Layer 3 Multi-Chassis EtherChannel (MEC)

[m_schmidt1]

The company I recently started working for is undergoing a network refresh. Geographically, it's just a single but extremely large campus LAN with zero branch offices. The current LAN architecture is a full cisco shop, classic 3 tier core, distribution, access layer hierarchical model. The core consists of two VSS linked 6509's where all Inter-VLAN routing is done, these aren't being touched. The distribution layer in each building/area consists of a single old Catalyst 3560 which currently has two 1Gig fibre uplinks back to each core VSS 6509. All these distribution switches are in scope for this refresh. Layer 2 is currently being spanned all the way up to the core. A friend outside of work has convinced me to bring routing down into the distribution layer.. if not to the access layer even, to protect the core VSS 6509 pair from any broadcast storms or layer 2 issues that could magically happen downstream in the access layer.

I'm planning to put two stacked units to act as the new distribution layer switch for each distribution area of the campus. I've decided I want a Layer 3 link between the core and distribution layer, but I also want resilience, so what I wondered was this - can I run a Layer 3 port channel from the core VSS pair (one interface from each VSS 6509 unit) down to a Layer 3 port channel (cross stack) on a distribution layer stack? Will this work?

Picture of current topology: https://imgur.com/JPAnFLy

Picture of new topology idea here: https://imgur.com/GLZCAIC

Anyone done anything like this? or am I just talking rubbish and this won't work/stupid. I want to know if this is something Cisco recommends doing, or not before proceeding. Will it be okay to do?

[Reza Sharifi]

There is really no need to distribution switches all all. You can simply connect the access switches to the core directly as long as the core switches have enough port capacity. Keep the access switches as layer-2 and the core as layer-2/layer-3. Keep it simple.

HTH

[m_schmidt1]

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@Reza Sharifi wrote:

There is really no need to distribution switches all all. You can simply connect the access switches to the core directly as long as the core switches have enough port capacity. Keep the access switches as layer-2 and the core as layer-2/layer-3. Keep it simple.

HTH

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Hi Reza, the trouble with what you say is this; all those access layer switches are spread out far and wide across the campus, we don't have enough fibre to connect them all back to the two buildings where each core switch is housed directly. That's why all the access layer switches connect back to one larger capacity switch in that particular access layer building/area which consolidates all access layer switches and connects back to the core via fibre optics. Do you see what I mean? So these switches are a single point of failure.

[Reza Sharifi]

Also, in your design, I see you are connecting some devices directly to the VSS core. If these are servers and this is a new design, you should not use the Catalyst for server, storage, VM connectivity, look at the nexus series for that part. If they are not servers, than they should connect to an access switch. If you have to connect a device to the core directly and the core is VSS, that device needs to connect to both VSS switches  and not just one of them.

HTH

[Deepak Kumar · Accepted solution]

Hi,

After considering all your comments, I think you can go with layer 3 architects. Like your plan, Keep L2 and L3 boundary on the distribution layer. If possible send summarization toward the Core Switch only.

Here is VSS recommendation: https://www.cisco.com/c/dam/global/da_dk/assets/docs/presentations/VSS_0109.pdf

[Joseph W. Doherty · Accepted solution]

Answers to your two question are yes (works for L2 too) and yes (well for 2 layer topology, edges directly connected to core and were stacks).