Stacking two 3750 switchs in production

[Lapecandcie]

Hello,

I work for a customer who needs the network must be up all the time.

They have two 3750 switchs that act at the core network level. They want to create a stack with these two switchs.

These two switches deliver trunks for access switches.

We have a new 3750 switch that is not again in the network but will populate the stack.

Firstly I will create a two-switch stack.

My first switch will act as master switch and the second one will be slave.

I will prepopulate the second switch on the master and I will give them the appropriate priority.

If I have understood the stack procedure the slave switch must be powered off before plugging the cable to avoid a new election process.

If there's no mean to avoid this state or shut down and restart, the switch which is not in the stack could receive the trunk interface from the slave switch, because almost of the access switchs have one trunk from the first 3750 switch and  another one from the second switch.

After I will transfer the trunk interfaces back to the origin switch, and add the third switch to the stack after populating it in the master switch configuration.

Do you think that is a good procedure? Is there another mean to do it more simply?

I have an other question : in the two 3750 switches configuration we have configured HSRP between them  do we need to keep this configuration after creating the stack?

Thank you for your support

Kind regards,

Chris

[glen.grant]

  No you will not use hsrp after you stack the switches because everything will be considered a single switch once this is completed and you will only have one configured SVI for each subnet.  that is also something to consider as right now i assume all gateways are pointed to the virtual gateway of the hsrp interfaces , how are you going to handle that going forward?  Just change the lone remaining SVI with the virtual ip ?  If you are using dhcp you have to consider that as well .   this is assuming you are moving from 2 single 3750's and making a single stack out of them ?  If this is not correct then i apologize.

[Lapecandcie]

Hello Glen,

Thank you for your answer. It's what I have thought in my head. For the gateway I will use the virtual Ip because all hosts are configured with it. For the stack I would process in two steps : Firstly I would create the stack with one switch that is actually in production with the other migrating trunk interfaces to the third switch (this one that will be integrated further in the second step), and secondly I will integrate the third switch so I will form a stack with three 3750 switches at the end. I want to notice that the third standalone switch have been updated with the two switches version that are in production, so that I will reduce the downtime for this operation. The most important point in this operation is to reduce to the maximum the downtime of the network.

Thank you for your support,

Chris 

[Masoud Pourshabanian]

Hello,

To stack several switches, turn them off and connect them with the stack cables. Turn on the master and then slaves. Use command  "show switch" to see the ID and the priority of each switch. The higher priority is more likely to become master. Change the priority of each switch based on the ID you see.

switch ID priority 15 [for the master]

switch ID priority 10 or less [for the slaves].

If some switches are already in production, the new member will pick the lowets priority and become the slave. You should change its priority after.

Please check Glen answer for your second question.

Hope it helps,

Masoud

[Lapecandcie]

Hello Masoud,

Thank you for the answer. During this operation I would avoid to turn off switches, because I have hosts that are in production 24/24. During this operation If I define the good priority values for switches and populate the slave switch into the master confiuration before plugging the FlexStack Cable only the backup switch will be reloaded. Is it right?

Chris 

[Masoud Pourshabanian]

It is correct on the paper; however, in network, uexpected things might happen. You should ask for downtime to avoid any risk. Make sure all switches have the same IOS. 

I am not sure what will happen for hsrp. Because three ips with the same range has been set on the both switches so when you stack them, these ips with the same range can not be on a single switch. Please  look into before starting the operations.

[Ganesh Hariharan]

Hello Chris,

As you are going to build a stack from standalone switches, Check out the best practices and recommendation by cisco for more informtion.

Adding a switch to Stack & Creating and adding Switch

and to answer your second query, Glen pointed right once you have created stack , switches wont act as standalone switches for whole network rather it will be single cluster switch for others. To run an HSRP you need to have separate standalone switches to server traffic from servers to network.

Hope it Helps..

-GI

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[Lapecandcie]

Hello Ganesh,

Thank you for the answer. The problem is that I have two switches that are already in production. So that I want to reduce the downtime for the network. It's for this reason that I want to use the third switch as backup during the operation. Is there a mean to do this withoutshuting down switches during the operation?

Chris