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New Member

Collapsed core design and spanning tree

The recommended design for a collapsed core is a L3 link between the core/distribution switches. While I understand it removes a possible loop path, I'm having a hard time figuring out the proper spanning tree configuration. I just labbed the design below and this is what I got.

Traffic from the workstations on the access switches goes thru CLSMTL01 as expected. However, assuming the link between ALSMTL02 and CLSMTL01 breaks, based on the spanning tree topology, traffic would flow from ALSMTL02 to CLSMTL02 to ALSMTL01 to CLSMTL01 instead of routing between CLSMTL02 and CLSMTL01.

What am I missing here?             

Screen Shot 2012-01-13 at 10.17.21 AM.jpg

  • LAN Switching and Routing
New Member

Collapsed core design and spanning tree


I'm not sure if the layer 3 connection makes sense, in your stated design I would suggest a overall layer 2 topology.

Nevertheless let's have a look at your questions:

Let's assume we have two PCs: PC1 connected to ALSMTL01 and PC2 connected to ALSMTL02.

In a stable topology a frame from PC1 to PC to would go:

PC1 -> ALSMTL01 -> CLSMTL01 -> ALSMTL02 -> PC2

If the link between ALSMTL02 and CLSMTL01 breaks:

PC1 -> ALSMTL01 -> CLSMTL02 -> ALSMTL02 -> PC2

And vice versa.

Due to the fact that you have the same Layer 2 domain shared on all switches the Layer 3 connection

would never be used.



New Member

Collapsed core design and spanning tree

My initial design was a L2 etherchannel between the two core switches. I am currently studying for the CCNP and the CCNP Switch book says that a Collapsed Core topology should have a L3 link between the two core switches.

The actual design I posted here is just a subset/my test lab environment.

Each access switch will have it's own local VLAN for data, with SVIs configured at the core. So there is interVLAN routing involved... The traffic would not likely flow between the access switches but between the access switches - through the core switches - to data center switches

I had it all figured out, MST with 2 instances, split VLANs between the instances etc, the design was done. Then I read this thing about an L3 layer and I'm completely confused lol. I have to finish this architecture by next wednesday so I'm working hard at figuring this out

Thanks for your input!

Collapsed core design and spanning tree

I'm not sure what the book would be talking about with a L3 port-channel with a collapsed core design.

My take on a collapsed core design has already been your Core and Distrbution layer are basically the same thing. You don't have a campue lan enviornment in which you have access layer-> distribution layer -> core layer ->----Other Buildings etc..

You basically have a access layer->distribution/core.

For Inter-Vlan data, the traffic flow would be, access layer -> (vlan interface on core CLSMT01), and then it would arp out

whatever ip address that you want to contact.

For Intra-vlan data, it would just ARP out the address of the destination.

As always, L2 traffic will follows the spanning-tree loop-free path to the destination.

New Member

Collapsed core design and spanning tree

You are right. Collapsed concept is simply core/distribution in one. But they recommend the L3 port-channel between the core/distribution switches to reduce loops risks, which is understandable. My problem is figuring out which spanning tree topology is needed/would work best. Obviously the 2 core switches would be the root bridges...

I'm starting to see the light here lol

Most of my traffic will be routed (interVLAN) and L2 traffic will be local to the switches (except the management network), so that's where the routed interface would come into play?

I think I still need to go with MST to limit the number of instances of spanning tree... This is quite a challenge for me! I'm an IT vet of 22 years but have had limited expose to the telecom side. I'm thoroughly enjoying this

Collapsed core design and spanning tree

Yeah, I understand what you're saying about the L3 ethechannel. Personally, if it were me, I would just stick with RPVST+, I would guess if you have 60 or less vlans or maybe more, RPVST+ should be just fine. When you get into having large amounts of VLANs I would for sure look into MST. Another reason you might want to use MST, is it you have multiple switch vendors, you should probably run MST.

New Member

Re: Collapsed core design and spanning tree

Here's the whole design, might give a better picture as to what I'm trying to do... a work in progress mind you lol

I do have quite a few VLANs, and a non Cisco switch in the design (BNT switch in an IBM Bladecenter H chassis).

New Member

Collapsed core design and spanning tree

Thanks for brainstorming with me about this, I really appreciate