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Up-link 2 x 3750 Stack to 4 x 3850 stack

All - 

Sorry is this is a rather simple question but I’ve never worked with Stack switches before? I have a remote office that has 3 floors. On one floor we have a 2 3750s in a stack. On another we have a single 3560 and I’m going to be installing a new 3850 stack – the new stack will have 4 x 3850’s...

My question is how should I uplink the other switches to the new stack? In the new stack I have a master a slave and two member switches. Should I create a port channel from the 3750’s and use all available links and spread them across all switches in the new stack, and then do the same with the single switch on the other floor? Or should I create two port-channels from the 3750 and the 3560 to the new 3850 stack?

My thought is that if I create a single port channel form each switch to the new stack I would be basically be removing spanning tree from the environment and basically daisy chaining the switches? Is this a valid solution? Also does the stack act like a chassis where the control plan would be on the master – if so can I spread a interfaces in a port channel across all members in the stack?

Thanks in advance


Hall of Fame Super Gold

The newest "fad" around LAN

The newest "fad" around LAN switching is stacking.  There are so many benefits to this feature:  etherchannel across multiple physical switches that are members to the stack without the need for STP, HSRP, VRRP, etc.  


So let us say you've got a stack (of two) 3750:  3750-1 and 3750-2.  Let's also presume your "core" is a stack of four 3850:  3850-1, 3850-2, 3850-3 and 3850-4.  


Now you've got soooo many ways to choose from.  But you have to bear in mind this factor:  Your SFP ports are LIMITED.  You don't want to "burn" so many SFP ports unnecessarily ... unless you happen to have 3850-12S or 3850-24S.  


If it was up to me, I would set each unit (not stack) of 3750 a single fibre link to the 3850.  It doesn't matter which SFP port you're going to use.  The most important thing is you've got TWO redundant link to the core.  


3750-1 goes to 3850-1

3750-2 goes to 3850-2


Now you've got a 3650 in another floor.  In this particular case, in the name of redundancy, I'd set up two links to the core:  3650 goes to both 3850-3 and 3850-4. 


You configure etherchannel, of course.  

New Member

Hi Leo, Thanks for the

Hi Leo, 

Thanks for the response. 


So you're saying a single port-channel from the 3750 stacks with at least two members to the 3850 stack. 

That was my original thought but I've just never configure a stack before so I wasn't sure about the spanning-tree implications but If it's  single PC then i should have to worry about that. 


Question, Why wouldn't I simply up-link a single PC from the 3560 using both of the GIG up-links to the 3850 stack? I could string it across the members for redundancy, right? 


Also the Cisco LAN CVD says for th 3850 to not use the mast switch for any up-links, rather use the slave and the members because the control plane runs solely on the master switch and it's extra work for that switch. Any thoughts on that?




Hall of Fame Super Gold

That was my original thought

That was my original thought but I've just never configure a stack before so I wasn't sure about the spanning-tree implications but If it's  single PC then i should have to worry about that. 

To configure Etherchannel, you need to put the port(s) into a Trunk.  This eliminates STP.  


One trick to ease you into configuring a switch stack:  DO NOT think of the stack as "multiple switches".  Think of it as configuring ONE SWITCH with multiple slots, like configuring a 6509 or 4510.

Why wouldn't I simply up-link a single PC from the 3560 using both of the GIG up-links to the 3850 stack? I could string it across the members for redundancy, right? 

If you have a server with two NICs and you home each NIC to multiple members of the stack, if one of the switch dies, then the other switch will take over.  They will take over seamlessly because they all share the same configuration.  You configure and/or make changes to the configuration and the entire stack will copy the config.  It's not just redundancy, it's the ease of management.   Look at Cisco's Catalyst portfolio, all the new switch models are now stack-capable.  2960X/XR, 3650, 3850 are now capable of stacking up to 9 switches (not recommended, but that's another topic).  The 4500 and the 6500/6800X can support VSS, another form of stacking.  

Super Bronze

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To take advantage of fewest (logical) devices and redundancy, run a dual Etherchannel from your new 3850 to the 3750 stack and the 3650.  On both the 3850 and 3750 stacks, insure the same Etherchannel links are on different stack members.

Logically, you would no longer need STP, but keep in running in case anyone accidentally creates a L2 loop.

As to what stack members to terminate uplinks/downlinks on, on 3750s, Cisco recommends avoiding the stack master (not possible on your dual 3750 stack).  I don't know what Cisco's recommendations, if any, is for 3850 stacks.

Yes, stacks "appear" much like chassis devices, i.e. stack member ports "appear" like chassis line cards ports.

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