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Cisco® CCNA Exam Cram Notes : Stp, Switching Methods, Port Numbers, Wan Devices, And Dod To Osi Mapping.

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21. Spanning Tree Protocol (STP) IEEE Specification 802.1d is used to prevent routing loops. In Cisco Catalyst 5000 series switches, use BDPUs (Bridge Protocol Data Units) to determine the spanning tree topology. STP uses a Tree Algorithm (STA) to prevent loops, resulting in a stable network topology.

22. 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.


1.The term "Segments" is usually associated with Transport layer

2. The term "Packets" is usually associated with Network Layer and

3. The term "Frames" is usually associated with Data Link Layer

24. The port number used by TCP is 6 and that of UDP is 17.

TCP: TCP is a full-duplex, connection-oriented protocol. It incorporates error checking as well.

UDP (User Datagram Protocol): UDP is a thin protocol. UDP is a connectionless protocol. It doesn't contact the destination before sending the packet and doesn't care whether the packet is reached at the destination. UDP uses port number 6.

25. Switching methods:

  • Store-and-Forward switching: Here the LAN switch copies the entire frame into its buffers and computes the CRC. The frame is discarded if there are any CRC errors. Giant ( more than 1518 bytes0 and Runt (less than 64 bytes) frames are also dropped, if found.
  • Cut-Through (Real-Time) switching: Here, the LAN switch copies only the destination address into its buffers. It immediately looks up the switching table and starts forwarding the frame. The latency is very less because, the frame is forwarded as soon as the destination address is resolved.
  • Fragment-Free switching: Here, the switch waits for the collision window before forwarding the entire frame. The collision window is 64 bytes long.

26. 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:

  • Routers: Routers are responsible for routing the packets in an internetwork.
  • 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.
  • Communication Servers: These are used for dial in/out to remote users. Provides RAS Remote Access Server) functionality.
  • Multiplexers (mux): Multiplexers combine two or more signals before transmitting on a single channel. Multiplexing can be done by sharing "time" or "frequency".

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

28. DOD Model maps to OSI model as below:

DoD Model OSI Model
Process/ Application map 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)

29. Routers can make alternate route decisions based on ICMP messages, if appropriate. Routers send an ICMP message if the destination is unreachable.

30. Port numbers are used by TCP/ UDP to set up sessions with other hosts. The following are the recommended port numbers:

  • Numbers 0 - 255 are used for public applications.
  • Numbers 255 - 1023 are assigned to companies so that they can use these port numbers in their applications.
  • Numbers above 1023 are used by upper layers to set up sessions with other hosts and by TCP to use as source and destination addresses.

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