03 - Ethernet Basics Flashcards
Preamble
- 7 byte series of alternating ones and zeros, followed by a 1 byte start frame delimiter
- gives receiving NIC time to realize a frame is coming and when it starts
- added by sending NIC
MAC Address Length
48 bit or 6 byte
Frame Pad
Frame is 64 bytes, but if not full, NIC will add data to bring it to 64
What do parts of 10BaseT mean
10 - speed - 10 Mbps
Base - signaling type - Baseband or Broadband
T - type of cabling - twisted pair
Half-duplex vs Full-duplex
Half - can send or receive at one time
Full - can send or receive at same time
Different between 568A and 568B
Remember “GO”
- green/orange pairs are swapped
- blue/brown stay in the same place
10BaseT Summary
- 10 Mbps
- Baseband
- 100 meter length
- 1024 node limit
- star-bus topology
- cat-3 or better with RJ-45 connections
10BaseFL Summary
- 10 Mbps
- Baseband
- 2 km length
- 1024 node limit
- star-bus topology
- MMF with SC or ST connectors
Media convertor
Used to connect different Ethernet types
CSMA/CD
Carrier Sense Multiple Access / Collision Detection
- determines with computer should use a shared cable at a given moment
- Carrier sense - examines cable before sending data frame, if network is used, waits a few seconds and tries again
- Multiple Access - all machines have equal access to the wire
- when a collision occurs, both machines generate a random number to determine how long to wait before trying again
Collision Domain
group of nodes that have the capability of sending frames at the same time
Bridge
- acts as a repeater to connect two networks
- filters and forwards traffic based on MAC addresses
How do switches handle collisions
- each port is on its own collision domain
- the switch can send signals at the same time without collisions
Broadcast Domain
a network with a switch
How do you connect switches
- uplink port - uses straight-through cable
- crossover cable
- modern switches have no uplink port, switch autodetects when another switch is plugged in. any port can be used
Crossover Cable
One end is 568A and the other is 568B
Spanning Tree Protocol
- connecting switches can cause redundant connections called bridging loops or switching loops
- uses special STP BDUs called Bridge Protocol Data Units (BPDU) to communicate with other switches to prevent loops
- one switch elected as root bridge
- configuration BPDU’s establish the topology
- certain ports placed in “blocking” state and only hear BPDUs, fowarded by root bridge every 2 seconds
Mitigation Techniques
Making bad things not as destructive
What happens when a switch with STP goes down?
TCN (Topology Change Notification) BPDU’s enable the switches to rework themselves around the failed switch
When to use STP
When a port on a switch is connected to another switch, not when the port is connected to a PC`
PortFast
- for ports that have PCs connected
- allows interface to come up right away without the latency from STP
- also prevents TCNs from being sent out everytime a computer is powered on or off
Root Gaurd
- will move a port into a root-inconsistent state if BPDUs coming from a certain direction indicate that another switch is trying to become the root bridge
- will automatically return to its forwarding state once the BPDUs stop
- helps determine where the root bridge should never be
Troubleshoot a switch
1 - device cant connect to network
2 - examine switch for obvious damage
3 - examine link lights, if not on try a different port
4 - check cables
