Rapid Spanning-Tree Quiz

1.) What best describes an edge-type connection in RSTP?
a.) connection to a hub
b.) connection to another switch
c.) connection to a host computer
d.) none of these are an edge-type

2.)Which of the following are RSTP port states? Choose 3
a.) blocking
b.) forwarding
c.) listening
d.) discarding
e.) forwarding

3.) The RSTP port role alternate is used on what link type?
a.) link type point-to-point
b.) edge-type
c.) link-type shared

4.) When is the backup port role in RSTP used?
a.) on links connecting to host computers
b.) on a single link connecting two switches
c.) a single switch with two links to a hub
d.) a switch connected to a router

5.) What port state are alternate port roles in once RSTP has converged and is stable?
a.) learning
b.) listening
c.) blocking
d.) discarding

6.) What is the default hello timer of RSTP?
a.) 2 seconds
b.) 4 seconds
c.) 6 seconds
d.) 1 second

7.) RSTP edge-type are similiar to what STP feature?
a.) BPDUguard
b.) portfast
c.) trunk ports
d.) BPDU filter

8.) In RSTP what switch is responsible for sending BPDUs?
a.) root switch
b.) designated switches
c.) access switches
d.) all switches on the LAN segement

9.) How does a switch running RSTP respond when it recieves a BPDU with a higher value than its current root?
a.) discards the BPDU
b.) places all other link type connections in discarding except the port which recieved the new BPDU
c.) places all ports into forwarding that recieve BPDUs
d.) Starts negotionation, but keeps all ports in forwarding

10.) Which of the following is the IEEE standard for Rapid Spanning-tree?
a.) 802.1d
b.) 802.1q
c.) 802.11b
d.) 802.1w

RSTP Notes

Answer Key
1.)c
2.)c,d,e
3.)a
4.)c
5.)d
6.)a
7.)c
8.)d
9.)b
10.)d

Root Switch & Port Exam Question Tactics

In this section we will learn how to approach questions on the exam pertaining to the root port and root switch in an STP configuration. Below shows an overview of the three step process.

Tactical Overview

  1. Issue show spanning-tree and or show spanning-tree root commands on available switches
  2. Examine the show spanning-tree output, because it lists the root cost in two places.Once at the top of the output and once at the bottom of the output. Remember the bottom section is the interface's cost not the total root cost. Use the CDP neighbor command to verify the device name of devices.
  3. For problems that require root cost calculations, do the following
    1. memorize default STP cost values
      100 for 10Mbps19 for 100Mbps4 for 1Gbps2 for 10Gbps
    2. look for indications of spanning-tree cost configuration commands, do not assume default values are used
    3. when default values are used check the actual speed of the link as well. Cisco switches choose the STP default cost based upon the speed of the link, not the maximum speed allowed on the link

Example Problem

Three Cisco 2960 switches are connected together on one LAN segment. Switch0, Switch1 and Switch2 are all connected and running a fully converged STP configuration. From Switch 2 determine which switch has been elected the root and it's cost from Switch2.


Step 1 - Console into the device and issue the show spanning-tree command

Step 2 - Examine the Output

By examining the output we can quick notice that this switch is not the root. The first indication is that the Root ID in the top section of the output lists a different BID than the Bridge ID output, this is our first clue. The second is at the bottom in the columns labeled Role and Sts, if this switch was the root it would not have a root port and it would also have all ports in a forwarding state and none in a blocking state. This confirms that Switch2 is not the root. Fa0/1 however has a Role of Root meaning that Fa0/1 was elected the root port and off that interface is the Root Switch, Switch0. To confirm this theory we can use the cdp neighbors command.


Step 3 - Determine the cost to the root

By examining the output of the show spanning-tree command again we can easily find the cost. Listed in the column labeled cost in the row for Fa0/1 is the root cost of 19. This value was chosen by STP because the cable connecting the two switches is a FastEthernet cable with a speed of 100mbps.

Additional Questions

Root Switch Election & Cost Calculation Quiz

Short LAN Switching Quiz


1.) Which of the following are valid Spanning Tree Modes? (Choose 3)





2.) What settings much match for an Ethernet Channel to form? (Choose 4)





3.) Which of the following are STP transition states?




4.)Which of the following are STP stable states? (Choose 2)




5.) If a switch port is inactive what STP state is it in?




6.) Which of the following would be in a STP forwarding state? (Chose 3)





7.) Which commands would list the STP interface state of fa0/2?




8.) Which of the following ways removes the need for spanning tree in a two switch network? (Choose 2)




9.) Which of the following command lists the STP mode currently being run on a switch?




10.) The length of time it takes for a switch port to move from listening to forwarding is determined by what STP timer?




Must Knows about Spanning-Tree Roles, Ports, and States

There many things to know about Spanning-Tree, however knowing the difference between a few key terms will help you earn some points on the exam. The three things we'll be covering in this post is the difference between STP Roles, Modes and States.

Spanning-tree Port Roles

Spanning-tree roles describe what the switch's responsibility is for the LAN segment running STP. For example if a switch was elected the Root Switch, it's role would be to promote it's self as Root and start sending out BPDUs to other switches indicating the timers of STP such as the forward delay timer. There are only four types of Roles; Root, designated, alternate, and backup.

Root Port Role - The port that receives the best BPDU on a bridge is the root port. This is the port that is the closest to the root bridge in terms of path cost. The STA elects a single root bridge in the whole bridged network (per-VLAN). The root bridge sends BPDUs that are more useful than the ones any other bridge sends. The root bridge is the only bridge in the network that does not have a root port. All other bridges receive BPDUs on at least one port. 

Designated Port Role - A port is designated if it can send the best BPDU on the segment to which it is connected. 802.1D bridges link together different segments, such as Ethernet segments, to create a bridged domain. On a given segment, there can only be one path toward the root bridge. If there are two, there is a bridging loop in the network. All bridges connected to a given segment listen to the BPDUs of each and agree on the bridge that sends the best BPDU as the designated bridge for the segment. The port on that bridge that corresponds is the designated port for that segment. 

Alternate & Backup - These two port roles correspond to the blocking state of 802.1D. A blocked port is defined as not being the designated or root port. A blocked port receives a more useful BPDU than the one it sends out on its segment. Remember that a port absolutely needs to receive BPDUs in order to stay blocked. RSTP introduces these two roles for this purpose.

An alternate port receives more useful BPDUs from another bridge and is a port blocked.

Information from http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml#topic2

Spanning-Tree Modes

Spanning-Tree modes are simply the version of spanning-tree running. Over the years several different versions have been developed some Cisco Proprietary some created by the IEEE. Below lists these modes.

802.1D Spanning-tree 
PVST+ or PVSTP - Per-vlan spanning tree "Cisco Proprietary"
802.1W Rapid Spanning-tree
Rapid PVST+ or PVSTP Cisco Proprietary version of RSTP

Spanning-tree Port States

For this post we're listing only the states for interfaces that are running 802.1D normal spanning tree protocol RSTP 802.1W has different names for these ports.

Transition States
Listening
Learning  

Stable States
Blocking
Forwarding
Disabled 

ICND2 200-101 Exam Topic Mind Map


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