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STP Questions

February 6th, 2017 Go to comments

Question 1

Explanation

First by comparing their MAC addresses we learn that switch B will be root bridge as it has lowest MAC. Therefore all of its ports are designated ports -> C & D are correct.

On the link between switch A & switch C there must have one designated port and one non-designated (blocked) port. We can figure out which port is designated port by comparing their MAC address again. A has lower MAC so Fa0/1 of switch A will be designated port while Fa0/1 of switch C will be blocked -> B is correct.

Question 2

Explanation

The switches compare their Bridge ID with each other to find out who will be the root bridge. The root bridge is the bridge with the lowest bridge ID.

Bridge ID = Bridge Priority + MAC Address

In this question the bridge priority was not mentioned so we suppose they are the same. Therefore the switch with lowest MAC address will become the new root bridge.

Question 3

Explanation

First by comparing their MAC addresses we learn that switch B will be root bridge as it has lowest MAC. Therefore all of its ports are designated ports -> C & D are correct.

On the link between switch A & switch C there must have one designated port and one non-designated (blocked) port. We can figure out which port is designated port by comparing their MAC address again. A has lower MAC so Fa0/1 of switch A will be designated port while Fa0/1 of switch C will be blocked -> B is correct.

Question 4

Explanation

This is a tricky question. We know the switch with lowest value of priority is elected the root switch. Therefore in this question the switches with priority of 4096, 8192, 12288 (which are lower than the current value of the root bridge 16384) are not joining the root bridge election by somehow. The only suitable answer is the switch with priority 20480 will become the root bridge.

Question 5

Explanation

Multiple Spanning Tree (MST) rides on top of RSTP so it converges very fast. The idea behind MST is that some VLANs can be mapped to a single spanning tree instance because most networks do not need more than a few logical topologies.

Question 6

Explanation

Cisco developed PVST+ to allow strolling numerous STP instances, even over an 802.1Q network via the use of a tunneling mechanism. PVST+ utilizes Cisco gadgets to hook up with a Mono Spanning Tree area to a PVST+ region. No particular configuration is needed to attain this. PVST+ affords assist for 802.1Q trunks and the mapping of a couple of spanning trees to the single spanning tree of popular 802.1Q switches strolling Mono Spanning Tree.

Question 7

Explanation

SPT must performs three steps to provide a loop-free network topology:

1. Elects one root bridge
2. Select one root port per nonroot bridge
3. Select one designated port on each network segment -> Answer B is correct.

Question 8

Explanation

For PVST and PVST+, any change in the STP topology will result in a Topology Change Notification (TCN) BPDU. The TCN tells the switches that a change in the topology table has occurred, and they must therefore flush their Content-Addressable Memory (CAM) tables. Switches will set their CAM tables to age out after ForwardDelay seconds, which is 15 seconds by default. In other words, if a host doesn’t send traffic within 15 seconds to update the CAM table, the switch will have to begin flooding traffic to that host. This can lead to excessive amounts of flooded traffic.

For more information please read: http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/12013-17.html

Question 9

Explanation

From the output, we see that all ports are in Designated role (forwarding state) -> A and E are correct.

The command “show spanning-tree vlan 30″ only shows us information about VLAN 30. We don’t know how many VLAN exists in this switch -> B is not correct.

The bridge priority of this switch is 24606 which is lower than the default value bridge priority 32768 -> C is correct.

All three interfaces on this switch have the connection type “p2p”, which means Point-to-point environment – not a shared media -> D is not correct.

The only thing we can specify is this switch is the root bridge for VLAN 30 but we can not guarantee it is also the root bridge for other VLANs -> F is not correct.

Question 10

Explanation

First by comparing their MAC addresses we learn that switch B will be root bridge as it has lowest MAC. Therefore all of its ports are designated ports -> C & D are correct.

On the link between switch A & switch C there must have one designated port and one non-designated (blocked) port. We can figure out which port is designated port by comparing their MAC address again. A has lower MAC so Fa0/1 of switch A will be designated port while Fa0/1 of switch C will be blocked -> B is correct.

Question 11

Explanation

PortFast BPDU guard prevents loops by moving a nontrunking port into an errdisable state when a BPDU is received on that port. When you enable BPDU guard on the switch, spanning tree shuts down PortFast-configured interfaces that receive BPDUs instead of putting them into the spanning tree blocking state.

In a valid configuration, PortFast-configured interfaces do not receive BPDUs (because PortFast should only be configured on interfaces which are connected to hosts). If a PortFast-configured interface receives a BPDU, an invalid configuration exists. BPDU guard provides a secure response to invalid configurations because the administrator must manually put the interface back in service.

Reference: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst4000/8-2glx/configuration/guide/stp_enha.html

Question 12

Explanation

When you enable PortFast on the switch, spanning tree places ports in the forwarding state immediately, instead of going through the listening, learning, and forwarding states.

Comments (6) Comments
  1. Sisi
    February 21st, 2017

    Q8 is referring to RSTP so it would be answer “D”

  2. Anonymous
    March 10th, 2017

    Question 4 throughs me off, why is it not answer C since is the lowest bridge ID 4096? can someone be kind and explain this to me please…

  3. Anonymous
    March 12th, 2017

    @anonymous because the roots bridges had higher priority than those so the lower IDs are disregarded as they are not part of the election.

  4. Anonymous
    March 16th, 2017

    any1 with latest icnd1 dumps please email {email not allowed} im writing end of march please

  5. Anonymous
    March 16th, 2017

    {email not allowed}

  6. Anonymous
    March 28th, 2017

    Q3 is equal to Q1, please update