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Key Elements of Ethernet Networks:

CSMA/CD – Only used with half-duplex (hubs).
Full-duplex switches do not use CSMA/CD.
Ethernet signals are sent to every host in the LAN.

Carrier Sense:

All devices must listen before transmitting.
If signal detected (CS), wait before transmitting.
If no tfc detected, can transmit.
While transmitting – must listen for collisions.

Multi-access:

If 2 devices transmit at the same time = collision.

Collision Detection:

Other devices (not involved in collision), and transmitting devices, detect collision.

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Tags: study, vtp, 802.1q, guides, switch, cisco

Routing & Packet Forwarding

Routing Protocols & Concepts

Router as a Computer:

Computers that specialize in sending packets over the data network.
Responsible for interconnecting networks.
Routers are the network center.
Generally have a min of 2 connections:
WAN connection (Connection to ISP).
LAN connection.
In addition to packet forwarding, routers provide other services as well.
A router connects multiple networks.
This means that it has multiple interfaces that each belong to a different IP network.
Each network that a router connects to typically requires a separate interface.

Routing Table:

Used to determine the best path.
Incoming packets’ dest IP address is looked up in the table.
The table includes an exit interface or next hop.
Once a match is found, the router encapsulates the IP packet into the L2 frame of the exit interface.

Router as a computer:

Router components and their functions:
CPU – Executes OS instructions.
RAM – Contains the running copy of config, routing table, buffers, etc. Contents lost when pwr is off.
ROM – Holds diagnostic software used @ startup and the bootstrap prgm.
NVRAM – Stores startup config.
Flash – Contains the OS, non-volitile.
Interfaces – usually multiple physical interfaces. Examples of interface types:
Ethernet / Fast Ethernet interfaces.
Serial interfaces.
Management interfaces (concole).

Internetwork Operating System:

IOS
A multitasking OS integrated with routing, switching, internetworking, and telecomm functions.
Stored as an image. Many different IOS images available.
IOS interface is CLI
On boot, the startup-config file in NVRAM is copied into RAM and stored as the running-config file.
Changes entered by the admin are stored in the running-config and are immediately implemented by the IOS.

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Tags: switch, ip, 802.1q, guides, router, study

LAN Design:

The Hierarchical Network Model:

CCNA focuses on networks for SMBs (small, medium businesses).
A hierarchical design model is recommended.
Easier to manage and expand.
Problems are solved more quickly.

Hierarchical design divides the network into 3 layers.
Core, (CL).
Distribrution, (DL).
Access, (AL).

Each layer provides specific functions.
This “modularity” facilitates scalability and performance.

Access Layer, (AL): lowest

Interfaces with the end device (user).
Includes routers, switches, bridges, hubs and wireless APs.
Provides a means of connecting devices to the network and controlling which one communicate on the network.

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Tags: vlan, wan, ip, guides, vtp, switch

Interframe Spacing:

Minimum spacing between 2 frames following each other on the line.
Time for media to stabilize and devices to process frame.
From the last bit of FCS to the first bit of the Preamble.
96 bit times (9.6 microsec)– time shortens as speed increases.
As we speed up, gap gets shorter as the bits get shorter.
Allows time for slow hosts to process frames.

A = Start Frame Field.
B = Address Field.
C = Type/Length Field.
D = Data Field.
E = FCS Field.
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Tags: switch, ip, study, router, 802.1q, interface