41 terms

Spanning Tree Protocol

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What STP is enabled by default on Cisco switches?
PVST+
You receive the following output from a switch:
S2#sh spanning-tree
VLAN0001
Spanning tree enabled protocol rstp
Root ID Priority 32769
Address 0001.42A7.A603
Cost 4
Port 26(GigabitEthernet1/2)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
[output cut]
Which are true regarding this switch?
A. The switch is a root bridge.
B. The switch is a non-root bridge.
C. The root bridge is four switches away.
D. The switch is running 802.1w.
E. The switch is running STP PVST+.
B, D. The switch is not the root bridge for VLAN 1 or the output would tell us exactly that. We can see that the root bridge for VLAN 1 is off of interface G1/2 with a cost of 4, meaning it is directly connected. Use the command show cdp nei to find your root bridge at this point. Also, the switch is running RSTP (802.1d), not STP.
If you want to effectively disable STP on a port connected to a server, which command would you use?
A. disable spanning-tree
B. spanning-tree off
C. spanning-tree security
D. spanning-tree portfast
D. If you have a server or other devices connected into your switch that you're totally sure won't create a switching loop if STP is disabled, you can use something called PortFast on these ports. Using it means the port won't spend the usual 50 seconds to come up while STP is converging. Understand this doesn't completely disable STP, just stops any port configured with PortFast from transitioning the different port states, and immediately puts the port into forwarding state.
Which of the following would you use to find the VLANs for which your switch is the root bridge?
A. show spanning-tree
B. show root all
C. show spanning-tree port root VLAN
D. show spanning-tree summary
A, D. It is important that you can find your root bridge, and the show spanning-tree command will help you do this. To quickly find out which VLANs your switch is the
root bridge for, use the show spanning-tree summary command.
You want to run the new 802.1w on your switches. Which of the following would enable this protocol?
A. Switch(config)#spanning-tree mode rapid-pvst
B. Switch#spanning-tree mode rapid-pvst
C. Switch(config)#spanning-tree mode 802.1w
D. Switch#spanning-tree mode 802.1w
A. 802.1w is the also called Rapid Spanning Tree Protocol. It is not enabled by default on Cisco switches, but it is a better STP to run because it has all the fixes that the Cisco
extensions provide with 802.1d. Remember, Cisco runs RSTP PVST+, not just RSTP.
Which of the following is a layer 2 protocol used to maintain a loop-free network?
A. VTP
B. STP
C. RIP
D. CDP
B. The Spanning Tree Protocol is used to stop switching loops in a layer 2 switched
network with redundant paths.
Which statement describes a spanning-tree network that has converged?
A. All switch and bridge ports are in the forwarding state.
B. All switch and bridge ports are assigned as either root or designated ports.
C. All switch and bridge ports are in either the forwarding or blocking state.
D. All switch and bridge ports are either blocking or looping.
C. Convergence occurs when all ports on bridges and switches have transitioned to either the forwarding or blocking states. No data is forwarded until convergence is
complete. Before data can be forwarded again, all devices must be updated.
Which of the following modes enable LACP EtherChannel?
A. On
B. Prevent
C. Passive
D. Auto
E. Active
F. Desirable
C, E. There are two types of EtherChannel: Cisco's PAgP and the IEEE's LACP. They are basically the same, and there is little difference to configuring them. For PAgP,
use auto or desirable mode, and with LACP use passive or active. These modes decide which method you are using, and they must be configured the same on both sides of the EtherChannel bundle.
Which of the following are true regarding RSTP?
A. RSTP speeds the recalculation of the spanning tree when the layer 2 network topology changes.
B. RSTP is an IEEE standard that redefines STP port roles, states, and BPDUs.
C. RSTP is extremely proactive and very quick, and therefore it absolutely needs the 802.1 delay timers.
D. RSTP (802.1w) supersedes 802.1d while remaining proprietary.
E. All of the 802.1d terminology and most parameters have been changed.
F. 802.1w is capable of reverting to 802.1d to interoperate with traditional switches on a per-port basis.
A, B, F. RSTP helps with convergence issues that plague traditional STP. Rapid PVST+ is based on the 802.1w standard in the same way that PVST+ is based on 802.1d. The operation of Rapid PVST+ is simply a separate instance of 802.1w for each VLAN
What does BPDU Guard perform?
A. Makes sure the port is receiving BPDUs from the correct upstream switch.
B. Makes sure the port is not receiving BPDUs from the upstream switch, only the root.
C. If a BPDU is received on a BPDU Guard port, PortFast is used to shut down the port.
D. Shuts down a port if a BPDU is seen on that port.
D. BPDU Guard is used when a port is configured for PortFast, or it should be used, because if that port receives a BPDU from another switch, BPDU Guard will shut that port down to stop a loop from occurring.
How many bits is the sys-id-ext field in a BPDU?
A. 4
B. 8
C. 12
D. 16
C. To allow for the PVST+ to operate, there's a field inserted into the BPDU to accommodate
the extended system ID so that PVST+ can have a root bridge configured on a per-STP instance. The extended system ID (VLAN ID) is a 12-bit field, and we can even see what this field is carrying via the show spanning-tree command output.
There are four connections between two switches running RSTP PVST+ and you want to figure out how to achieve higher bandwidth without sacrificing the resiliency that
RSTP provides. What can you configure between these two switches to achieve higher bandwidth than the default configuration is already providing?
A. Set PortFast and BPDU Guard, which provides faster convergence.
B. Configure unequal cost load balancing with RSTP PVST+.
C. Place all four links into the same EtherChannel bundle.
D. Configure PPP and use multilink.
C. PortFast and BPDU Guard allow a port to transition to the forwarding state quickly, which is great for a switch port but not for load balancing. You can somewhat load balance with RSTP, but that is out of the scope of our objectives, and although you can use PPP to configure multilink (bundle links), this is performed on asynchronous or synchronous serial links. Cisco's EtherChannel can bundle up to eight ports between switches.
In which circumstance are multiple copies of the same unicast frame likely to be transmitted in a switched LAN?
A. During high-traffic periods
B. After broken links are reestablished
C. When upper-layer protocols require high reliability
D. In an improperly implemented redundant topology
D. If the Spanning Tree Protocol is not running on your switches and you connect them together with redundant links, you will have broadcast storms and multiple frame copies; you will have broadcast storms and multiple frame copies being received by the same destination device..
You want to configure LACP. Which do you need to make sure are configured exactly the same on all switch interfaces you are using?
A. Virtual MAC address
B. Port speeds
C. Duplex
D. PortFast enabled
E. VLAN information
B, C, E. All the ports on both sides of every link must be configured exactly the same or it will not work. Speed, duplex, and allowed VLANs must match.
Which of the following modes enable PAgP EtherChannel?
A. On
B. Prevent
C. Passive
D. Auto
E. Active
F. Desirable
D, F. There are two types of EtherChannel: Cisco's PAgP and the IEEE's LACP. They are basically the same, and there is little difference to configure them. For PAgP, use the auto or desirable mode, and with LACP use the passive or active mode. These modes decide which method you are using, and they must be configured the same on both sides of the EtherChannel bundle.
Which of the following would put switch interfaces into EtherChannel port number 1, using LACP?
A. Switch(config)#interface port-channel 1
B. Switch(config)#channel-group 1 mode active
C. Switch#interface port-channel 1
D. Switch(config-if)#channel-group 1 mode active
A, D. To configure EtherChannel, create the port channel from global configuration mode, and then assign the group number on each interface using the active mode to
enable LACP, although just configuring the channel-group command under your interfaces will enable the bundle, but options A and D are the best Cisco objective answers.
Which two commands would guarantee your switch to be the root bridge for VLAN 30?
A. spanning-tree vlan 30 priority 0
B. spanning-tree vlan 30 priority 16384
C. spanning-tree vlan 30 root guarantee
D. spanning-tree vlan 30 root primary
A, D. You can set the priority to any value from 0 through 61,440 in increments of 4,096. Setting it to zero (0) means that the switch will always be a root as long as it has
a lower MAC than another switch with its bridge ID also set to 0. You can also force a switch to be a root for a VLAN with the spanning-tree vlan vlan# root primary command.
Why does Cisco use its proprietary extension of PVST+ with STP and RSTP?
A. Root bridge placement enables faster convergence as well as optimal path determination.
B. Non-root bridge placement clearly enables faster convergence as well as optimal path determination.
C. PVST+ allows for faster discarding of non-IP frames.
D. PVST+ is actually an IEEE standard called 802.1w.
A. By using per-VLAN spanning tree, the root bridge can be placed in the center of where all the resources are for a particular VLAN, which enables optimal path determination.
Which are states in 802.1d?
A. Blocking
B. Discarding
C. Listening
D. Learning
E. Forwarding
F. Alternate
A, C, D, E. Each 802.1d port transitions through blocking, listening, learning, and finally forwarding after 50 seconds, by default. RSTP uses discarding, learning, and forwarding only.
Which of the following are roles in STP?
A. Blocking
B. Discarding
C. Root
D. Non-designated
E. Forwarding
F. Designated
A, C, D, E, F. The roles a switch port can play in STP are root, non-root, designated, non-designated, forwarding, and blocking. Discarding is used in RSTP, and disabled
could be a role, but it's not listed as a possible answer.
What STP feature causes an interface to be placed in the forwarding state as soon as the interface is physically active?
a. STP
b. EtherChannel
c. Root Guard
d. PortFast
d. PortFast
Which of the following facts determines how often a nonroot bridge or switch sends an 802.1D STP hello BPDU message?
a. The hello timer as configured on that switch.
b. The hello timer as configured on the root switch.
c. It is always every 2 seconds.
d. The switch reacts to BPDUs received from the root switch by sending another BPDU 2 seconds after receiving the root BPDU.
b. The hello timer as configured on the root switch.
Which of the following bridge IDs win election as root, assuming that the switches with these bridge IDs are in the same network?
a. 32769:0200.1111.1111
b. 32769:0200.2222.2222
c. 4097:0200.1111.1111
d. 4097:0200.2222.2222
e. 40961:0200.1111.1111
c. 4097:0200.1111.1111
Which of the following are transitory IEEE 802.1D port states used only during the process of STP convergence?
a. Blocking
b. Forwarding
c. Listening
d. Learning
e. Discarding
c. Listening
d. Learning
Which of the following IEEE 802.1D port states are stable states used when STP has completed convergence?
a. Blocking
b. Forwarding
c. Listening
d. Learning
e. Discarding
a. Blocking
b. Forwarding
Consider the following command output:
SW1# show interfaces f0/11 status
Port Name Status Vlan Duplex Speed Type
Fa0/11 connected 3 a-full 100 10/100BaseTX
A frame arrives on this same switch's port F0/1, an access port in VLAN 3. The frame is destined to MAC address FFFF.FFFF.FFFF. Which of the following statements is true, based
on the information about F0/11 in the show command output, about whether SW1 will forward
the frame out port F0/11?
a. The output confirms that SW1 will definitely forward the frame out F0/11.
b. The output confirms that SW1 will definitely not forward the frame out F0/11.
c. The output confirms that it is possible for SW1 to forward the frame out F0/11, but not definitely.
c. The output confirms that it is possible for SW1 to forward the frame out F0/11, but not definitely.
An Ethernet frame arrives at switch SW1, entering on port F0/1. F0/1 does not trunk. SW1 forwards the frame out F0/2. Which of the following statements is true about the logic SW1 uses when forwarding the frame?
a. Forward based on MAC table entries for the F0/1's access VLAN
b. Forward based on MAC table entries for the F0/2's access VLAN
c. Forward based on MAC table entries for the F0/1's native VLAN
d. Forward based on MAC table entries in all VLANs
A. The switch forwarding process considers MAC table entries for one VLAN only. That VLAN is based on the inbound interface, and for an access interface, that VLAN is the access VLAN assigned to the inbound interface. The VLAN is never determined by the outbound interface (F0/2 in this case). The native VLAN could not matter because the inbound interface
was not a trunk.
How is a incoming Frame process without STP?
Step 1. Determine the VLAN in which the frame should be forwarded, as follows:
A. If the frame arrives on an access interface, use the interface's access VLAN.
B. If the frame arrives on a trunk interface, use the VLAN listed in the frame's trunking header.
Step 2. Add the source MAC address to the MAC address table, with incoming interface and VLAN ID.
Step 3. Look for the destination MAC address of the frame in the MAC address table, but only for entries in the VLAN identified at Step 1. Follow one of the next steps depending on whether the destination MAC is found:
A. Found: Forward the frame out the only interface listed in the matched address table entry
B. Not found: Flood the frame out all other access ports in that same VLAN and out all trunk ports that list this VLAN as fully supported (active, in the allowed list, not pruned, STP forwarding)
What does it mean when a port in blocking state?
The port is not learning mac address and ignoring frames entering that port. This affect what is displayed on the mac address-table.
What will a switch do when it receives a frame destine for a different vlan?
The switch must determine the VLAN in which the frame arrived. Then forwards the frame out ports in that same VLAN only or out a trunk ports that supports that VLAN.
A _____ port or a ______ port determines how the switch determines the incoming frame's VLAN
access , trunk
The show interfaces status display what information?
the interfaces status, vlan, speed and duplex and if it's a trunk or access port
What is MAC table instability?
This occurs when a loop occurs. The switches' MAC address tables keep changing the information listed for the source MAC address of the looping frame.
What 3 problems caused by switching loops?
broadcast storms, mac table instability, multiple copies of a frame.
In what mode will a interface not forward user frames, learn MAC addresses and do not process received user frames?
Blocking
If a tie occurs in a root election, what will be the deciding factor who the winner is?
1. Root bridge with the lowest bridge ID
2. Root bridge with the lowest MAC
How is a Root port designated?
There can be only one RP and it's the interface with the least STP cost to reach the root switch (least root cost).
What is the default hello BPDU timer?
The root switch sends a new hello BPDU every 2 seconds by default.
What are the steps in BPDU hello process?
Step 1. The root creates and sends a hello BPDU, with a root cost of 0, out all its working interfaces (those in a forwarding state).
Step 2. The nonroot switches receive the hello on their root ports. After changing the hello to list their own BID as the sender's BID, and listing that switch's root cost, the switch forwards the hello out all designated ports.
Step 3. Steps 1 and 2 repeat until something changes.
What are the 3 timers used by STP?
Hello (defaults 2 sec), Max Age (defaults 10 times hello), Forward Delay (defaults 15 sec)
How is the Hello timer determine?
It is set by the root bridge. All switches use the timers as dictated by the root switch, which the root lists in its periodic hello BPDU messages.
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