- Cisco Networking: STP Switch Sim | Spanning Tree priority Configuration

Question:

A company has an existing network comprised of 5 switches; 

·         CoreSwitch

·         DSW1

·         DSW2

·         AcessSwitch1

·         AcessSwitch2

This switch network has pre-VLAN spanning tree mapping. You has the following tasks to complete this Lab:

– CoreSwitch should be the root bridge for VLAN 20, but currently DSW1 is the root bridge for VLAN 20.
– Traffic for VLAN 30 should be forwarding over the gig 1/0/6 trunk port between DSW1 and DSW2. However VLAN 30 is currently using gig 1/0/5.
– Traffic for VLAN 40 should be forwarding over the gig 1/0/5 trunk port between DSW1 and DSW2. However VLAN 40 is currently using gig 1/0/6.

You task is complicated by the fact that you only have full access to DSW1, with the enable password cisco. You are allowed and provided only limited show commands access is provided on CoreSwitch, and DSW2 using the enable 2 level with a password of Cisco. You are not allowed to made configuration changes on these Switches.  Also you don’t have access to ASW1 or ASW2. So you are only limited to make configuration on DSW1.

Required Configurations and Explanation:  

1.  “CoreSwitch should be the root bridge for VLAN 20.

DSW1 is currently the root bridge for VLAN 20″ our task to make CoreSwitch the root bridge for VLAN 20. For making the CoreSwitch as a root bridge we need to assign the lowest priority value for VLAN 20 on CoreSwitch, but we are not allowed to make changes on this Switch we have only access to DSW1 therefore we can increase the priority of DSW1 so that it become higher than CoreSwitch. For this use the “show spanning-tree” on CoreSwitch and on DSW1 for checking the priority of both switches for VLAN 20. Currently DSW1 is the root bridge for VLAN 20 (notice the line “This bridge is the root” on below figure).

DSW1>enable
DSW1#show spanning-tree

To make the CoreSwitch the root bridge we need to increase the DSW1′s priority value, the best value should be you can use another value but make sure it is higher than the CoreSwitch priority value by checking if the CoreSwitch becomes the root bridge or not; and that value must be in increments of 4096.

DSW1#configure terminal
DSW1(config)#spanning-tree vlan 20 priority 61440 

2.“Traffic for VLAN 30 should be forwarding over the gig 1/0/6 trunk port between DSW1 and DSW2.

VLAN 30 is currently using gig 1/0/5″ and gig 1/0/6 is in blocking state, you can find these information by using command “show spanning-tree”. We know that spaning tree protocol make the forwarding port base on

• Bridge-ID
•  Port-Cost
• Interface-ID

Since both ports have same bridge-ID and both interfaces are fastethernet therefore they will have the same port-cost. When the both ports have same port-cost and bridge-ID then the port with the lower interface will be in forwarding state while other will be in blocking state. Same happen in this case that both switches DSW1 and DSW2 have same bridge-ID and Cost therefore gig 1/0/5 is forwarding port for VLAN 30. Now you can make the gig 1/0/6 as forwarding port by lowering the Priority Number of the port gig 1/0/6 you can see the currently priority numbers of ports by show “show spanning-tree” 

Notice that we only need to change this value for VLAN 30, not for all VLANs.

DSW1(config)#interface g1/0/6
DSW1(config-if)#spanning-tree vlan 30 port-priority 64
DSW1(config-if)#exit

“3. Traffic for VLAN 40 should be forwarding over the gig 1/0/5 trunk port between DSW1 and DSW2.

However VLAN 40 is currently using gig 1/0/6″ and our task is to make the forwarding of VLAN 40 through gig 1/0/5. It is a similar job, but we are not allowed to make any configurations on DSW2, and in this case we can’t change the port-priority for VLAN 40. There is another solution for this you can change the cost (set the lower value) on an interface of DSW1 then only DSW1 will learn the change.

By default, the cost of a 100Mbps link is 19 but we can change this value to make sure that VLAN 40 will use interface Gig1/0/5.

DSW1(config)#interface g1/0/5
DSW1(config-if)#spanning-tree vlan 40 cost 1

DSW1(config-if)#exit

You should re-check to see if everything was configured correctly:

DSW1#show spanning-tree

DSW1#copy running-config startup-config