Ethernet Basics Flashcards
What committee defines wired network standards?
802.3
preamble of an ethernet frame
7 byte series of alternating ones and zeroes, followed by a 1 byte start frame delimiter
- it always precedes a frame
- give receiving NIC time to realzie a frame is coming and exactly when it starts
- added by sending NIC
type field of ethernet frame
tells the receiving NIC what type of data the frame contains
pad of ethernet frame
Minimum ethernet frame is 64 bytes
pad brings up the data to 64 bytes if it is smaller
what does each part of “10BaseT” mean
10 - 10 Mbps
Base - baseband
T - Twisted Pair
What is a 10BaseT network?
two or more computers connected to a central hub
Which wires trasmit on 10BaseT?
1 and 2 to send data
3 and 4 to receive data
Half duplex & Full duplex
Half duplex - NIC can only send or recive, not both
Full Duplex - NIC can send and recieve at same time
Memorize Wire Standards 568a/568b
1 - Green/White 1 - Orange/White
2 - Green 2 - Orange
3 - Orange/White 3 - Green/White
4 - Blue 4 - Blue
5 - Blue/White 5 - Blue/White
6 - Orange 6 - Green
7 - Brown/White 7 - Brown/White
8 - Brown 8 - Brown
10BaseT summary
Speed 10mbps signal type baseband distance 100m node limit 1024 topology star bus (physcal star, logical bus) Cat 3 or better
10BaseFL summary
speed 10mbps
signal type baseband
distance 2000 m
node limit 1024
topology star bus (physcal star, logical bus)
used multimode fiber and SC or ST connector
What is CSMA/CD
Carrier Sense Multiple Access / Collision Detection
- carrier sense means each node examines the cable before sending data
- if traffic is detected, it waits a few milliseconds then rechecks
- maps to IEEE 802.3 standard
- multiple access means all machines have equal access to the wire
What is a bridge?
- acts like a repeater to connect 2 networks
- filters and forwards based on MAC addresses
What is a collision domain?
a group of nodes that have the capability of sending frames at the same time as each other
How do switches handle collisions?
- each port is on its own colision domain
- two nodes on a switch can send data at the same time without collisions
How do switches send broadcast messages?
will broadcast to all ports on the switch except the originating port
How do you connect switches?
uplink port or crossover cables
How do uplink ports work?
- enable you to connect two switches using a straight-through cable
- modern switches dont have a dedicated uplink port, but instead auto-sense when another switch is plugged in
What kind of techniques are STP, BPDU and root guard?
Mitigation techniques - making bad things not as destructive
What is STP?
Spanning Tree Protocol
- uses BDPU that eliminates accidental bridging loops
- establishes topology and elects one switch to be the root bridge
- each switch uses root bridge as a reference point
- puts ports in blocking state to stop redundant connections
- blocked ports will hear configuration BPDUs sent by the root bridge every 2 seconds
- if link goes down, STP sends a TCN that enables switches to rework themselves around the failed device
What is a BPDU?
- Bridge Protocol Data Unit
- used by switches
What is PortFast?
- enables interface to come up right away, without the normal latency introduced by STP
- prevents TCN BPDUs being sent out of that switch every time a PC is powered on or off (which causes all switches to flush their source address table and relearn MAC addresses)
- these ports should never receive a BPDU or it could start a bridging loop
What is TCN?
Topology Change Notification
- a BDPU that causes the network to rework itself around a broken link
What does BPDU guard do?
move a port configured with PortFast into an errdisable state (error occurred, disabled) if a BPDU is received on that port
- requires an admin to bring the port back up
What is root guard?
will move a port into a root-inconsistent state if BPDUs coming from a certain direction indicate another switch is trying to become the root bridge
- will automatically return to its forwarding state once these BPDUs stop
- helps define locations where the root bridge should never be located
What is RSTP?
Rapid Spanning Tree Protocol
- significantly faster convergence time following a network change
Categories of switch problems
- obvious physical damage
- dead ports
Steps to troubleshooting a switch
1 - Recognize the problem (no network on device)
2 - Examine switch for obvious damage
3 - Look for link lights
4 - Look at cables
How does a switch direct packets?
By MAC address
MTU
Maximum Transmission Unit
