ICND2 Section 1 - LAN Switching

Question 1

3 problems caused by not using STP

Answer 1

1. Broadcast Storms
2. MAC table instability
3. Multiple copies of the same frame delivered to a host.

Question 2

What types of packets are processed by a port in Blocking?

Answer 2

STP control traffic only

Question 3

4 general steps for a switched network to get from nothing to full STP convergence

Answer 3

1. Elect a root switch
2. Select the root ports
3. Select the Designated ports
4. Select the Blocked ports

Question 4

What does BPDU stand for?

Answer 4

Bridge Protocol Data Unit

Question 5

What is the BID?

How is it calculated?

Answer 5

Bridge ID

2 byte priority field + 6 byte system ID (BIA)

Question 6

For STP, is higher or lower priority better?

Answer 6

Lower

Question 7

In STP, if the priority is equal, what is used to determine the root bridge?

Answer 7

The lowest System ID, aka MAC address used in the BID

Question 8

How is cost to the root bridge calculated?

Answer 8

The sum of all ports exited to reach the root bridge

Question 9

How does a switch determine it’s neighbor’s cost to get to the root bridge?

Answer 9

It’s a value listed in the neighbor’s BPDU root cost.

Question 10

What is the default bridge priority?

Answer 10

32768

Question 11

STP IEEE standard?

Answer 11

802.1D

Question 12

Time interval for BPDUs

Answer 12

2 seconds

Question 13

4 STP port states

Answer 13

Listening
Learning
Forwarding
Blocking

Question 14

Roughly speaking, what is used to calculate STP port cost

Answer 14

Port Speed

Question 15

What is a switch doing while a port is in Listening

Answer 15

Listening for and processing BPDUs, trying to see if it gets any info that would cause it to go to a Blocking state.

Question 16

Does a switch populate the MAC table while in Listening?

Answer 16

No

Question 17

What is a switch doing in Learning?

Answer 17

Learning source MAC addresses and populating the MAC address table, preparatory to beginning Forwarding.

Question 18

Default STP port costs for the following speeds:
10M
100M
1G
10G

Answer 18

10M = 100
100M = 19
1G = 4
10G = 2

Question 19

When a non-root switch receives a Hello BPDU, what does it change before forwarding it on?

Answer 19

It adds it’s root cost to the root cost present in the BPDU.

Question 20

What are the 3 types of BPDUs?

Answer 20

Hello BPDU
CBDPU - Configuration BPDU
TCN BPDU - Topology Change Notification BPDU

Question 21

Command to configure an etherchannel

Answer 21

conf t
interface int
channel-group group number mode on

Question 22

Command to show the status of an etherchannel

Answer 22

show etherchannel group number

Question 23

What are the 2 protocols that can be used to create dynamic etherchannels?

Answer 23

PAgP

LACP

Question 24

What does PAgP stand for?

Answer 24

Port Aggregation Protocol

Question 25

What does LACP stand for?

Answer 25

Link Aggregation Control Protocol

Question 26

What is the IEEE standard for PAgP and LACP?

Answer 26

PAgP is Cisco proprietary

LACP is 802.1ad

Question 27

Syntax to enable PAgP on a interface

Answer 27

conf t
int gi1/0
channel-group group number mode {desirable | auto}

Question 28

Syntax to enable LACP on a interface

Answer 28

conf t
int gi1/0
channel-group group number mode {active | passive}

Question 29

In PAgP configurations, what does the keyword desirable tell the switch to do vs auto?

Answer 29

It tells the switch “use this protocol and begin negotiations”
versus “use this protocol and wait for the other switch to begin negotiations.”

Question 30

In LACP configurations, what does the keyword active tell the switch to do vs passive?

Answer 30

It tells the switch “use this protocol and begin negotiations”
versus “use this protocol and wait for the other switch to begin negotiations.”

Question 31

What happens with an etherchannel where both sides are set to either auto or passive?

Answer 31

It won’t come up, because both sides are waiting for the other to begin negotiations.