BCMSN Lab - Spanning Tree

Scenario:
Acme is a small export company that has an existing enterprise network comprised of 5 switches; CORE,DSW1,DSW2,ASW1 and ASW2. The topology diagram indicates their desired per-VLAN spanning tree mapping.


Previous configuration atempts have resulted in the following issues:

1.-CORE should be the route bridge for VLAN20; however, DSW1 is currentely the root bridge for VLAN20.
2.-Traffic for VLAN 30 should be forwarding over the gig 1/0/6 trunk port between DSW1 and DSW2. However VLAN 30 is currentely using gig 1/0/5.
3.-Traffic for VLAN 40 should be forwarding over the gig 1/0/5 trunk port between DSW1 and DSW2. However VLAN 40 is currentely using gig 1/0/6.

You have been tasked with isolating the cause of these issues and implementing the appropriate solutions. Your task is complicated by the fact that you only have full access to DSW1, with the enable secret password cisco. Only limited show command access is provided on CORE, and DSW2 using the enable 2 level with a password of acme. No configuration changes will be possible on these routers. No access is provided to ASW1 or ASW2.

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Solution:
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1. You can see that currently the DSW1 is Root bridge for VLAN20. To make CORE the route bridge for VLAN20, increase the priority on DSW1 from 28692 to highest possible i.e 61440 -
(because the lower priority bridge become root)

DSW1#sh spanning-tree vlan 20


DSW1#conf t
DSW1(conflg)#spanning-tree vlan 20 priority 61440

2. "When a switch experiences a tie in regard to the cost to reach the root, the switch first uses the interfaces' port priority values as a tiebreaker. If the port priority values tie, the switch uses the lowest internal interface number."

In this configuration all Gig interfaces has same cost and priority. So switch DSW1 will compare interface number of Gig1/0/5 ( .5) and Gig1/0/6 (.6) for Vlan 30 traffic and send traffic on lower i.e. gig 1/0/5.

DSW1#sh spanning-tree vlan 30

You can see that currently the Gig 1/0/5 interface has lower priority(128.5 «128.6), thats why the traffic is going through it.


To make the traffic for VLAN 30 forward over the gig 1/0/6 trunk port from DSW1 to DSW2, you should change the port priority of Gig 1/0/6 from 128 to 64.

DSW1(config)#int g1/0/6
DSW1(config-if)#spannlng-tree vlan 30 port-priority 64
DSW1(config-if)#no shut
DSW1(config-if)#end

3. To make the traffic for VLAN 40 forward over the gig 1/0/5 trunk port from DSW2 to DSW1, we should change the cost of Gig 1/0/5 from 19 to 1.

You can see that currently the G1/0/6 is Root port for Vlan 40 and G1/0/5 port is in blocked(BLK) state. We must change its state to forward(FWD) by lowering the port cost.

DSW1#sh spanning-tree vlan 40


DSW1(config)#int g1/0/5
DSW1(config-if)#spanning-tree vlan 40 cost 1
DSW1(config-if)#no shut
DSW1(config-if)#exit

Finally don't forget to save your configuration-

DSW1#wr

You can issue sh spanning-tree commands to verify the configuration-

DSW1# show spanning-tree vlan 20
DSW1# show spanning-tree vlan 30
DSW2# show spanning-tree vlan 40


FAQ.-Why can't we configure port-priority or cost for both vlan 30 and vlan 40 ?

Ans.- This is very huge topic to explain. I'll try to summarize it here without complicating it. For this we need to understand the concept of Upstream and Downstream switches.

Switch DSW1 is root bridge (Upstream) for Vlan30 and Downstream for vlan 40.
Switch DSW2 is root bridge (Upstream) for Vlan40 and Downstream for vlan 30.

To influence which port is elected the root port, the two user configurable values to change are port cost and port priority. Port-priority is set on an upstream switch to influence a downstream switch and Cost is set on downstream switch to influence a upstream switch.

Moreover, Changing port cost will effect both the local bridge and all downstream bridges. Changing the port priority will only affect the directly connected downstream bridge.

So we'll configure port-priority on DSW1 for vlan 30, as it is a upstream switch for that vlan.

For task 3, if we want port Gi1/0/5 to forward vlan 40 BPDUs, we can force this to happen by either manipulating cost on the downstream switch(DSW1) or increasing the port priority on the upstream switch(DSW2). We could not set the port priority on DSW1 for vlan40, as it would not make any difference.
Moreover you can see that the priority of Gi1/0/5 is already lower than 1/0/6, so we must decrease the cost. Also note that we only have limited show command level access to DSW2 switch, so we can't configure the DSW2 using port-priority command.

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