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Conceptual easy question on Trunk/load balance

Hi, can you help me with this question:

I understand that I could configure 'spanning-tree vlan x port-priority' and

make a trunk load balance. However I don't understand why the answer below is a). Can you please clarify ?

SWITCH-A (root) SWITCH-B

PortFa0/1------------------------- Fa0/1

PortFa0/2------------------------- Fa0/2

Question:

VLAN1 and VLAN2 are configured between switches SWITCH-A

and SWITCH-B. How can you make the Siwtches load balance

VLAN traffic between SWITCH-A and SWITCH-B ?

a) Lower the port priority for VLAN1 on port 0/1 for SWITCH-A

b) Lower the port priority for VLAN1 on port 0/2 for SWITCH-A

c) Make the bridge ID of Switch SWITCH-B lower than Switch-A.

d) Enable ports 0/1 and 0/2 on Switch-A and 0/1 and 0/2 on switch

SWITCH-B to be trunk ports.

1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

Re: Conceptual easy question on Trunk/load balance

Hi Friend,

When you have configured fa0/1 and fa0/2 both as trunk one of them will go into blocking and because they are parallel link portcost, port priority all will be same so the only option left is PORT ID which can be a deciding factor which port should go into blocking state.

Now because fa0/2 has a higher port id it will go into blocking stae and fa0/1 will be fwding state.

I hope I am clear till here.

Now when for both the vlans fa0/1 is fwding and fa0/2 is blocking and you want a load balancing for vlans on trunk the only option left is to change or lower the port priority for VLAN 1 on fwding link which is fa0/1 and once done the port priority for vlan 1 will be having higher value on fa0/1 and fa0/1 will become blocking for vlan 1 and will stay fwding for vlan 2 and fa0/2 will fwd vlan 1 and will stay blocking for vlan2.

The trunk port with the lower priority (higher values) for the same VLAN remains in a blocking state for that VLAN.

HTH

Ankur

*Pls rate all helpfull post.

6 REPLIES
Silver

Re: Conceptual easy question on Trunk/load balance

Based on this info. I cannot identify the correct ans. unless there is more info. I wonder if the ans. is (a), there should be more info. that related to the config. then we can discover the hint.

Moreover, where do you find the ans., is it a reliable source ?

Hope this helps.

Re: Conceptual easy question on Trunk/load balance

Hi Friend,

You are right. The answer should be b) and not a)

Since the trunk is between two switches the port ID is the only parameter that is different and is taken for loadbalancing.

As a result all traffic will be through port fa0/1 as it is the lowest ID

In order to load balance, the VLAN 1 priority needs to be reduced on trunk port fa 0/2 so that vlan 1 traffic is forwarded via 0/2 while traffic to VLAN 2 remains through fa 0/1

check this link for more details

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00800ae96a.shtml

HTH, rate if it does

Narayan

Cisco Employee

Re: Conceptual easy question on Trunk/load balance

Hi Narayan,

The answer will definetely be (a).

The port priority and and its value are 2 different things and may a times get confusing.

When we say lower priority its a comparison between the 2 values which is set on 2 different trunks for a particular vlans.

So if we set a port priority for vlan 1 example as 10 on one trunk and 20 on another trunk the lower value is 10 which will be having a higher priority so it will be in fwding state and when we say lower priority it means higher value.

In the example posted by original poster when option 1 says lower prioriy means higher value assigned to vlan 1 on fa0/1 which will make vlan 1 blocking on fa0/1.

Hope I am able to clear my point.

Ankur

Cisco Employee

Re: Conceptual easy question on Trunk/load balance

Hi Friend,

When you have configured fa0/1 and fa0/2 both as trunk one of them will go into blocking and because they are parallel link portcost, port priority all will be same so the only option left is PORT ID which can be a deciding factor which port should go into blocking state.

Now because fa0/2 has a higher port id it will go into blocking stae and fa0/1 will be fwding state.

I hope I am clear till here.

Now when for both the vlans fa0/1 is fwding and fa0/2 is blocking and you want a load balancing for vlans on trunk the only option left is to change or lower the port priority for VLAN 1 on fwding link which is fa0/1 and once done the port priority for vlan 1 will be having higher value on fa0/1 and fa0/1 will become blocking for vlan 1 and will stay fwding for vlan 2 and fa0/2 will fwd vlan 1 and will stay blocking for vlan2.

The trunk port with the lower priority (higher values) for the same VLAN remains in a blocking state for that VLAN.

HTH

Ankur

*Pls rate all helpfull post.

Silver

Re: Conceptual easy question on Trunk/load balance

Thanks all that I overlooked the *root* in the diagram.... :(

Re: Conceptual easy question on Trunk/load balance

Thanks ankur,

Its really confusing.

Lowering priority doesn't really mean lowering the values.

Narayan

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