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35 Cards in this Set

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  • Back

What are the three switching methods used in Cisco LAN switches?
1. Store-and-Forward
2. Cut-Through
3. Modified Cut-Through (or Fragment Free)

Define device "latency".
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.

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

Order by latency (highest to lowest) the three Cisco swiching methods.
1. Highest latency - Store-and-Forward
2. Mid - Modified Cut-Through (Fragment Free)
3. Lowest latency - Cut-Through
Exam Keys:

How many questions and how much time do you have on the test?
Approx. 65 questions in 90 minutes

How many bytes is a runt frame?
Less than 64 bytes including the CRC

How many bytes is a giant frame?
More than 1518 bytes.

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

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

How does a switch build its switching/forwarding table?
Either manually configured or learned automatically.

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

Describe how Modified Cut-Through works.
1. First, 64 bytes (called the collision window) is saved in the onboard buffer of the switch.
2. Checks for collisions. Discards if true.

Desribe the fields of an Ethernet frame.
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

What is a broadcast storm?
This occurs when network broadcasts use up all available bandwidth, bringing the network to a complete stop.

What is a CRC?
Cyclic Redundancy Check

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

What is the Spanning Tree Protocol (STP)?
A Layer 2 protocol used in switches to block data loops.

What is the payload size range for a L2 Ethernet frame?
46-1500 bytes.

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

Which of the L2 forwarding method(s)have constant latency?
1. Cut-through - First 6 bytes following the preamble are read.
2. Modified Cut-through - First 64 bytes are read.

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

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

How long does convergence in switches typically take?
50 seconds.

Only after convergence can switches forwarded frames.

What is forward delay?
The time it takes for a port to transition from listening --> learning or learning --> forwarding states.

How are designated ports determined in STP?
First, all root ports are designated ports.

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

How are root ports determined?
Cost of link is looked at.

Lowest cost = root

Cost = f(bandwidth)

What are the current IEEE costs for Ethernet networks (10Gig, Gig, 100Mbps, 10Mbps)?
10Gig = 2
Gig = 4
100Mbps = 19
10Mbps = 100

How is the root bridge determined?
f(priority + MAC) lowest best

default priority - 32768

What is the IEEE Specification for STP?

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

What are the advantages of VLANs?
1. Breakup large broadcast domains.
2. Segment network into logical groups: departments, locations, functions.

What is a trunk?
Ports used to connect switches to switches.

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

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

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

What are a group of switches called?
Switch block

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

802.1Q and ISL operate at what layer in the ISO/OSI model?
Layer 2

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