allows multiple devices to share media.
Logical Link Control (LLC) Starts and maintaining links between devices.
Control Transmission method is used to access media.
- maintains physical device addresses ( MAC addresses ).
- provides flow control and error control for single links between devices.
Flow control Determines how much data to send to avoid overwhelming receiver.
Error control Detects errors in received frames and requests retransmission.
3.
Network – ROUTERS, BROUDERS, IPX, IP protocols operate in Network
Layer.
Network Layer, also handles addressing and delivering packets on complex
20
network.
4.
Transport – TCP, UDP, NETBEUI, SPX protocols, also GATEWAY is operate
in Transport Layer.
Transport Layer can implement procedures to ensure reliable delivery of
messages to destinations.
5.
Session – TELNET and GATEWAY are operate in this layer.
Session Layer - Manages dialogs between two computers by establishing,
managing, and
terminating communication.
There are three forms of dialogs:
Simplex – One-way data transfer.
Half-duplex – Two way data transfer, but data can only flow in one direction at a time.
Full-duplex – Two way simultaneous data transfer.
A session is a formal dialog between a requestor and provider and must
have three phases:



6.
Connection establishment
Data transfer
Connection release
Presentation – Deals with syntax for communication between two
computers.
7.
Application – APPLE TALK, NFS protocols operate in this layer, also
GATEWAY.
Application Layer - Provides services on the network, such as file/print, email,
databases, etc.
Digital Signaling 0 is base unit for all greater sized digital lines ( DS0 ).
One DS 0 = 64 Kbps
T1=DS1, DS1 has 24 DS 0 x 64 Kbps = 1.544 MB. The size of T1 is 1.544 MB or 1544 Kbps.
T2=DS2, DS2 has 96 DS 0 x 64 Kbps= 6.312 MB. The size of T2 is 6.312 MB or 6312 Kbps.
T2 is 4 times bigger than T1.
21
T3=DS3, DS3 has 672 DS 0 x 64 Kbps = 44.736 MB. The size of T3 is 44.736 MB or 44736 Kbps.
T3 is 7 times bigger than T2 and 28 times bigger than T1.
T4=DS4, DS4 has 4032 DS 0 x 64 Kbps = 274.76 MB. The size of T4 is 274.76 MB or 27476 Kbps.
T4 is 6 times bigger than T3, 42 times bigger than T2 and 168 times bigger than T1.
Broadcast storm can result when the bridge sends the broadcast packets to each
segment on the network, and if this is the case router can solve the problem. Router
only pass on packets if they have a destination network address. By doing that, router
stops broadcast storms because broadcast packets have no destination network
address.
NONSWITCHED HUB:
When one port opens communication to another port in a nonswitched hub, all ports
“SENSE“ this communication because they shared the same medium. That medium is
unavailable for other ports at the moment that those two ports communicate.
SWITCHED HUB:
A switched hub fights this problem by turning on a switch between the two ports,
thereby connecting the two communication ports directly. The other ports do not
“SENSE” this communication and are free to contact other ports.
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