54)The syntax for ping command is : "ping <destination_address>"
The following are possible responses to "ping" command:
Ping Explanation
Response
U destination Unreachable
C Congestion
experienced
! Successful receipt of echo reply
. Time out
? Packet type unknown
& Packet time to live exceeded
Ping
sends ECMP echo. Ping can be used with almost any type of Network
layer protocols including IPX, IP, VINES, AppleTalk etc.
55)The system LED shows whether the system is receiving
power and functioning properly.
LED Off: System is not powered
up.
LED color is Green: System is operating normally.
LED color is Amber: System is receiving power but is not functioning
properly. The amber color on a Cisco2950 series switch indicates
that the system is malfunctioning.
The user data is first segmented, converted to packets (with logical address), packed in a frame (with MAC address), and then transmitted over the media as a sequence of bits. Note that the media can be cable, wireless, or optic.
56)The cable determines the DTE/DCE combination. The router with DCE cable needs to be configured with clock rate for proper functioning of the WAN link.
There are different types of cables:
1. Straight through cable:
This is also known as patch cable. A straight through cable is used
to connect a computer to a hub or a switch.
2. Crossover cable:
A crossover cable is used to connect one computer to the other computer
B without using a switch or hub.
3. Rollover cable: A rollover
(also known as console cable) is used to connect to routers for
configuration purpose.
Also, you can't connect the Ethernet port
on PC to another Ethernet port on a router using a straight through
cable. You need crossover cable for this purpose. Also, the PC need
to have a network card, and appropriate networking protocol stack
properly installed and configured.
57)There are three ways a router learns how to forward
a packet:
1. Static Routes - Configured by the administrator
manually. The administrator must also update the table manually
every time a change to the network takes place. Static routes are
commonly used when routing from a network to a stub (a network with
a single route) network.
The command is
ip route network mask
address/interface [distance]
ex: ip route 165.44.34.0 255.255.255.0
165.44.56.5
Here, 165.44.34.0 is the destination network or subnet
255.255.255.0 is the subnet mask
165.44.56.5 is the default gateway.
2. Default Routes - The default route (gateway of last resort)
is used when a route is not known or is infeasible. The command
is
ip route 0.0.0.0 0.0.0.0 165.44.56.5
The default gateway
is set to 165.44.56.5
3. Dynamic Routes - As soon as dynamic
routing is enabled, the routing tables are automatically updated.
Dynamic routing uses broadcasts and multicasts to communicate with
other routers. Each route entry includes a subnet number, the interface
out to that subnet, and the IP address of the next router that should
receive the packet. The commands to enable rip are:
router rip
network <major network number>.
58)There are broadly three types of routing protocols:
1. Distance Vector (Number of hops) - Distance vector routing
determines the direction (vector) and distance to any link in the
internetwork. Typically, the smaller the metric, the better the
path. EX: Examples of distance vector protocols are RIP and IGRP.
Distance vector routing is useful for smaller networks. The limitation
is that any route which is greater than 15 hops is considered unreachable.
One important thing that differentiates distance vector with Link
state is that distance vector listens to second hand information
to learn routing tables whereas, Link state builds its routing tables
from first hand information. Distance vector algorithms call for
each router to send its entire routing table to each of its adjacent
neighbors.
2. Link State Routing: Link State algorithms are
also known as Shortest Path First (SPF) algorithms. SPF recreates
the exact topology of the entire network for route computation by
listening at the first hand information. Link State takes bandwidth
into account using a cost metric. Link State protocols only send
updates when a change occurs, which makes them more attractive for
larger networks. Bandwidth and delay are the most heavily weighed
parts of the metric when using Link-State protocols. EX: OSPF and
NLSP.
Benefits of Link State protocols:
Allows for
a larger scalable network
Reduces convergence time
Allows
"supernetting"
3. Balanced Hybrid - Balanced Hybrid combines
some aspects of Link State and Distance Vector routing protocols.
Balanced Hybrid uses distance vectors with more accurate metrics
to determine the best paths to destination networks. EX: EIGRP.
59)There may be times that you have lost the passwords
for accessing the router and need to recover from the lost passwords.
ROM Monitor (rommon), stored ROM is used for password recovery.
By hitting special Break key within the first 60 seconds of the
router power-on, it is possible to recover from lost passwords.
Broadly, the sequence of operations is as below (For a 2501 router):
a. Power cycle the router by turning it OFF and ON.
b. Press
Break key (key configuration depends on the router type) within
60 seconds of power on.
C. Use the rommon command O/R 0X2142
d. Use rommon command INITIALIZE to load IOS
e. Skip the setup
mode (by choosing NO to configuration commands)
f. Enter privileged
mode
g. After entering the privileged mode, you can view the
passwords by issuing appropriate config commands.
To change the enable password, you use "enable password
password".
To change the enable secret, you use "enable
secret password".
To configure your router to do look-up for domain name server,
the correct syntax is:
ip name-server <ipaddress_of_server>
Here the ipaddress_of_server is the ip address of the DNS server.
To enable the Cisco IOS to forward packets destined for obscure subnets of directly connected networks onto the best route, you use "ip classless" command.
60)Trace command has syntax, "trace ip <destination_address> ".
The trace command traces the path to the destination hop by hop.
This is achieved by setting the TTL (Time To Live) to 1 for the
first packet to send and increasing TTL in the intervals of 1. This
causes the successive next hop devices to send a time out message.
This message is used for finding out the route that the packet takes
to reach the destination.
Important Trace responses are:
Trace
Explanation
Response
P ---- The Protocol was Unreachable
N ---- The Network was Unreachable
U ---- The port was Unreachable
* ---- The packet timed out
!H ---- Could not be forwarded due
to access list violation
Tracert, Ping use ICMP as their base protocol.
Traditionally, the router interface on a serial link is a 60-pin D-shell connector. The CSU/DSU may have EIA/TIA 232 interface to the router. The CSEU/DSU interface to the service provider could be RJ-11, RJ-45 etc
Transport Layer is responsible for multiplexing the packets received from upper layers. It also makes the network transparent to the upper layers (Session, Presentation, and Application layers)..
61)Typical WAN interfaces, that one come across frequently
are:
EIA 232, EIA 449, EIA 530, V.35, and X.21. The Cisco router
serial interface will be a 60-pin D-shell connector. This in turn
is connected to service provider equipment, usually through a CSU/
DSU. Therefore, appropriate conversion cable need to be used when
interfacing with a WAN device such a CSU/DSU. CSU/ DSU in turn is
connected to the service provider through the cable supplied by
the service provider.
Typically, a Cisco router has the following types of memory components:
1. RAM (or DRAM): This type of memory is used for working storage.
This is where part of IOS image, routing tables, packets, and others
are stored during normal working of a router. RAM is erased when
the router is powered off.
2. ROM (Read Only Memory): ROM stores
bootstrap code for booting Cisco IOS. It points to the functional
IOS image so that the router can complete its start-up configuration
successfully. It is somewhat analogous the BIOS in PC. ROM also
contains a small portion of IOS.
3. Flash memory: Flash memory
contains the fully functional IOS images. It is the default location
for getting IOS image at boot time. You can loosely compare the
IOS image with operating system (say DOS).
4. NVRAM: NVRAM stands
for Non Volatile RAM. NVRAM stores the initial or startup configuration
file.
Typically, a Cisco router has the following types of memory components:
1. RAM (or DRAM): This type of memory is used for working storage.
This is where part of IOS image, routing tables, packets, and others
are stored during normal working of a router. RAM is erased when
the router is powered off.
2. ROM (Read Only Memory): ROM stores
bootstrap code for booting Cisco IOS. It points to the functional
IOS image so that the router can complete its start-up configuration
successfully. It is somewhat analogous the BIOS in PC. ROM also
contains a small portion of IOS.
3. Flash memory: Flash memory
contains the fully functional IOS images. It is the default location
for getting IOS image at boot time. You can loosely compare the
IOS image with operating system (say DOS).
4. NVRAM: NVRAM stands
for Non Volatile RAM. NVRAM stores the initial or startup configuration
file.
62)UDP is known as unreliable protocol because there is no way for the source to know if any packets are lost during transit to the destination. Important fields that are available with TCP header, but not available with UDP header include sequence number, acknowledgement number, and window size.
Variable-Length Subnet Masking:
Variable-length subnet masking
(VLSM) means implementing more than one mask on the same major class
of a network. It allows for a more efficient use of IP address space
both in terms of hosts and subnets, and can be essential on a network
that does not have an abundance of IP address space.
The following
routing protocols support VLSM:
1. RIP v2
2. EIGRP
3. IS-IS
4. OSPF
5. BGP
VLAN port assignments can be configured either of two ways:
Static VLANs: The administrator statically configures VLAN port
assignment. VLAN memberships on the switch ports are assigned on
a port-by-port basis.
Dynamic VLANs: A VMPS (VLAN Management
Policy Server) can dynamically assign VLAN ports. The MAC address
of the node is used to determine the VLAN assignment. A separate
server or a Catalyst 5000 can function as a VMPS server. When a
frame arrives on a dynamic port at the switch, it queries the VMPS
for the VLAN assignment based on the source MAC address of the arriving
frame.
63)WAN (Wide Area Network) devices extend the reach of
LAN (Local Area Network) devices. WANT typically span over a wide
area, such over multiple cities / countries. WANS are connected
over serial lines that operate at lower speeds than LANs. Some of
the WAN devices are:
1. Routers: Routers are responsible for
routing the packets in an internetwork.
2. Modems: Modems connect
to public telephone circuits through dial-up.
3. CSU/DSU: Stands
for Channel Service Unit / Data Service Unit. CSU/DSUs are used
for connecting to Central Office of a Telephone switching company
and provides serial WAN connections.
4.Communication Servers:
These are used for dial in/out to remote users. Provides RAS Remote
Access Server) functionality.
5. Multiplexors (mux): Multiplexors
combine two or more signals before transmitting on a single channel.
Multiplexing can be done by sharing "time" or "frequency".
When designing an ESS WLAN, APs with overlapping coverage areas should be configured to use non-overlapping channels. This is true whether the APs are using DSSS (Direct Sequence Spread Spectrum) or FHSS (Frequency Hopping Spread Spectrum) technology.
While a packet travels through an Internetwork, it usually involves multiple hops. It is important to know that the logical address (IP address) of source (that created the packet) and destination (final intended destination) remain constant, the hardware (Interface) addresses change with each hop.
Windowing, Buffering, and Congestion avoidance are three different types of flow controls used frequently.
64)Wired Equivalent Privacy (WEP) is a protocol used to
secure IEEE 802.11 wireless networks. It is part of the IEEE 802.11
wireless networking standard. Wireless networks broadcast messages
using radio, and therefore more susceptible to eavesdropping than
wired networks. WEP was intended to provide confidentiality comparable
to that of a traditional wired network.
Wi-Fi Protected Access
(WPA and WPA2) is a class of systems to secure wireless (Wi-Fi)
computer networks. WPA is forward compatible with the IEEE 802.11i
standard, and was intended as an intermediate measure to take the
place of WEP while 802.11i was prepared.
802.11i includes dynamic
key exchange, stronger encryption, and user authentication. It is
not backward compatible with WPA. The 802.11i standard is widely
known as WPA2.
WPA stands for Wi-Fi Protected Access. The key features of the
protocol are given below:
1. It supports both static and dynamic
key distribution
2. It provides Device Authentication, as well
as User Authentication.
3. It uses TKIP (Temporal Key Integrity
Protocol) encryption for dynamic key exchange. Note that WPA2 uses
AES encryption where as WPA uses TKIP. AES encryption is a stronger
encryption protocol.
4. WPA is forward compatible with WPA2.
65)You can use "show controllers serial [port number]" to see if the interface detects a DCE or DTE cable.
You need to assign a different network number for each subnet. Also, you need to set aside one network number for each WAN connection.
You need to enter global configuration mode to change the name
of the router.
The three different modes are:
Mode Prompt
1. User EXEC mode ... Router>
2. Privileged EXEC mode ... Router#
3. Global Configuration mode ... Router(Config)#
Global configuration
mode allows you to perform tasks that affect the entire router,
such as naming the router, configuration of banner messages, enabling
routed protocols, and generally anything that affects the operation
of the entire router.
When you first switch on a router, you
enter Setup mode. Setup mode is different from configuration mode
in that setup mode appears when there is no configuration file present.
Upon entering setup mode, you can supply some basic configuration
parameters to Cisco router.
You use show vlan or shor vlan vlan# command to see the configuration details of VLANs. The command "sh vlan" will display the configuration information for all VLANs, where as the command "sh vlan vlan#" shows only the configuration information pertaining to that vlan. For example, if you want to see the configuration information for vlan2, you give the command "sh vlan 2".