Ccna Chapter1

Front 1

Bridging/Switching:

What are the three switching methods used in Cisco LAN switches?

Back 1

1. Store-and-Forward
2. Cut-Through
3. Modified Cut-Through (or Fragment Free)

Front 2

Bridging/Switching:

Define device "latency".

Back 2

The amount of time for a frame or packet to enter a network device, for the device to process and make a decision on it, and which port/ports it should exit from.

Front 3

Bridging/Switching:

Define network latency.

Back 3

The time for data to get from the sending host to the destination device.

Front 4

Bridging/Switching:

Order by latency (highest to lowest) the three Cisco swiching methods.

Back 4

1. Highest latency - Store-and-Forward
2. Mid - Modified Cut-Through (Fragment Free)
3. Lowest latency - Cut-Through

Front 5

Exam Keys:

How many questions and how much time do you have on the test?

Back 5

Approx. 65 questions in 90 minutes

Front 6

Bridging/Switching:

How many bytes is a runt frame?

Back 6

Less than 64 bytes including the CRC

Front 7

Bridging/Switching:

How many bytes is a giant frame?

Back 7

More than 1518 bytes.

Front 8

Bridging/Switching:

Describe what a switch does to a frame when one is received.

Back 8

First, the frame (partially or fully) is copied into its onboard buffer.

1. Fully copied with Store-and-Forward.
2. Copies only destination MAC (6 bytes following preamble) into the buffer with Cut-Through.
3. Copies the collision window which are the first 64 bytes into buffer for Modified Cut-Through / Fragment Free. ERRORS ALMOST ALWAYS OCCUR WITHIN THE FIRST 64 BYTES.

Front 9

Bridging/Switching:

Describe the basic steps peformed by a switch operating in Store-and-Forward method.

Back 9

1. Receive frame and fully store in onboard buffer.
2. Compute CRC and compare it with CRC of frame.
3. If correct, check switching/forwarding table and look-up destination address and match to the correct interface on the switch. Otherwise the frame is discarded.

Front 10

Bridging/Switching:

How does a switch build its switching/forwarding table?

Back 10

Either manually configured or learned automatically.

Front 11

Bridging/Switching:

Describe how a switch uses Cut-Through method of frame forwarding.

Back 11

1. First, destination header (6 bytes) is stored in the onboard buffer.
2. The switching/forwarding table is checked to determine the correct interface to forward the packet to.

NOTE: Some switches can be configured to auto-switch to Store-and-Forward if a certain threshold is reached for errors. Subsequently, auto return to Cut-Through when the error rate falls.

Front 12

Bridging/Switching:

Describe how Modified Cut-Through works.

Back 12

1. First, 64 bytes (called the collision window) is saved in the onboard buffer of the switch.
2. Checks for collisions. Discards if true.

Front 13

Bridging/Switching:

Desribe the fields of an Ethernet frame.

Back 13

1. 6 Bytes - Preamble
2. 1 Byte - SFD
3. 6 Bytes - Destination MAC
4. 6 Bytes - Source MAC
5. 2 Bytes - Length field
6. Up to 1500 Bytes - DATA
7. 4 Bytes - FCS or CRC

Front 14

Bridging/Switching:

What is a broadcast storm?

Back 14

This occurs when network broadcasts use up all available bandwidth, bringing the network to a complete stop.

Front 15

Bridging/Switching:

What is a CRC?

Back 15

Cyclic Redundancy Check

A mathematical algorithm used to check for errors when a frame, packet or segment has been transmitted through the network.

Front 16

Bridging/Switching:

What is the Spanning Tree Protocol (STP)?

Back 16

A Layer 2 protocol used in switches to block data loops.

Front 17

Bridging/Switching:

What is the payload size range for a L2 Ethernet frame?

Back 17

46-1500 bytes.

Runt frames are <64 bytes
Giant frames are >1518 bytes

Front 18

Bridging/Switching:

Which of the L2 forwarding method(s)have constant latency?

Back 18

1. Cut-through - First 6 bytes following the preamble are read.
2. Modified Cut-through - First 64 bytes are read.

Front 19

Bridging/Switching:

What are the four different states a port can be in running STP?

Back 19

1. Blocked - default for each port / listens for BPDUs
2. Forwarding
3. Listening - listens to BPDUs to make sure no loops before passing frames.
4. Learning - learns MAC addresses and builds filter table / no forwarding of frams

Front 20

Bridging/Switching:

What is convergence?

Back 20

When all bridges have the same devices and have transitioned fully into a blocked or forwarding state.

Front 21

Bridging/Switching:

How long does convergence in switches typically take?

Back 21

50 seconds.

Only after convergence can switches forwarded frames.

Front 22

Bridging/Switching:

What is forward delay?

Back 22

The time it takes for a port to transition from listening --> learning or learning --> forwarding states.

Front 23

Bridging/Switching:

How are designated ports determined in STP?

Back 23

First, all root ports are designated ports.

Bridge ID is used to determine designated ports. Lower Bridge ID has higher priority.

Front 24

Bridging/Switching:

How are root ports determined?

Back 24

Cost of link is looked at.

Lowest cost = root

Cost = f(bandwidth)

Front 25

Bridging/Switching:

What are the current IEEE costs for Ethernet networks (10Gig, Gig, 100Mbps, 10Mbps)?

Back 25

10Gig = 2
Gig = 4
100Mbps = 19
10Mbps = 100

Front 26

Bridging/Switching:

How is the root bridge determined?

Back 26

f(priority + MAC) lowest best

default priority - 32768

Front 27

Bridging/Switching:

What is the IEEE Specification for STP?

Back 27

802.1D

L2 Protocol to prevent loops. STA determines the cost of links and how to handle secondary links. When to enable and disable.

Front 28

Bridging/Switching:

What are the advantages of VLANs?

Back 28

1. Breakup large broadcast domains.
2. Segment network into logical groups: departments, locations, functions.

Front 29

Bridging/Switching:

What is a trunk?

Back 29

Ports used to connect switches to switches.

Front 30

Bridging/Switching:

What term is used to describe the VLAN links where devices are connected to but unaware of any special tagging?

Back 30

Access Links

Front 31

Bridging/Switching:

What are the VLAN links that connect switches to switches/routers/servers?

Back 31

Trunk Links - they may carry a single or multiple VLAN(s)

Trunk protocols supported by Cisco are
1. ISL (Inter-Switch Link)
2. 802.1Q

Front 32

Bridging/Switching:

Do access link devices know what VLAN they are apart of?

Back 32

No, the switches strip out all VLAN tagging before forwarding frames to access link devices. Only the switch is aware of the VLANs that are available and which devices are apart of each.

Front 33

Bridging/Switching:

What are a group of switches called?

Back 33

Switch block

Front 34

Bridging/Switching:

What are the highway of wires, ports, ASICs called that a frame goes through to get from one station to the end station called?

Back 34

Switch fabric

Front 35

Bridging/Switching:

802.1Q and ISL operate at what layer in the ISO/OSI model?

Back 35

Layer 2

802.1Q - VLAN Tag (or Color) in header
ISL - encapsulates frame with VLAN identifier