CCNA learning: Layer 2 Exchange Tech

Part 1 Ethernet

Ethernet type:

1. point to point

2. broadcast

NBMA (noon broadcast multiple access)

Ethernet use CSMA/CD technology (Carrier Sense Multiple Access with Collision Detection)

Ethernet use IEEE802.3 standard --> Physical Layer and MAC sub layer

10BASE-5: 500M

10BASE-2: 200M

10BASE-T: 100M

10BASE-F: 2000M

Ethernet use IEEE802.2 standard --> LLC sub layer

Switcher Function:

1. Address Learning

2. Frame transfer and filter

3. Prevent loop

Address Learning Process

1. When start, the MAC address table of switcher is empty.MAX mac address table is different.

2. Switcher learn from source of message

3. Switcher will transfer frames by flooding method when the target is not in the MAC address table

Frame transfer and filter

After learning the address for a while, Switcher will send directly to target instead of flooring transfer.

Broadcast frame

When transfer broadcast frame, switcher will transfer to all ports except the source.

Prevent Loop

Fail-over structure

Advantage: prevent network accident due to one point problem

Disadvantage: broadcast storm, loop, depulated frame, MAC address table unstable

Solution: Spanning-Tree Protocol

Work method: Only keep the best path, temporary block the other path. Once the best path broke, recover the other path

Switcher default priority: 32768

Best path choose method: Switcher (priority + MAC address smallest) will be defined as root bridge. The ports of the switcher will be defined as root ports. Root ports will be set transfer port(no block). Other switcher will has one port as root port(cost lowest + priority + MAC address + port id smallest), other port of this switcher will be blocked.

Network        Cost

10M                 100                            

100M               19

1G                    4

10G                  2

Root bridge send out BPDU(bridge protocol data unit) every 2 seconds

Example:

1. According to MAC address, Switcher Z is root bridge;

2. According to MAC address, Switcher Y port 1 is blocked

Port status:

Blocking --> Listening --> Learning --> Forwarding

Bridge vs Switcher

1. Bridge is software, Switcher is hardware

2. Bridge only has 16 ports, Switcher has more ports

Switcher Frame transfer mode:

1. Cut-through: switcher transfer when switcher found the target

2. Fragment free: switcher check the first 64bytes before transfer (default) 

3. Store and forward: switcher check the whole message before transfer

Multi-layer switcher:

1. layer 2 switcher: bridge, make vlan, network seperate

2. layer 3 switcher: almost like router

3. layer 4 switcher: support TCP, UDP protocol

Deplux

HUB --> half deplux

Collision area --> half deplux

No collision area --> full deplux