STP Questions 3

Question 1

Explanation

The command “spanning-tree port priority 0” is better than the command “spanning-tree port priority 16” as interface G1/0 is preferred over other interfaces with lowest priority port ID. So answer E is better than B.

Question 2

Question 3

Explanation

If you configure “spanning-tree portfast” then it will only takes effect on access ports (portfast is still disabled on trunk ports). To enable portfast on a trunk port you need the trunk keyword (“spanning-tree portfast trunk”).

Note: The command “spanning-tree portfast default” can only be configured under global configuration mode but this question requires to “apply on the interfaces”.

Question 4

Explanation

The full macro command is “spanning-tree vlan <vlan-id> root primary”. This command will adjust its priority to a lower value (than the value of the current root bridge), so it will become the new root bridge for the specified VLAN, so this command basically generates a macro. But this keyword does not adjust STP timers so maybe this requirement does not exist in the exam.

If the question asks about “automatically adjust STP timers” only then the answer should be “diameter”:

Use the diameter keyword to specify the Layer 2 network diameter (that is, the maximum number of switch hops between any two end stations in the Layer 2 network). When you specify the network diameter, the switch automatically sets an optimal hello time, forward-delay time, and maximum-age time for a network of that diameter, which can significantly reduce the convergence time. You can use the hello keyword to override the automatically calculated hello time.

Reference:

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3560/software/release/12-2_50_se/configuration/guide/scg/swstp.html

Question 5

Question 6

Explanation

From the second command output (show spanning-tree mst) we learn that MST1 includes VLANs 10 & 20. Therefore if we want DSW1 to become root bridge for these VLANs we need to set the MST 1 region to root -> The command “spanning-tree mst 1 root primary” can do the trick. In fact, this command runs a macro and sets the priority lower than the current root.

Also we can see the current root bridge for these VLANs has the priority of 32769 (default value + sysid) so we can set the priority of DSW1 to a specific lower value. But notice that the priority must be a multiple of 4096. Therefore D is a correct answer.

Question 7

Explanation

By calculating and assigning the port cost of the switch ports, you can ensure that the shortest (lowest cost) distance to the root switch is used to transmit data. You can calculate and assign lower path cost values (port costs) to higher bandwidth ports by using either the short method (which is the default) or the long method. The short method uses a 16-bit format that yields values from 1-65535. The long method uses a 32-bit format that yields values from 1-200,000,000.

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

The port cost depends on the port speed; the faster interface speeds indicate smaller costs. MST always uses long path costs.

Reference: http://www.cisco.com/c/m/en_us/techdoc/dc/reference/cli/nxos/commands/l2/spanning-tree-mst-cost.html

Question 8

Question 9

Question 10

Explanation

We suppose there is no Etherchannel configured between ASW1 and DSW1.

For STP, the cost of 10Gbps is 2 while the cost of 1Gbps is 4 so the paths from ASW1 -> DSW1 -> DSW2 and ASW1 -> DSW2 have the same accumulate STP Path cost (= 4). If multiple paths are available to reach the Root Bridge (Root Switch) with the same accumulated Spanning Tree Path Cost in a Non-Root Switch, select the port connected to the neighbor switch which has the lowest Switch ID value as the Root Port. Therefore the 1Gbps port on ASW1 will be chosen as Root port (as DSW2) has the lowest Switch ID value -> traffic will go from ASW1 -> DSW2 -> core. 

Question 11

Explanation

From the output we see DSW2 is running in RSTP mode (in fact Rapid PVST+ mode as Cisco does not support RSTP alone). When a new switch running PVST+ mode is added to the topology, they keep running the old STP instances as RSTP (in fact Rapid PVST+) is compatible with PVST+.

Question 12

Question 13

Explanation

Use the diameter keyword to specify the Layer 2 network diameter (that is, the maximum number of switch hops between any two end stations in the Layer 2 network). When you specify the network diameter, the switch automatically sets an optimal hello time, forward-delay time, and maximum-age time for a network of that diameter, which can significantly reduce the convergence time. You can use the hello keyword to override the automatically calculated hello time.

Reference: https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3560/software/release/12-2_50_se/configuration/guide/scg/swstp.html