CCNA
WHAT
I'm trying to study to get my CCNA (640-802) degree :D I'm using my wiki to note what I'm trying to learn ! Much of the stuff here are from SYBEX CCNA Study guide (6th edition). Hope it'll help other people.
Software
I'm using GNS3 to emulate a Cisco environment. Get it from GNS3 Homepage
Internetworking Devices
Switches
They work at Layer 2
They break up "collision domains" only.
Routers (Layer 3 switches)
They work at Layer 3
Cisco calls them multiport bridges.
Advantages:
- They break up "broadcast domains" by default ( 1 broadcast domain per interface ).
- They break up "collision domains".
- Can filter network based on Layer 3 ( Network Layer ) information ( e.g., IP Address )
Useful at:
- Packet switching
- Packet filtering
- Internetwork communication
- Path selection
Bridges
They break up collision domains.
Hubs
They keep all hosts in the same collision domain.
They often cause traffic congestion if on a busy network.
Traffic Congestion
Can be caused by:
- Too many hosts in a broadcast domain
- Broadcast storms
- Multicasting
TEST
Test 1
Do you _SEE_ the "collision domains" and "broadcast domains" here :
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9 collision domains
- All hub network - 1 collision domain
- The bridge network - 3 collision domains
- The switch network - 5 collision domains
3 broadcast domains
- Only the router breaks up broadcast domains ! It has 3 connections.
Test 2
Do you _SEE_ the "collision domains" here:
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10 "collision domains"
OSI Layered Reference Model
L1-L4 Define how data is transmitted end to end L5-L7 Define how applications within the end stations will communicate with each other and with users.
Physical Layer (L1)
Function: Physical topology
- Movies bits between devices
- Specifies voltage, wire speed and pin-out of cables
Data Link Layer (L2)
Function: Framing
- Combines packets into bytes and bytes into frames
- Provides access to media using MAC address
- Performs error detection not correction
Network Layer (L3)
Function: Routing
- Provides logical addressing, which routers use for path determination
Transport Layer (L4)
Function: End-to-end connection
- Provides reliable or unreliable delivery
- Performs error correction before retransmit
The Transport layer segments and reassembles data into a data stream.
UDP and TCP protocols work at this layer. UDP is a unreliable service. Reliable means that acknowledgments, sequencing and flow control will be used.
The Transport layer is responsible for providing mechanisms for multiplexing upper-layer applications, establishing sessions and tearing down virtual circuits.
It hides details of any network-dependent information from the higher layers by providing transparent data transfer.
Can be connectionless or connection-oriented.
Flow Control
Ensures Data integrity.
Prevents a sending host on one side of the connection from overflowing the buffers on the receiving host.
The purpose is to provides the means for the receiver to govern the amount of data sent by the sender.
Connection-Oriented Communication
In reliable transport operation, a device that wants to transmit sets up a connection-oriented communication with a remote device by creating a session. This is the only concerned with the connection-oriented portion of the Transport layer.
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- The first "connection agreement" segment is a request for synchronization.
- The second and third segments acknowledge the request and establish connection parameters—the rules—between hosts. These segments request that the receiver's sequencing is synchronized here as well so that a bidirectional connection is formed.
- The final segment is also an acknowledgment. It notifies the destination host that the connection agreement has been accepted and that the actual connection has been established. Data transfer can now begin.
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A service is considered connection-oriented if it has the following characteristics:
- A virtual circuit is set up ( e.g., a three-way handshake )
- It uses sequencing.
- It uses acknowledgements.
- It uses flow control.
The types of flow control are buffering, windowing and congestion avoidance.
Windowing
The quantity of data segments ( measured in bytes) that the transmitting machine is allowed to send without receiving an acknowledgement for them is called a window
Windows are used to control the amount of outstanding, unacknowledged data segments.
If a receiving host fails to receive all the segments that it should acknowledge, the host can improve the communication session by decreasing the window size
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Acknowledgements
Positive acknowledgement with retransmission ensures the integrity of a stream of data, it ensures the data won't be duplicated or lost.
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Session Layer (L5)
Function: Dialog control
- Keeps different applications' data separate
The Session Layer is responsible for setting up, managing and then tearing down sessions between Presentation layer entities.
It serves to organize the communication between systems in different modes: simplex, half duplex and full duplex
The Session Layer basically keeps different applications' data separate from other applications' data.
Presentation Layer (L6)
Function: Data encryption, compression and translation services
- Presents data
- Handles processing such as encryption
The Presentation layer presents data to the Application layer and is responsible for data translation and code formating.
Application Layer (L7)
Function: File, print, message, database and application services
- Provides a user interface
Applications residing in the Application Layer: FTP, TFTP
The Application layer is acting as an interface between the actual application programs
