Volume 1 - Chapter 9: Spanning Tree Protocol Concepts Flashcards
What term best describes the following?
The continual updating of a switch’s MAC address table with incorrect entries, in reaction to looping frames, resulting in frames being sent to the wrong locations.
MAC table instability
Describe multiple frame transmission in regards to STP issues.
When multiple copies of the same frame arrive at the destination
Describe the attributes of a STP blocked port.
Interfaces in a blocking state:
1) Do not forward user frames
2) Do not learn MAC address
3) Do not process received user frames
What process is used by Spanning Tree to determine what interfaces to put in a forwarding state?
Spanning-Tree Algorithm (STA)
What is the size of the bridge ID?
8 bytes
What fields does the spanning-tree bridge ID consist of?
The 2-byte priority field and a 6-byte system ID
What is the value of the system ID portion of the spanning-tree bridge ID?
The switch’s universal (burned-in) MAC address
Describe how the root bridge is elected.
The root bridge is elected based on the lowest bridge ID in the topology
If two switches have the same priority value in their BID, how is the root bridge determined?
By the MAC address (system-ID). The lowest MAC wins
In regards to the root bridge election:
What happens when a switch receives a hello message from another switch with a lower BID?
The switch stops claiming to be the root and forwards the BDPU from the superior switch
How is the designated switch chosen on a LAN segment?
The switch with the lowest cost to the root switch
Describe the tiebreaker process on a single switch to choose a root port when the cost to the root switch is the same.
1) Choose the root port based on the neighbor with the lowest bridge ID
If the neighbor priorities are the same, then both ports are connected to the same switch, then:
2) Choose the root port based on the lowest neighbor port priority
3) Choose the root port based on the lowest neighbor internal port number
What state are all the ports on the root switch?
Designated and in a forwarding state.
When 2 switches are electing a DP, how is the DP chosen when the cost to the root switch is tied?
The switch with the lowest BID is elected as the designated switch and its non-root port becomes a DP
An interface that is not elected to be a root port or designated port is put into a _____ state.
Blocking
What is the command to change the spanning-tree method for determining a ports cost?
spanning-tree pathcost method short/long
What are the long and short port costs associated with a 10 Mbps interface?
Short: 100
Long: 2,000,000
What are the long and short port costs associated with a 100 Mbps interface?
Short: 19
Long: 200,000
What are the long and short port costs associated with a 1 Gbps interface?
Short: 4
Long: 20,000
What are the long and short port costs associated with a 10 Gbps interface?
Short: 2
Long: 2,000
What are the long and short port costs associated with a 100 Gbps interface?
Short: N/A
Long: 200
What are the long and short port costs associated with a 1 Tbps interface?
Short: N/A
Long: 20
If an interface is capable of speeds of 10/100/1000. What is the cost of the link?
The cost of the link will be whatever the current speed of the link is.
What is the default STP hello interval?
2 seconds
When a switch receives a BDPU, it changes the _____ in the BDPU and adds its own ____ before forwarding it on
Bridge ID (BID)
Cost
Describe the purpose of the spanning-tree Max Age timer.
It determines how long a switch should wait after it stops detecting BDPU hello messages before trying to change the topology
What is the default STP Max Age timer on a switch?
10 times the hello interval
Describe the purpose of the spanning-tree Forward Delay timer.
It determines how long a switch will sit in the interim states of listening and then learning when switching from a blocking state to a forwarding state
Describe what happens when the Max Age timer expires.
The entire STP process runs again from scratch. the root switch is elected and the RP/DP ports are chosen
Switches using STP can move immediately from a _____ state to a ____ state.
Forwarding
Blocking
Switches using STP take extra time to transition from a _____ state to a ____ state.
Blocking
Forwarding
What interim states does a port transition to before moving from a blocking to forwarding state?
Listening, then Learning
Describe the function of the STP Listening state.
In the listening state:
- The interface does not forward frames
- The interface removes stale MAC address table entries
Describe the function of the STP Learning state.
In the listening state:
- The interface does not forward frames
- The interface begins to learn MAC address received on it
Assuming the default forward delay timer, what is the amount of time it takes for an interface to move from the blocking to forwarding state in STP.
30 seconds:
15 seconds in listening
15 seconds in learning
What is the original IEEE standard for STP?
802.1D
What was the IEEE amendment of RSTP to the original STP standard?
802.1W
What port state in RSTP is equivalent to the original STP blocking state?
Discarding
Assuming all default timers, what is the total time required for STP convergence?
50 seconds:
20 for Max Age Timer
15 for Listening state
15 for Learning state
How does RSTP differ from STP in regards to electing a new RP or DP?
RSTP adds a mechanism so a switch can replace its RP or DP without waiting to reach a forwarding state.
What is the default Max Age timer in RSTP?
3 times the hello interval
How does RSTP improve on STP in regards to determining problems in the topology?
A switch using RSTP can relay information to its neighboring switches that it is having a problem before the max age timer expires
Which port role in RSTP acts as a backup in case the root port on a non-root switch fails?
The alternative port
Which port role in RSTP acts as a backup in case the designated port on a switch fails?
The backup port
The backup port role is only used in network designs that utilize ____.
Network hubs
Describe the difference between how hellos operate in STP versus RSTP.
With STP, only the root switch sends a hello. The other non-root switches update and forward that hello.
With RSTP, each switch can generate and send their own hellos to their neighbors
Alternative ports do not need to transition to the _____ & ____ states before reaching a forwarding state.
Listening & Learning
Which interim state does not exist in RSTP?
Listening
What RSTP state replaces the Blocking state in regular STP?
Discarding
For STP, what state is associated to an interface that is in non-working state (Down/Down)?
Disabled
For RSTP, what state is associated to an interface that is in non-working state (Down/Down)?
Discarding
Describe the function of the RSTP Backup port role.
To take over as the Designated Port (DP), but only if the DP and Backup Port are connected to a hub.
Describe the RSTP “Point-to-Point” port type.
A link that directly connects 2 switches together
Describe the RSTP “Point-to-Point edge” port type.
A link that directly connects a switch to a PC or other end device
Describe the RSTP “Shared Port” port type
A link that directly connects a network hub
Describe how STP logic works on an EtherChannel link.
STP treats the interface as a single link
Which STP feature is used as an alternative to BDPU Guard?
BPDU Filter
Describe the difference between BPDU Guard and BPDU Filter.
BPDU Guard puts an interface into an err-disabled state if it detects a BPDU.
With BPDU Filter, the switch disables PortFast on the interface and treats it as a normal port and applies STP logic and processes.
What is the command to enable BPDU Filter on all PortFast enabled ports by default
In global config mode:
spanning-tree portfast bpdufilter default
What spanning-tree feature is used to prevent a new switch from becoming the root switch if it receives a superior BPDU
Root Guard
What spanning-tree feature is used to prevent a RP or ALT port from becoming a DP due to a one way communications failure?
Loop Guard
In what specific scenario would you enable Loop Guard on an interface
A switch to switch link connected by a fiber cable
