Unanswered Question
Sep 18th, 2007
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Still working through most of the BCMSN book and, while I thought I was clear on the majority of the stuff, looks like I still have a lot to learn.

Say I have one access layer switch (with only one vlan) that has dual uplinks to two distribution switches (one dist switch is root while the other dist switch is the backup root). From what I can tell, it is not possible to load balance this traffic over both of these links using the default CST, correct? This is because normal STP will block one of the ports like it is supposed to in order to prevent loops. Ok, I know that...

So, how is one supposed to load balance the vlan (remember it is just one) using say, GLBP? Can PSVT+ be used or would MST be a better choice?

Just reading over MST once really has me pretty lost here. It may be one of the more complicated things to learn in this course...

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Edison Ortiz Tue, 09/18/2007 - 18:52
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You can't load balance one Vlan with Layer2 per your description.

First Hop Redundancy Protocol such as GLBP works at Layer3. The switchport will remain blocked.

With PVST, you can load balance if you have more than one Vlan. You do so by playing with per-vlan priority from the root.

rodman.frowert Tue, 09/18/2007 - 18:57
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So there is essentialy no way to load balance an access layer switch with one vlan, huh?

PVST looks fairly simple to configure, but MST is a whole other story...

At any rate, redundancy can still be maintained with GLBP and STP/RSTP... I guess that is really most important...

rodman.frowert Wed, 09/19/2007 - 06:01
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Something just occured to me and I wanted to run it by the forum...

If one had two Vlans, Vlan 10 on access layer switch 1 and Vlan 20 on access layer swtich 2, then load balancing could only be accomplished uplinking to two distribution switches with PVST+ or MST, correct?

When you say "play with the per-vlan priority on the distributuion switch", you are actually saying that you would set Dist. switch 1 as the root bridge for Vlan 10 and then Dist. switch 2 as the root bridge for Vlan 20. This should allow load balancing for both Vlans over both switches while keeping one port blocked for redundancy, correct? At least, this is how I see it working for PVST+.

CST wouldn't be able to do this because only ONE instance of STP is allowed to run for the entire switch. Doing it the way I proposed, we actually have two version of STP running on each switch...

And, of course, MST could also be used for this example, but I haven't got a good grasp on it quite yet.

Just wanting to make sure my logic is good here....

Kevin Dorrell Wed, 09/19/2007 - 06:07
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That's correct.

Futhermore, if both of your access switches have ports on both VLANs, and each has two uplinks, one to each distribution switch ... then make one distribution switch the root for VLAN 10, and the other the root for VLAN 20, in each case the other being the secondary root.

This way all four links will be used. each switch will pass VLAN 10 to one distribution switch, and VLAN 20 traffic to the other.

Kevin Dorrell


rodman.frowert Wed, 09/19/2007 - 06:50
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Doing it the you and I described, how can GLBP be utilized? One port will always be blocked on so how can both uplinks to both distributution switches be utitilized to provide true load balancing?

jorgenolla Wed, 09/19/2007 - 12:26
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GLBP works at Layer 3 on router ports or SVIs on L3 switches. STP, MSTP, CST, RSTP, PVRSTP, PVSTP, are all layer 2 protocols and run on switchports and not on router ports.

Note that GLBP is only available on the 6500s only, at the moment, and on 12.4T IOS for routers.

Thus the only implementation available in other L3 switches is HSRP. HSRP has the same limitations for load balancing, you manually have to assign what traffic you want in each link; very much like MST, RSTP, PVRSTP and so on.


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