Chapter 3 - Ethernet Basics

Question 1

Ehternet hubs take an incoming packet and ____ it out to the other connected port.

Answer 1

Repeat.

Question 2

What is appended to the beginning of the Ethernet frame?

Answer 2

The preamble, which is a 7 bytes sequence meant to synchronise the NIC.

Question 3

What tyoe of bus does 10BaseT use?

Answer 3

Star bus.

Question 4

What is the maximal distance that can separate a 10BaseT node from its hub?

Answer 4

100m.

Question 5

When used for Ethernet, UTP pair uses what type of connector?

Answer 5

RJ-45.

Question 6

What is the maximum number of nodes that can be connecter to a 10BaseT hub?

Answer 6

1024.

Question 7

What two connectors are used for 10BaseFL?

Answer 7

SC and ST.

Question 8

Which Networking device can use STP?

Answer 8

The switch can use the Spanning Tree Protocol.

Question 9

When did DIX (Digital/Intel/Xerox) give intellectual property of Ethernet to IEEE?

Answer 9

In 1980. The committe 802.3 oversees the Ethernet since then.

Question 10

How is a frame structured?

Answer 10

Destination MAC address, Source MAC Address, Type, Data and FCS. The type indicates what is the higer level protocol in use (e.g. Layer 3 IPv4 or IPv6).

Question 11

What is a pad?

Answer 11

The minimal size of a packet is 64 bytes. If the packet cannot reach this size, a padding of zeros is added.

Question 12

What is 10BaseT?

Answer 12

10 Mbps ethernet over UTP (usually Cat3). It only uses 2 of the 4 pairs of UTP. It is half-duplex. It is baseband. It is used over star bus topology.

Question 13

What is TIA/EIA?

Answer 13

Telecommunication Industry Association/Electronics Industries Alliance.

Question 14

What is the TIA/EIA 568A standard?

Answer 14

Crimping standard most commonly used :

Green/White
Green
Orange/White
Blue
Blue/White
Orange
Brown/White
Brown

Question 15

What is the TIA/EIA 568B standard?

Answer 15

Crimping standard used for crossover cables :

Orange/White
Orange
Green/White
Blue
Blue/White
Green
Brown/White
Brown

Question 16

What is 10BaseFL?

Answer 16

It’s the fiber optic equivalent to 10BaseT. It’s got a maximal speed of 10 Mbps, it emits in baseband. It can reach a maximal distance of 2 km. It uses star bus topology. It uses multimode fiber with SC or ST connectors (two fibers required).

Question 17

What is CSMA/CD?

Answer 17

CSMA/CD (Carrier Sense Multiple Access with Collision Detection) is a network protocol used in Ethernet networks to manage access to the shared communication medium, such as a coaxial cable. In CSMA/CD, a device first senses the carrier (the shared communication channel) to check if it is idle. If the channel is clear, the device can transmit data. However, if multiple devices attempt to transmit simultaneously and a collision is detected, the devices involved stop transmission and initiate a backoff period before retrying.

This collision detection mechanism helps manage network contention and reduces the chances of data collisions, ensuring efficient data transmission in Ethernet networks.

Question 18

Compare the convergence time of STP versus RSTP.

Answer 18

About 50 second for STP and 6 for RSTP.

Question 19

What Level 2 protocol is used by switches to implement STP?

Answer 19

BPDU(Bridge Protocol Data Unit).

Question 20

What is a TCN BPDU?

Answer 20

A TCN BPDU (Topology Change Notification Bridge Protocol Data Unit) is a type of Bridge Protocol Data Unit used in spanning tree protocols, such as IEEE 802.1D (STP - Spanning Tree Protocol) and its variants. TCN BPDUs are sent by a bridge (switch) to notify the other devices in the network about a change in the network topology.

When a bridge detects a topology change, such as a link going up or down, it sends TCN BPDUs to inform the other bridges in the network about the change. This allows the network to converge and adapt to the new topology by recalculating the spanning tree.

TCN BPDUs are crucial for maintaining a loop-free and efficient network by signaling topology changes and triggering the necessary adjustments in the spanning tree algorithm.

Question 21

What is portFast?

Answer 21

PortFast is a feature in spanning tree protocol implementations, such as Cisco’s Spanning Tree Protocol (STP) or Rapid Spanning Tree Protocol (RSTP). PortFast is used to reduce the time it takes for a switch port to transition from the blocking state to the forwarding state.

In a traditional spanning tree environment, ports go through the listening and learning states before reaching the forwarding state. PortFast allows designated switch ports to skip these states, immediately transitioning to the forwarding state when the link comes up. This is particularly useful for end devices, such as workstations or servers, where the delay introduced by the traditional spanning tree states is unnecessary.

PortFast should only be configured on ports connected to end devices and not on ports connecting to other switches or network segments, as it bypasses the normal spanning tree process and could lead to network loops if misapplied.

Question 22

What is a BPDU guard?

Answer 22

BPDU Guard is a feature in network switches that helps prevent network loops by protecting against the unintentional connection of rogue switches. When BPDU Guard is enabled on a switch port, the port monitors for the receipt of Bridge Protocol Data Units (BPDUs), which are messages exchanged between switches to determine the network topology.

If the switch port receives a BPDU, indicating the connection of another switch, BPDU Guard puts the port into an error-disabled state. This prevents the potential for loops in the network caused by the introduction of unauthorized switches.

BPDU Guard is commonly used in conjunction with PortFast on access ports to ensure that end-user devices, such as computers, printers, or IP phones, do not inadvertently introduce switching loops by acting as rogue switches.