CCENT (Cisco Certified Entry Network Technician) Certification Exam Notes

 K.Miscellaneous

i. Cables

1. 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.

2. For connecting a switch to another switch, a crossover cable is required. For connecting a host to a switch, a straight-through cable is used. For connecting a Cisco Router/Cisco Switch console port, a rollover cable is required.

3. 100BaseT (Fast Ethernet) uses IEEE803.2u standard which incorporates CSMA/CD protocol.

4. 10Base2, and 10Base5 use coaxial cable for transmission of digital signals. Therefore, it can’t support simultaneous transmission and reception. On the other hand, 10BaseT and 100BaseT use RJ-45 connectors and Cat 3/4/5 cables. These cables have multiple pairs of wires, and hence support full duplex transmission (Note that the other parts of the network should also support full duplex, like the network card).

5. In normal course, a DTE device is expected to communicated with a DCE device. In such event, the connections between the DCE and DTE devices is straight. However, if the communication need to happen between two DTEs or two DCEs, the signals need to be rolled. This is achieved by rolling the pins in the cable or in the DB-25 adapter. Typically, a DTE to DTE communication can be achieved by:

1. By connecting DTE to a rolled RJ-45 cable to a straight DB-25 adapter to DTE, OR
2. By connecting a straight RJ-45 cable to a rolled DB-25 adapter to DTE.
Similarly, a DTE to DCE communication can be achieved by:
1. Connecting a DTE to a straight RJ-45 cable to a straight DB-25 adapter to DCE, OR
2. Connecting a rolled RJ-45 cable to a rolled DB-25 adapter to DCE.
Note that rolling the signals twice is same as using straight through cables or adapters without any rolling at all.

6. The maximum allowed cable lengths are as given below:
1. 10Base5: 500 meters
2. 10Base2: 185 meters
3. 10BaseT: 100 meters
4. 100BaseT: 100 meters
5. 1000BaseT: 100 meters
7. 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.

ii. ARP and RARP

1. ARP (Address Resolution Protocol): IP works at network layer. IP address is a logical address. If a packet is to be delivered to a destination machine, its physical address (MAC address) needs to be known. ARP is a protocol, which enables a machine to obtain its MAC address from a known IP address.

2. RARP (Reverse Address Resolution Protocol): RARP is used to obtain IP address from a known MAC address.

iii. Static and Default Routing

1. 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>.

2. ISP router is normally configured with static route.

3. A static route is one that is hard coded into the routing tables. Here, the destination network/host, and the next hop information are entered by hand. A static route on a Cisco router is most commonly defined by using the command:
Router(config)# ip route A.B.C.D (destination network/host) A.B.C.D (subnet mask) A.B.C.D (next hop)
You can also use the port identifier such as e0, s1 etc. to define the next hop address.
Optionally, the "distance metric" can be added at the end of the command to change the default weight.

4. The command to configure a static route to network 192.204.1.64/28 from assuming that the default gateway is 192.204.1.2 is:
· ip route 192.204.1.64 255.255.255.240 192.204.1.2
Syntax: ip route <remote_network> <mask> <default_gateway> [administrative_distance]

5. Default route is used when there is no other known route to a given IP packet’s destination. Default route is also known as gateway of last resort.

6. The correct syntax for setting default route is:
· Router(config)#ip route 0.0.0.0 0.0.0.0 192.157.1.1
You can also set default route by specifying the interface as below:
· Router(config)#ip route 0.0.0.0 0.0.0.0 s0
The syntax is:
· ip route 0.0.0.0 0.0.0.0 <next_hop_address>

iv. WAN

1. 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".

2. WANS are connected over serial lines and typically operate at lower speeds than LANs, and more expensive. Some of the WAN terms are:

1. Modems: Modems connect to public telephone circuits through dial-up.
2. CPE stands for Customer Premise Equipment.
3. Demarc: Demarcation point between carrier equipment and CPE.

3. 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.

4. You need to assign a different network number for each subnet. Also, you need to set aside one network number for each WAN connection.

v. Basic Utilities (ping, traceroute)

1. 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.

2. 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. 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.

3. ICMP (Internet Message Control Protocol) messages are used for basic error reporting between host to host, or host to gateway. It is not used for error reporting between Gateways. ICMP messages are encapsulated using the IP protocol. For example, the command “ping” uses ICMP protocol. In the OSI Reference model, ICMPs are generally considered part of the IP layer.
ICMP (Internet Control Message Protocol) sends an ICMP message if the destination is unreachable. Routers can make alternate route decisions based on ICMP messages, if appropriate.

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