Review - Chapter 4

Question 1

STP

Answer 1

• Layer 2 protocol

* Provides network link redundancy while eliminating potential problems

Question 2

STP Limitation

Answer 2

Convergence delay after a topology change

Question 3

BPDUs (STP)

Answer 3

○ Used to determine current topology information and how to react if any devices are added/removed/changed in the topology
○ Sent every 2 seconds by default on all switch ports

Question 4

STP Rules

Answer 4

○ One root bridge
○ One root port on each non-root switch
○ One designated port per LAN segment
○ Non designated ports are blocked

Question 5

STP Timers

Answer 5

○ Hello time (2s)
○ Forward delay (15s)
○ Max age (20s)

Question 6

Key Types of STP

Answer 6

○ STP (original IEEE 802.1D version)
○ Common Spanning Tree (CST)
○ PVST and PVST+ (Cisco proprietary)
○ Multiple Spanning Tree (MST)
○ Rapid Spanning Tree (RSTP)
○ RPVST+ (Cisco implementation of RSTP based on PVST+)

Question 7

Root Bridge

Answer 7

Selected based on lowest Bridge ID (BID)

Question 8

Bridge ID (BID)

Answer 8

Two parts:
○ Priority
○ Mac address

Question 9

Root Port Factors

Answer 9

○ Path Cost
○ Lowest Sender BID
○ Lowest Port ID

Question 10

Bridge Protocol Data Units (BPDUs)

Answer 10

• Used to exchange STP information, specifically for root bridge election and loop identification
• Sent every 2 seconds by default

Question 11

Types of BPDUs

Answer 11

○ Configuration BPDUs

○ TCN (topology change notification) BPDUs

Question 12

STP Port States

Answer 12

○ Blocking
○ Listening (15s)
○ Learning (15s)
○ Forwarding
○ Disabled

Question 13

Per VLAN STP+ (PVST+)

Answer 13

• Runs one spanning-tree instance for each VLAN
• The original 16-bit bridge priority is split into two fields, resulting in the following components in the BID:
○ Bridge priority
○ Extended system ID
○ MAC address

Question 14

Bridge priority

Answer 14

○ 4-bit field carrying bridge priority
○ Default is 32,768 (midrange)
○ Conveyed in discrete values in increments of 4096

Question 15

Extended system ID

Answer 15

12-bit field carrying VLAN ID

Question 16

MAC address

Answer 16

6-byte field with MAC address of the switch

Question 17

Types of Topology Changes

Answer 17

• Direct topology change
• Indirect topology change
• Insignificant topology change

Question 18

Direct Topology Change

Answer 18

Can be detected on an interface

Question 19

Indirect topology change

Answer 19

Detected because no BPDUs are received from the root bridge

Question 20

Insignificant topology change

Answer 20

Occurs if an event causes a switch to send out TCNs (no actual topology change occurs)

Question 21

Rapid Spanning Tree Protocol (RSTP)

Answer 21

• 802.1W

* Speeds up the recalculations after topology change

Question 22

RSTP Port Roles

Answer 22

• Root
• Designated
• Alternate
• Disabled
• Backup

Question 23

RSTP Port States

Answer 23

• Discarding
• Learning
• Forwarding

Question 24

RSTP Rapid Transition

Answer 24

Rapid transitions to forward state on:
• Edge ports
• Point-to-point links

Question 25

RSTP Timers

Answer 25

Designated ports wait 2x forwarding delay before forwarding

Question 26

UplinkFast

Answer 26

Fast uplink failover on access switch

Question 27

BackboneFast

Answer 27

Fast convergence in distribution/core layer when STP change occurs

Question 28

PortFast

Answer 28

Configures access port to transition directly to forwarding state

Question 29

BPDU Guard

Answer 29

Disables PortFast-enabled port if a BPDU is received

Question 30

BPDU Filter

Answer 30

Suppresses BPDUs on ports

Question 31

Root Guard

Answer 31

Prevents external switches from becoming roots

Question 32

Loop Guard

Answer 32

Prevents an alternate port from becoming the designated port if no BPDUs are received

Question 33

Unidirectional Link Detection (UDLD)

Answer 33

Detects when a unidirectional link exists and shuts down the affected interface

Question 34

UDLD Packets

Answer 34

Sent to an STP neighbor every 15s

Question 35

Flex Links

Answer 35

Layer 2 availability feature which provides link redundancy with a convergence time of less than 50ms

Question 36

Multiple Spanning Tree Protocol (MST)

Answer 36

• Main purpose is to reduce the total number of spanning-tree instances to match the physical topology of the network (Reduces the CPU cycles of a switch)

Question 37

MST Configuration Attributes

Answer 37

○ An alphanumeric name (32 bytes) ○ A configuration revision number (2 bytes) ○ A 4096-element table that associates each VLAN to an instance

Question 38

Extend System ID

Answer 38

12-bit field used in MST and carries the MST instance number

Question 39

MST Regions

Answer 39

A group of switches must share the same configuration attributes to be part of a common MST region

Question 40

STP Instances with MST

Answer 40

MST supports a number of instances but doesn't send BPDUs for every active STP instance separately

Question 41

Instance 0 (STP with MST)

Answer 41

The internal spanning tree (IST) ○ Designed to carry all STP-related information ○ MSTI information is carried in the IST's BPDU in so-called M-Record fields (one for every active MSTI)

Question 42

Potential STP Problems

Answer 42

* Duplex mismatch * Unidirectional link failure * Frame corruption * Resource errors * PortFast configuration error