Day 25 - STP (Spanning Tree Protocol) Flashcards
This original IEEE standard allowed for only one instance of STP to run for an entire switched network
IEEE STP 802.1D
STP helps prevent __________ in a redundant switched network
Loops
Without STP, redundancy in a switched network can introduce what types of issues?
Broadcast storms
Multiple frame transmission
MAC database instability
STP creates a __________ that ensures that only one path exists for each network segment at any one time
Tree
For any interfaces not chosen to be in a forwarding state, STP places the interfaces into a __________ __________
Blocking state
Switches exchange STP configuration every ___ seconds, by default
2
The configuration messages exchanged every 2 seconds (by default) are using what protocol and also, what are these messages called?
Multicast
BDPUs (Bridge Protocol Data Units)
True or False: Blocked ports listen for BPDUs to listen for whether or not the other side of link is down
True
What does the Bridge ID (BID) consist of?
Priority value
Bridge MAC address
What is the default bridge priority value?
32768
The __________ __________ is the bridge with the lowest BID
Root bridge
If the BID default priority value isn’t changed, the switch with the lowest __________ becomes the root bridge
MAC address
This is the process by which switches collectively realize that something has changed in the LAN topology
STP convergence
What are the 4 steps for STP convergence?
Elect a root bridge
Elect root ports
Elect designated ports
Root ports and designated ports transition to forwarding and the other ports transition to blocking
What is the IEEE cost in STP for a 10 Mbps link?
100
What is the IEEE cost in STP for a 100 Mbps link?
19
What is the IEEE cost in STP for a 1 Gbps link?
4
What is the IEEE cost in STP for a 10 Gbps link?
2
What four states does STP use?
Blocking
Listening
Learning
Forwarding
What is the fifth state that STP uses when a network admin manually disables the port or when a security violation occurs?
Disabled
Solve for definition:
The original specification of __________ defined as 802.1D
STP
Solve for definition:
Cisco’s enhancement of STP that provides a separate 802.1D spanning tree instance for each VLAN configured in the network
PVST+
Solve for definition:
This is 802.1w. It’s an evolution of STP that provides faster convergence. However, it still provides for only a single instance of STP
RSTP
Solve for definition:
A Cisco enhancement of RSTP. Provides a separate instance of 802.1w per VLAN
RPVST+
PVST+’s tree calculation is for All VLANs or Per VLAN?
Per VLAN
STP’s tree calculation is for All VLANs OR Per VLAN?
All VLANs
RPVST+’s tree calculation is for All VLANs for Per VLAN?
Per VLAN
What STP operation is the default on all Cisco Catalyst switches?
PVST+
Switched networks running PVST+ have what characteristics?
Per VLAN configuration allows redundant links to be fully utilized
Each additional spanning tree instance for a VLAN adds more CPU cycles to all switches on the network
True or False:
Ports in a blocking state can still receive and process BPDUs
True
True or False:
Ports in a listening state can forward data frames received on the interface
False
True or False:
Ports in a listening state can learn MAC addresses
False
True or False:
Ports in a learning state can receive and process BDPUs
True
True or False:
Ports in a learning state can forward data frames switched or received on the interface
False
True or False:
Ports in a forwarding state can forward data frame switched from another interface or received on the interface
True
PVST+ requires what for each VLAN?
A separate instance of spanning tree
In PVST+ the extended Bridge ID with System ID equals what?
VLAN identifier in the bridge ID
In a PVST+ BID, what 3 fields are included?
Bridge priority
Extended system ID (VLAN)
MAC address
In RPVST+ a single instance of __________ runs for each VLAN
RSTP
With RSTP, IEEE improved the convergence performance of STP from 50 seconds to less than ____________
10 seconds
With RSTP, what is the blocking state called?
Discarding state
Solve for definition:
A single port on each nonroot switch in which the switch hears the best BPDU out of all the received BPDUs
Root port
Solve for definition:
Of all switch ports on all switches attached to the same segment/collision domain, this is the port that advertises the best BPDU
Designated port
Solve for definition:
An RSTP role for a port on a switch that receives a suboptimal BPDU
Alternate port
RSTP edge (portfast) ports should immediately transition to a __________ __________, thereby skipping 802.1D listening and learning port state
Forwarding state
Portfast links must be what kind of links?
Point to point links with an end device
What is the default hello time for PVST+
2 seconds
What is maximum aging time for PVST+
20 seconds
What two configuration options will help you achieve load balancing with PVST+
Bridge ID manipulation
Port cost manipulation
What commands would you use to change the bridge ID for VLAN 1?
spanning-tree vlan 1 root primary (or secondary)
spanning-tree vlan 1 priority (priority)
The primary keyword in an stp configuration automatically sets the bridge priority value to what?
24576
The secondary keyword in an stp configuration automatically sets the bridge priority value to what?
28672
The bridge priority value goes up in increments of what?
4096
Alternatively, what commands would you use to manually alter the cost of bridge ID priority values?
spanning-tree vlan 1 priority 24576
What command would you use to verify spanning tree protocol?
show spanning-tree
What commands would you use to configure portfast and BPDU guard on Fa0/1?
int fa0/1
sw mode acc
spanning-tree portfast
spanning-tree bdpuguard enable
What can the Bridge Priority range be?
0-65535 but in reality it’s only 0-61440 because it goes in increments of 4096
What is a Root Bridge elected based on?
Bridge ID values (MAC Address and Bridge Priority Value)
True or False: When a switch calculates its best path towards the Root Bridge, higher bandwidth links are preferred?
True
True or False: If a switch has multiple equal cost paths towards the Root Bridge, it will select the neighbor switch with the lowest Bridge ID?
True. It will select it based on the lowest MAC Address or a lower assigned Bridge Priority value
Ports on the neighbor switch opposite the root port are called what?
Designated Ports
Root Ports point toward what?
Root Bridge
The Root Ports and Designated Ports are the most direct path(s) to this?
Root Bridge
What IEEE standard is MSTP (Multiple Spanning Tree Protocol)?
802.1s
Does PVST+ allow load balancing?
Yes
If you had 20 VLANS on a switch running RPVST+ how many Spanning Tree instances would be running?
- One for each VLAN
PVST+ will assign the _____, ______ or _______ role to ports?
Root, Designated or Alternate
What is the Short-Mode Cost (IEEE) of 20 Gbps?
1
What is the command to verify Spanning Tree for VLAN 1?
show spanning-tree vlan 1
On a show spanning tree command, what is the Root ID section telling you about?
Tells you about the Root Bridge, which might not necessarily be the switch you’re looking at the output on
On a show spanning tree command, what is the Bridge ID section telling you about?
This is actually giving you information about the switch you’re running the command on
If you entered a Priority value of 5000 what would happen?
It would error out since it’s not an increment of 4096
If a portfast port has been errdisabled by receiving BPDUs what should you do to bring it back?
First, correct the issue
then:
shut
no shut
If you had two switches - one primary and one secondary what command could you use to make sure no other switch ever got introduced that had a superior priority value and thus became the root bridge?
int {int}
spanning-tree guard root
Switch port enters the __________ state at time of election process, when a switch receives a BPDU on a port that indicates a better path to the Root Switch or if a port is not a Root Port.
Blocking
________ state is the first transitional state after blocking state.
Listening
This state of an STP port does not participate in any operation of STP because it is considered non-operational.
Disabled
Switch port enters __________ state after passing through all the states involved in creation of network topology.
Forwarding
Where would you enable Root Guard?
Use Root Guard only on uplink ports that connect to switches or devices that should never become the root bridge. These are typically connections to distribution or core switches.
What STP priority setting is NOT permitted on a Cisco switch?
1
0
4096
32768
28672
1 because it’s not an increment of 4096
How is a root bridge elected when there are multiple switches with a default configuration?
switch with highest priority
switch with lowest priority
switch with lowest MAC address
switch with highest MAC address
switch with lowest MAC address
Which of these MAC addresses would be chosen in the case of default config on multiple switches?
00:1A:2B:3C:4D:5E
A1:B2:C3:D4:E5:F6
08:00:27:00:FF:AA
2C:33:7F:9A:BC:DE
00:1A:2B:3C:4D:5E
What two features of the Spanning-Tree Protocol contribute to the time it takes for a switched network to converge after a topology change occurs? (Choose two.)
The max-age timer
The spanning-tree hold down timer
The forward delay
The spanning-tree path cost
Max-Age Timer
Forward Delay
What three link types have been defined for Rapid Spanning-Tree Protocol? (Choose three.)
Shared
End-to-end
Edge-type
Boundary-type
Point-to-point
Shared
Edge-Type
Point-to-Point
