3750 Stacking and SPT

Unanswered Question
Jun 13th, 2007
User Badges:


I have a collapsed backbone design with a single C3570 G switch and 3 C3750 access switches.

I plan to have 2 uplinks from the access switches to core so I will stack the 3 access switches.

Q1. If the 3 access switches are stacked and if cross stack uplink fast is used. Will one of the 2 uplinks be blocking ?

Q2. If one of the uplinks in the stack is blocking, will I be able to configure vlan load balancing to get both the uplinks up ?

Q3. Should I use port cost to do the vlan load balancing ?

Any help appreciated.



  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
chriswill Wed, 06/13/2007 - 23:00
User Badges:

Hi Paul,

Is the intention that the 3 stacked switches are going to run vlans? And if so does the uplinks have to be layer 2?

Depending on how many vlans you are intending to deploy you could do port cost to determine which ports will forward each vlan in PVST. You could group together a set of vlans as an instance and use MST or you could put the links at layer 3 and use a routing protocol or even static routing.

With the links given that you are going from a stack to another 3750 you could use etherchannel and let it do a course load balancing for you. The advantage of etherchannel would be that if a link fails it will transmit data over the other link instanaeuosly .



PAUL CHEN Thu, 06/14/2007 - 01:50
User Badges:

Hi Chris,

Thanks for the advise. The 3 stacked switches will have vlans. The customer wants only L2 on the access switches.

So, I can do STP vlan load balancing and that will ensure that both uplinks to the core are utilized ?

Not sure how to do etherchannel because I have 2 separate uplinks at 1st and the 3rd switch to the core ?


Are you are talking about figure 4 in the above CCO link ?

I suppose I can use the

'spanning-tree vlan x cost xxx ' command to load share and get both uplinks to the core forwarding ??

As for MST, is that how you would do it

Switch(config)# spanning-tree mst configuration

Switch(config-mst)# instance 1 vlan 1-20

If I can't do routing, do I configure a default gateway in the access stack to point to the core ?

Thanks again,


Wilson Samuel Thu, 06/14/2007 - 05:20
User Badges:
  • Gold, 750 points or more
  • Community Spotlight Award,

    Mobile User, July 2015

Hi Paul,

After going through your link (which you pasted in the above post) and going through your posts, I would requst you to take a second look in the work Stacking and StackWise Technology, as both of the words resemble a lot but are dramatically different as Cisco StackWise Technology is available only in 3750s.

Please have a look on the following url


Secondly, whenver you have a StackWise technology deployed all the switches act like one single unit, and hence one of the uplinks from a total of 2 would be in blocked mode.

In order to achieve Faul-Tolerance and Load Sharing (if you have more than 1 VLANS) then you may prioritize each VLAN on each of the uplink hence effectively you will achieve that.

However there is an easier solution to it, and thats simply EtherChannel them and both will act like one single link and you get both FT and LS.

Please rate if it helps.

Kind Regards,

Wilson Samuel

PAUL CHEN Wed, 06/27/2007 - 20:18
User Badges:

Hi Wilson,

Thanks for the help, much appreciated. It worked , I tested the stack with MST and Etherchannel.

Best regards,



This Discussion