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Cisco Certified Network Associate CCNA
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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.
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23.
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:
1. 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.
2. 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.
3. 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:
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.
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:
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DoD Model
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OSI Model
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Process/ Application
maps
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Application, Presentation, Session
layers (layers 7,6,5).
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Host-to-Host
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ISO's Transport layer (layer 4).
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Internet
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Network layer (layer 3)
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Network Access
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Data Link and Physical Layers (layers
6,7)
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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:
1.
Numbers 0 - 255 are used for public applications.
2.
Numbers 255 - 1023 are assigned to companies so that they can
use these port numbers in their applications.
3.
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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