Cisco® CCENT Exam Cram Notes : 7 Layers Of OSI Model

About CCENT certification:CCENT -Cisco®Certified Entry Networking Technician is an entry level Cisco certification. Getting CCENT certified ensures that the candidate has enough knowledge to plan, configure, and operate simple WAN and switched LAN networks using Cisco products. No pre-qualification is necessary to take CCENT exam.  640-822 is the only exam necessary to pass for obtaining CCENT certification. 640-822 is required for achieving CCNA, which covers medium size enterprise branch networks with more complex connections.

1)The 7 layers of OSI model are:

1. The Application Layer: Application layer is responsible for identifying and establishing the availability of intended communication partner and verifying sufficient resources exist for communication. Some of the important application layer protocols are: WWW, SMTP,FTP,etc.

2. The Presentation Layer: This layer is responsible for presenting the data in standard formats. This layer is responsible for data compression, decompression, encryption, and decryption. Some Presentation Layer standards are: JPEG, MPEG, MIDI, PICT, Quick Time, TIFF.

3.The Session Layer: Session Layer is responsible for co-ordinating communication between systems/nodes. The following are some of the session layer protocols and interfaces: a) Network File System (NFS),SQL,RPC(Remote Procedure Call), X-Windows, ASP, DNA SCP.

4. The Transport Layer: The Transport Layer is responsible for multiplexing upper-layer applications, session establishment, and tearing-down of virtual circuits. This layer does "flow control" to maintain data integrity. Flow Control prevents the problem of a sending host on one side of the connection overflowing the buffers in the receiving host.

5. The Network Layer: There can be several paths to send a packet from a given source to a destination. The primary responsibility of Network layer is to send packets from the source network to the destination network using a pre-determined methods. Routers work at Network layer.

6. The Data Link Layer: Data Link Layer is layer 2 of OSI reference model. This layer is divided into two sub-layers:

  • The LLC sub-layer handles error control, flow control, framing, and MAC sub-layer addressing.
  • The MAC sub-layer is the lower of the two sub-layers of the Data Link layer. MAC sub-layer handles access to shared media, such a Token passing or Ethernet.

7. Physical Layer: The actual flow of bits take place through Physical layer. At Physical layer, the interface between the DTE and DCE is determined. The following are some of the standard interfaces are defined at Physical layer: A> EIA/TIA-232, EIA/TIA-449,V.24,V.35,X.21,G.703,HSSI (High Speed Serial Interface).

2)By implementing VLANs,

1. The effective broadcast traffic decreases, since VLANs do not forward the broadcast traffic from one VLAN to another.

2. The security can be improved by implementing a router (A layer 3 device) to route the packets among VLANs.

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.

3)To verify vtp configuration on a Cisco switch, "show vtp"privileged executive command can be used. This command displays, among other things, VTP domain name, VTP password if any, VTP pruning mode (enabled or disabled) and the IP address of the device that last modified the configuration.

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

  • Modems: Modems connect to public telephone circuits through dial-up.
  • 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.
  • Multiplexors (mux): Multiplexors combine two or more signals before transmitting on a single channel. Multiplexing can be done by sharing "time" or "frequency".
  • CPE stands for Customer Premise Equipment.
  • Demarc : Demarcation point between carrier equipment and CPE.

4)"sh int <interface_no> " is a very usefull command. It displays the following information:

  • Hardware address
  • Status of interface and the line protocol - carrier detect brings up the serial port(which means that physical layer connections are working) and keep alive bring up the line protocol (which means that Data link layer protocol is working)..
  • MTU, BW, DLY, rely, and load metrics.
  • Encapsulation type (layer 2, Data link layer) - HDLC is the default.

5)If a properly configured workstation is unable to access the network information, it is likely that the DHCP information available with the client workstation had become obsolete. Issuing ipconfig command with /release and /renew commands will enable the client workstation to obtain latest changes to the DHCP server.

  • A straight-through cable is what you normally use to connect a PC to a hub or switch
  • A crossover cable is a special type of Ethernet cable used to connect a PC to a PC or a switch to a switch
  • A rolled cable isn't actually an Ethernet cable, but looks like one. It is used to connect a PC to a switch's console port, for management. This cable is normally used when configuring a Cisco switch or a router for the first time.

Application layer is responsible for ensuring the availability of intended communication partner. This layer is also responsible for synchronizing sending and receiving applications.

Presentation Layer is responsible for negotiating data transfer syntax.

Session layer is responsible for co-coordinating communication between systems

Transport layer is responsible for multiplexing upper-layer applications. It is also responsible for establishment and tear down of virtual circuits.

6)100BaseT (Fast Ethernet) uses IEEE803.2u standard which incorporates CSMA/CD protocol.

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).

11 bit subnet mask is equivalent to; It will accommodate 2,046 subnets, each with 30 hosts. The host address of is in the 32 subnet range. The valid hosts for subnet 32 are 33-62. The broadcast address is 63.

7)A 12-bit subnet mask gives us; 4094 subnets, each with 14 hosts. (Host addresses of all zeros and all 1s can't be assigned). The is in the 48 subnet range. The valid range is through 49 - 62. 63 is a broadcast address.

Here is how you get the subnet range:

1. Find the subnet mask. In this case, default subnet mask for Class B address is There are additional; 12 bits in the subnet mask. Now the subnet mask looks like:

11111111.11111111.11111111.11110000. This is equal to

2. Now, deduct the lowest value octet (Which is non zero), from 256. Here, (256-240)=16.

This is the value that the subnets are incremented. Therefore, you will have hosts with values from:. to (All 0s and all 1s host addresses can not be used) to to to and so on.
It is important to know that subnets are incrementing by a factor of 16. is the local loopback address defined by the Internet Protocol and used for by a host to ping itself to verify the IP connectivity.

8)802.3 - Layer 2 protocol that defines Ethernet CSMA/CD protocol

802.5 - Layer 2 protocol that defines Token Ring protocol

Note that 802.3, and 802.5 are examples of layer 2 LAN protocols, whereas, HDLC, PPP are examples of layer 2 WAN protocols.