Newbie: 3750 Stack Question

I have two 3750 switches in the same rack interconnected by a cable from one 100mb port in one switch to another port in the other switch. Routing seems to be working fine because workstations on a VLAN on one switch can route to another VLAN on the other switch.

I want to create a stack with the two 3750's. Once I interconnect the two stack cables between the two switches - Will I lose the VLAN and other configurations previously established on each of the switches as I make them into one stack ? Also, Will I be able to unplug the cable that connects the two switches together (referenced in the 1st paragraph) and still have routing thru the new stack cables.

Thanks all

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Think of stacked switches as one big switch

When you migrate from unstack to stacked a master switch will be elected. The master switch gives each individual switch its configuration. All switches must be using the same IOS version.

You should grab a copy of the Cisoc Stakwise technology whiteppaer from

Also see

Catalyst 3750 Switch Software Configuration Guide, 12.1(14)EA1 Managing Switch Stacks

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One of the key things to note is that in a stacked configuration each switch is assigned a switch number and that number is used in the formation of the interface number

To configure 10/100 port 4 on stack member 3, enter this command: Switch(config)# interface fastethernet3/0/4

Do your homework ...

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Thank you Merv

I did come away from my readings of the Cisco docs that I would end up with one logical switch easier for management and failover redundancy and also regarding stack master elections and bypassing the election thru commands.

But I'm still unclear on the questions :

  1. Will my two operational separate switches lose their VLAN config's that are working now once they are combined into a single logical stack/switch (I think the answer is that they won't lose their original VLAN and other configs - but I want to be double sure before I combine them).

  1. Regarding VLAN / routing connectivity once I'm hooked together as a stack, do the Stack cables also provide traffic / throughput for routing between the switches as well as Stack communications or is routing only provided via port to port communications.

I would think the back plane / stack circuitry would provide the routing throughput. The way I have it setup now is that the two separate switches are connected from one port on one switch to another port on the other switch and it won't be needed once I make them into a stack because the stack cables will provide the 32gb/s throughput for routing and stack operations.

Of course, I'll save the original configs of each switch to a TFTP server in case I end up with a reinitialized Master Switch.

thxs - there is alot of homework out there but this will help me along

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If I recall the whitepapers correctly, on the 3750, internally each packet is tagged with a bitmap indicating all of its destination ports (for the entire stack), and then launched onto the bus. The ports indicated in the bitmap copy the packet into the local buffer for transmission. Each member of the stack is kept up to date with the (VLAN,MAC,port) tuples of all the other members.

Thus, the backplane and stack circuitry do not have anything to do with routing on the 3750: the decisions are made locally.

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Walter Roberson

I have not gone thru the conversion from unstacked to stacked.

The documentation clearly indicates that the interface numbers will change in a stacked topology.

I do not know if the conversion process is smart enought to migrate the vlan info from the old interface numbers to the new ones. I would assume that it is not and that you will have to re-configure once the switches are in a stacked configuration If it does well that will be a bonus.

Traffic between switches will flow over the stack cabling and thus you will not need to use ports for that pupose as you are currently doing.

good idea

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Thank you gentlemen !

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