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Exam notes for CCNA Exam

Cisco Certified Network Associate CCNA 2.0

K. Miscellaneous

i. Network Devices

1. Repeaters, Bridges, and Routers: 

The most frequently used network devices may be categorized as repeaters, hubs, switches, and routers. These devices let you connect computers, printers, and other devices to communicate with each other. The medium that is used for communication is usually cable (optical or copper) and air (Wifi, bluetooth, etc.).

A repeater is a basic device that simply amplifies the input signals and retransmits. It is used to extend the range of a network segment. 
For example, the range of a 10BaseT network segment is 100meters by default. If the end devices are at a distance more than 100 meters, you will require a repeater so that the transmitted signals are received at the destination device without losing any information.

A bridge/switch essentially forwards the frames that come from one port to other ports. A switch is used to connect two or more network segments. A switch learns the physical addresses of sending devices by reading the MAC address and mapping it to the port number through which the frame had arrived.
This way, it will quickly learn which MAC address belongs to which switch port, and stores the information in a table (called MAC table). Then onwards, it will send a frame only to the port that connects to the destination device (as specified in the frame). MAC addressesare layer-2 addresses. Because a switch works on MAC addresses, we can classify switches as Layer-2 devices.

A router is used to route packets by connecting two or more networks together.They work at layer-3 of the OSI model.They route packets based on the IP addresses where as a switch forwards packets based on the MAC addresses. A router needs to disseminate an incoming packet down to its IP address and route it to destination based on information available in its routing table.

I. Repeaters work at Physical layer (Layer 1),
II. Bridges and simple switches work at Data Link Layer (Layer 2),
III. Routers work at Network Layer (Layer 3) of ISO Reference Model.

2. CSU / DSU is an acronym for Channel Service Unit / Data Service Unit. CSU/DSU is part of Customer Premise Equipment (CPE). CSU / DSU connect to a Central Office (CO), a telephone switching company located nearer to the customer.

3. For using full duplex Ethernet transmission, a switch is required. A Hub cannot support full duplex transmission. In full duplex mode, there will not be any frame collisions.

ii. WAN Devices

1. WAN (Wide Area Network) devices extend the reach of LAN (Local Area Network) devices. WANs 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 provide serial WAN connections.
4. Communication Servers: These are used for dial in/out to remote users. Provides RAS Remote Access Server) functionality.
5. Multiplexers (mux): Multiplexers combine two or more signals before transmitting on a single channel. Multiplexing can be done by sharing "time" or "frequency".

iii. Wireless LAN

1. WEP uses RC4 stream encryption
WPA uses (as describe above) TKIP/MIC Encryption.
WPA2 uses AES-CCMP Encryption
2. In "ad-hoc" or Independent Basic Service Set (IBSS) configuration there is no backbone infrastructure. Mobiles can talk to each other without the use of an Access Point (AP). In the Extended Service Set (ESS) configuration, there will be two or more Access Points (APs), and users can freely roam between the Access Points without any disconnection or reconfiguration.

iv. Others

1. HTTP is the protocol used for accessing the World Wide Web services. HTTP operates over TCP/IP. TCP/IP is the protocol, which is used by all internet applications such as WWW, FTP, Telnet etc. IPX/SPX is proprietary protocol stack of Novell NetWare.

2. Route summarization is calculated as below:

Step 1: 
1. Take the first IP: 172.24.54.0/24 : 172.24. 0 0 1 1 0 1 1 0.0
2. Take the second IP: 172.24.53.0/24 : 172.24. 0 0 1 1 0 1 0 1.0
Note that we are not really concerned about the octets that have equal decimal values. This is because they donít come into play while calculating summarization route, in this case.

Step 2: 
Count the number of bits in the third octet that are aligned (or lined up) with same values. In this case 6 bits are lined up in the third octet. The summarization route is calculated by adding this number (6) to the octets preceding the third (first and second octets). 
Therefore, the number of bits in the summarized route is 8+8+6 = 22

Step 3:
Calculate the decimal equivalent for third octet with 6 bits as given in the matching binary. That is 0 0 1 1 0 1 x x. Note x is because it corresponds to non matching binary number. It is equal to 128*0 + 64*0 + 32*1 + 16*1 + 8*0 + 4*1 or 32+16+4 or 52.

Therefore, the summarized route is:
172.24.52.0/22

3. Debugging output takes priority over other network traffic. Also, the debug all command generates more output than any other debug command, and it can severely affect the router's performance. In practically all cases, it is best to use more specific debug commands.

4. Ethernet II has a type field to identify the upper-layer protocol. 802.3 has only a length field and can't identify the upper-layer protocol.

5. Hold down timers prevents regular update messages from reinstating a route that has gone bad. Here, if a route fails, the router waits a certain amount of time before accepting any other routing information about that route. Hold downs tell routers to hold any changes that might affect routes for some period of time. The holddown period is usually calculated to be just greater than the period of time necessary to update the entire network with a routing change.

6. Congestion avoidance, Windowing, and Buffering are three types of flow control.

7. Convergence is the term used to describe the state at which all the internetworking devices, running specific routing protocol, are having the same information about the internetwork in their routing tables. The time it takes to arrive at common view of the internetwork is called Convergence Time. 

8. IP helper addresses forward a client broadcast address (such as a DHCP or BOOTP requests) to a unicast or directed broadcast address. Helper-address is required due to the fact that routers do not forward broadcasts. By defining a helper-address, a router will be able to forward a broadcast from a client to the desired server or network. There can be more than one helper-address on a network. The helper-address must to be defined on the interface that receives the original client broadcast.
Note that "ip unnumbered" command is used to enable IP processing on a serial interface without assigning a specific IP address to the interface.

9. Runts are packets that are smaller than the medium's minimum packet size. For example, Ethernet has a minimum allowed packet size of 64 bytes. Any packet that is less than 64 bytes in size is considered a runt in Ethernet.
Giants are packets that bigger than the medium's maximum packet size. Fro example, Ethernet has a maximum allowed packet size of 1,518 bytes. Any packet that is bigger than 1,518 bytes is considered a Giant in Ethernet.
CRC error occurs when the check sum calculated at the receiving end of the frame does not match with the check sum calculated at the source end. 
The most probable reasons for runts, giants, and CRC errors is frame collisions while traveling from source to destination. It is also possible that a network card or device is bad and generating runts and giants.

10. Standard adopted for Ethernet CSMA/CD by IEEE Committee is 802.3. 100BaseT (Fast Ethernet) uses IEEE803.2u standard which incorporates CSMA/CD protocol.

11. DOD Model maps to OSI model as below:

DoD Model

OSI Model

Process/ Application maps

Application, Presentation, Session layers (layers 7, 6, 5).

Host-to-Host

ISO's Transport layer (layer 4).

Internet

Network layer (layer 3)

Network Access

Data Link and Physical Layers (layers 6,7)

12. While a packet travels through an Internetwork, it usually involves multiple hops. Note that the logical address (IP address) of source (that created the packet) and destination (final intended destination) remain constant, the hardware (Interface)                                                                                            

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