Chap 2 - Spanning Tree Protocol

Question 1

What happens on a port in Disabled state?

Answer 1

It is admin’ed down.

Question 2

What happens on a port in Blocking state?

Answer 2

• BPDUs: Receive only
• Modify MAC table: No

Question 3

What happens on a port in Listening state?

Answer 3

• BPDUs: Sends and receives
• Modify MAC table: No

Question 4

What happens on a port in Learning state?

Answer 4

• BPDUs: Send and receive
• Modify MAC table: Yes

Question 5

What happens on a port in Forwarding state?

Answer 5

• BPDUs: Sends and receives
• Modify MAC table: Yes
• Forwarding: Yes

Question 6

What are the 3 802.1D port types?

Answer 6

• Root Port (RP)
• Designated Port (DP)
• Blocking Port

Question 7

How many RPs are there on a switch?

Answer 7

No more than 1 RP per VLAN

Question 8

What is the Destination MAC of a BPDU?

Answer 8

01:80:c2:00:00:00

Question 9

What are the 2 types of BPDUs?

Answer 9

• Configuration BPDU
• Topology Change Notification (TCN) BPDU

Question 10

What 4 things does a Configuration BPDU do?

Answer 10

• Identifies the Root Bridge
• Identifies RP
• Identifies DP
• Identifies Blocking Ports

Question 11

What are the 7 fields in a Configuration BPDU?

Answer 11

• STP Type
• Root Path Cost
• Root Bridge Identifier
• Local Bridge Identifier
• Hello Timer
• Forward Delay Timer
• Max Age

Question 12

What does a Topology Change Notification (TCN) BPDU signify?

Answer 12

Communicates changes in the Layer 2 topology to other switches.

Question 13

What is the Max Age time?

Answer 13

If a switch loses communication with the generating switch of a BPDU the Max Age time is the length of time a switch considers that BPDU to be valid.

Question 14

What is the default Max Age time?

Answer 14

20 seconds

Question 15

How is the Root Bridge determined?

Answer 15

• The bridge with the lowest Priority
• if there is a tie then the bridge with the lowest MAC address

Question 16

What is the default Priority?

Answer 16

32768 + sys-id-ext

Question 17

What is the sys-id-ext?

Answer 17

A number equal to the VLAN number.

Question 18

What is the root path cost?

Answer 18

The cumulative outbound interface cost to get to the root bridge.

Question 19

How does a local switch calculate its root path cost?

Answer 19

It takes the root path cost advertised by its neighbor in a BPDU and then adds the interface cost of the local interface that BPDU was received on.

Question 20

How does a switch initially identify its Root Port (RP) along with 4 tie-breakers?

Answer 20

• Port with lowest path cost to the Root
• Port who’s neighbor has the lowest System Priority
• Port whose neighbor has the lowest base MAC address
• Port whose neighbor has the lowest port priority
• Port whose neighbor has the lowest interface index number

Question 21

What can a Designated port NOT be and how is the DP initially determined along with 2 tie-breakers?

Answer 21

• Cannot be a RP
• Switch with lowest root path cost gets the DP
• Switch with lowest System Priority wins
• Switch with lowest MAC address wins

Question 22

What is the difference between a point-to-point port and a point-to-point edge port?

Answer 22

Point-to-point Edge has Portfast

Question 23

What happens with a switch receives a BPDU with the TCN bit flipped on?

Answer 23

• All switches change their MAC address timer to the forward delay time (def 15 sec)
• This causes all MAC entries that aren’t heard during that 15 seconds to be flushed.

Question 24

What is the difference between a Direct Link Failure and an Indirect Link Failure?

Answer 24

• When a direct link failure occurs the links interfaces go down.
• When an Indirect Link Failure occurs the interfaces are still up but data is impaired or corrupted.

Question 25

By default how long is a MAC address in the MAC table before it times out?

Answer 25

300 seconds

Question 26

What do non-root switches do when they receive a Configuration BPDU with the TCN flag set?

Answer 26

They change their MAC address timeout from the default 300 seconds to the Forward Delay time (15 seconds) to flush the MAC table.

Question 27

What are the 3 port states in RSTP?

Answer 27

* Discarding * Learning * Forwarding

Question 28

In RSTP what is the Discarding State equivalent to in 802.1D STP?

Answer 28

Discarding combines the 802.1D port states of Disabled, Blocking, and Listening

Question 29

If a switch running RSTP detects a device connecting to it what does it do?

Answer 29

* Attempts a RSTP handshake. * If the device does not respond the switch enables 802.1D on that port causing the port to go through Listening and Learning states.

Question 30

What are the 4 RSTP Port Roles?

Answer 30

* Root Port (RP) * Designated Port (DP) * Alternate Port * Backup Port

Question 31

What does an Alternate Port do?

Answer 31

Is normally in blocking state but if the switch's RP goes down then it provides alternate connectivity toward the Root Bridge via a different switch

Question 32

What does a Backup Port do and when would you see one?

Answer 32

* Exists only when multiple links connect between same switches but a different port on the same switch was elected as the DP. * Normally in blocking state but if the DP goes down the backup port takes over as DP

Question 33

What are the 3 RSTP Port Types and how are they defined?

Answer 33

* Edge Port - port running portfast * Root Port - port with lowest cost to the root * Point-to-point Port - port running full duplex

Question 34

What is the difference between 802.1D and CST

Answer 34

* No difference. They are the same. * Both provide for only one STP instance. The opposite of PVST+.

Question 35

Which 'show' command identifies the Root Bridge?

Answer 35

show spanning-tree root

Question 36

What 'show' command will display the STP state (RP, DP, Blocking)

Answer 36

show spanning-tree vlan (vlan\_id)

Question 37

Display spanning-tree detail about a specific port.

Answer 37

show spanning-tree int (interface\_ID) detail

Question 38

Display information about TCNs.

Answer 38

show spanning-tree vlan (vlan\_id) detail

Question 39

What are the short mode port costs for 10 Mbps, 100 Mbps, 1 Gbps, 10 Gbps, 20 Gbps, 100 Gbps, 1 Tbps, 10 Tbps?

Answer 39

* 10 Mbps - 100 * 100 Mbps - 19 * 1 Gbps - 4 * 10 Gbps - 2 * 20 Gbps and higher - 1

Question 40

What are the long mode port costs for 10 Mbps, 100 Mbps, 1 Gbps, 10 Gbps, 20 Gbps, 100 Gbps, 1 Tbps, 10 Tbps?

Answer 40

* 10 Mbps - 2,000,000 * 100 Mbps - 200,000 * 1 Gbps - 20,000 * 10 Gbps - 2,000 * 20 Gbps - 1,000 (half of 10 Gbps) * 100 Gbps - 200 * 1 Tbps - 20 * 10 Tbps - 2

Question 41

What are the 5 factors when choosing the root port?

Answer 41

* Interface with the lowest root path cost * Interface with the neighbor that has the lowest system priority * Interface with the neighbor that has the lowest MAC address * Multiple links to same switch - neighbor's interface with lowest port priority * Multiple links to same switch - neighbor's interface that has lowest index

Question 42

How is the Designated Port chosen?

Answer 42

* Must not be a root port * Switch with the lowest root path cost has the DP, if there is a tie then: * Switch with lowest system priority has the DP, if there is a tie then: * Switch with lowest MAC address has the DP * If multiple connections to same switch then neighbor interface with lowest priority has the DP * If multiple connections to same switch then neighbor interface with lowest interface index has the DP

Question 43

When choosing the RP or DP what are the 2 tie-breakers when there are multiple connections to the same switch?

Answer 43

* choose the interface with the lowest port priority, if there's a tie then * choose the interface with the lowest interface index

Question 44

What is the RSTP negotiation process?

Answer 44

* sw1 sends bpdu with proposal bit set * sw2 * moves all its non-edge ports to blocking * sends proposal bpdu's to non-edge switches * replies to sw1 with bpdu with proposal bit cleared and agreement bit set * moves its port to sw1 to forwarding state * sw1 upon receiving agreement bpdu from sw2 moves its port to forwarding

Question 45

What are the 6 802.1D port states?

Answer 45

* Disabled * Blocking * Listening * Learning * Forwarding * Broken