Help stacking 2 3750's

Answered Question
Nov 12th, 2008

Hi,


I am using a Cisco 3750 for my VLAN's off a Cisco ASA firewall. These VLAN's are just for Internet websites, and some WAN's.


Anyway, I have a 2nd Cisco 3750 that is racked below and has the same config which I update manually through the serial port just in case this switch fails I can swap the cables out of the main 3750 and put them is this 2nd 3750.


A very manual failover.


I had a couple of Cisco guys in today for something else and they just said I should stack them both so I can manage both via telnet/ssh and there would be failover should one fail.


They said make sure both 3750's are using the same IOS which they are and when I add the stack cable they will both reboot.


One chap said on the second (spare) 3750 remove all the IP stuff to stop conflicts, is this right? Please look at my config as I would like to do this soon.



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Correct Answer by allan.thomas about 8 years 3 months ago

The simplest option is to auto-provision the switch to the first switch in the stack. The configuration of the second switch can either be auto-provisioned or defaulted when added:-


Take a look at the following document:-


http://www.cisco.com/en/US/products/hw/switches/ps5023/products_configuration_example09186a00807811ad.shtml


The first thing that you should do it ensure you have copies of your configurations.


Prior to provisioning the second switch, configure the first switch to Master, 'switch 1 priority 5', this will ensure that when you add the second switch it is not elected as Master.


The next step is to auto-provision the switch on this Master switch 'switch 2 provision ws-c3750-48ts' providing they are of the same type.


You will then discover that new ports have been added to the configuration starting 2/0/1 - 2/0/48.


Now you have the choice of auto-provising the configuration, by taking the configuration of the ports on the second switch and overlay them on the new port range.


If you don't do this now, all that will happen is that the second switch when added to the stack will have the default configuration applied to it and then you change the port configuration.


Hope this helps.

Allan.


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Correct Answer
allan.thomas Wed, 11/12/2008 - 14:31

The simplest option is to auto-provision the switch to the first switch in the stack. The configuration of the second switch can either be auto-provisioned or defaulted when added:-


Take a look at the following document:-


http://www.cisco.com/en/US/products/hw/switches/ps5023/products_configuration_example09186a00807811ad.shtml


The first thing that you should do it ensure you have copies of your configurations.


Prior to provisioning the second switch, configure the first switch to Master, 'switch 1 priority 5', this will ensure that when you add the second switch it is not elected as Master.


The next step is to auto-provision the switch on this Master switch 'switch 2 provision ws-c3750-48ts' providing they are of the same type.


You will then discover that new ports have been added to the configuration starting 2/0/1 - 2/0/48.


Now you have the choice of auto-provising the configuration, by taking the configuration of the ports on the second switch and overlay them on the new port range.


If you don't do this now, all that will happen is that the second switch when added to the stack will have the default configuration applied to it and then you change the port configuration.


Hope this helps.

Allan.


Pls rate helpful posts.

whiteford Thu, 11/13/2008 - 03:08

Hi Allan,


Thanks for finding the time to answer my question.


This is the show switch command from the main 3750:


H/W Current

Switch# Role Mac Address Priority Version State

----------------------------------------------------------

*1 Master 001j.f738.0d70 1 0 Ready



1.) So on switch 1 (Main live 3750) I simply need to add 'switch 1 priority 5' to the global config?


2.) Then in the global config of the main 3750 add 'switch 2 provision ws-c3750-48ts' as they are exactly the same.


3.) Should I wipe the current config of the 2nd switch first then auto-provising the configuration?


4.) I guess this would need to be done out of hours?


Thanks


allan.thomas Thu, 11/13/2008 - 03:22

Hi Andy,


You have summarised the plan exactly.


You can wipe the configuration of the 2nd switch as a precautionary measure if you like before adding it to the stack, and it is what I tend to do.


But you can still auto-provision the configuration before you add the 2nd switch, hence changing the priority of the 1st switch.


As you can see from show switch command that although the switch is currently Master the priority is 0 which will be the case on both switches. What we want to avoid is the 2nd switch being elected as Master, so providing the 1st switch has a higher priority, then this should be avoided.


I would recommend any changes within a production/live environment to be done OOH, as you cannot guarantee 100% that it will not be service affecting.


Let us know how you get on, best of luck.


Hope this helps.

Allan.


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whiteford Thu, 11/13/2008 - 03:42

Hi Allan,


For my understanding using this auto provision method will it mirror the VLAN's configured on the first switch, I was thinking I'd have to manually configure the ports to match the ports and vlans from the 1st switch to the 2nd?


Sorry for that bad output it says my priority is 1 for this master switch so if I set this to 5 like you said and the second to 1 would this be ok as the higher the better?


Do I set the second one to master or slave?


OOH's it will be.

allan.thomas Thu, 11/13/2008 - 03:58

Hi Andy,


If you have different or additional vlans on the 2nd switch, then ensure that you create these on the Master 1st switch, then you can configure and assign the appropriate VLAN access to your newly provision switchports 2/0/1 - 2/0/48.


Also ensure that these values are enabled on trunk uplinks, if not done so already?


The switch priority by default will be the same on both switches. The higher priority forces the switch to be elected Master during the negotiation.


Therefore when you add the second switch to the stack it will become the standby as it will have a lower priority.


You can change the priority if you like, just as long as the switch you are designating as your Master has a higher value. It should not be necessary though.


The switch will the lower priority will be the standby or slave as I have just mentioned.


Allan.

whiteford Thu, 11/13/2008 - 05:54

Hi Allan,


I just want the second switch to "mirror" the first. So I guess both switches will be master with the first have the higher priority setting of 5 and the 2nd at 1 then as you say the negotiation will take over.


I think I will wipe the config of the 2nd switch first, what is the best command for this?


This reminds be of the VTP server/client method where you set the vlans on the server then they are used on the client.

allan.thomas Thu, 11/13/2008 - 06:21

Hi Andy,


The concept of the switch stack is that it operates as a single switch instance. In this particular scenario both switches will have a copy of the exact same configuration.


When you make a change on the Master the nvram is also updated on the Standby. You will never have a situation in the stack where both are Master, unless they become partitioned, the cascade is broken between them say.


If this happens when you look at the configuration on both switches they will be exactly the same. For example the 2nd switch will have the configuration of all the ports including the ones from switch 1 and switch 1 will have all configurations of the ports from switch 2, the same VLANs, management IP etc...


Remember there is only one common configuration which is copied between switch stack members.


If you are running VTP I would expect both switches to have the same VLANs, if they were both VTP clients that is. So you should not need to add the VLANs from switch 2 to switch 1?


I would be inclined to change the VTP mode to transparent, this enables you to have more control over what VLAN are required.


VTP is only useful if you moving clients between different switches on different VLANs. As the VLAN on these switches remain static (never change), then either set the VTP mode as transparent of leave it disabled.


When you state 'I think I will wipe the config of the 2nd switch first', you only need to erase the config on the 2nd. You will creating all the necessary configuration on switch 1. So that when you join switch 2 to switch 1, switch 1 will then copy the config to it.


Use the write erase command to erase the NVRAM in switch 2 and reload. There


Hope this helps.

Allan

whiteford Thu, 11/13/2008 - 06:44

Thanks for all you time and help, so useful.


Both switches are VTP server mode, so I will leave them like that?


I will take a backup of the switches and plan to do this out of hours.


Great think is I will now have a proper failover scenario as my Active ASA uses switch one and my standby ASA uses switch 2. If the active ASA fails I have to move the cables from the failed ASA the the new active ASA. This would mean all VLANS will stay up.


I like the fact that I dont need to touch the first switch much at all apart from the priority and auto-provision to point to the 2nd switch.


I can't see too much going wrong, as the last thing I want to do is create problems with the first switch.

allan.thomas Thu, 11/13/2008 - 07:19

No problem Andy, glad to help :-)..


If you have spare 3750 switch(s), perhaps you could configure these in advance without having to touch the production ones if makes you feel more comfortable? Then at least you would have a fairly straight forward regression path if required.


Make sure that you do not save any changes until you are happy that everything is operating normally. Then if you do have problems you can simply break the cascade and reload the switch, so that it come ups with the original config.


Hope this helps.

Allan.


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whiteford Thu, 11/13/2008 - 07:43

Yeah good idea!


I only have the 2 3750's to use.


1.) I will make the changes to the first switch.

2.) write erase the 2nd.

3.) insert stac cable

4.) check the 1st switch is happy and working

5.) look at the ports on the 2nd switch and configure the switchport access vlans.



whiteford Thu, 11/20/2008 - 01:01

Hello,


My current main/master switch says "switch 1 provision ws-c3750-48ts"


do I need to change this to "switch 2 provision ws-c3750-48ts

"


allan.thomas Thu, 11/20/2008 - 04:48

Hi Andy, how has it be going?


My current main/master switch says "switch 1 provision ws-c3750-48ts" - This will always be the case with a single 3750 switch.


When a second switch is connected it will be provisioned as switch 2, then a third switch will provisioned as switch 3 and so on..


In either scenario, when you provision the switch manually or simply add the switch to the stack the 'switch 2 provision' is configured additionally. This is how the stack operates as a single entity.


Take a look at the example below, here I have a switch stack with four members, switch 1 will be acting as the master as it was configured with a higher priority:-


switch 1 provision ws-c3750g-24ts-1u

switch 2 provision ws-c3750g-24ts-1u

switch 3 provision ws-c3750g-24ts

switch 4 provision ws-c3750g-24ts


If I wanted to add another 5th switch before physically connecting it, the I would either console onto the stack or telnet to the single management IP address. I would then add the command 'switch 5 provision ws-c3750g-24ts'


After this command has been added, a further 24 ports will be added into the configuration starting from 5/0/1 - 5/0/24.


Hope this helps.

Allan.


Pls rate helpful posts..

whiteford Thu, 11/20/2008 - 06:13

Sorry I haven't rated this yet as I have yet to find the time, but will do.



So the "switch 1 provision ws-c3750-48ts" will become ""switch 2 provision ws-c3750-48ts" so this is done automatically upon connect of the stack cable?

allan.thomas Thu, 11/20/2008 - 06:32

When you add the second switch to the main switch which you have designated as the master, you will then see the command added to the configuration.


Before you add the switch the Config on the master will indicate:-


'switch 1 provision ws-c3750-48ts'


After you connect the second switch to the master the config will be shown as:-


'switch 1 provision ws-c3750-48ts'

'switch 2 provision ws-c3750-48ts


As I have mentioned you can add the 'switch 2 provision' command before you connect the second switch, this is if you wanted to configure the ports in advance.


As you will be creating the stack, it maybe straight forward to connect the switch to the Master, and allow the switch to auto-provision the ports as default.


Hope this helps.

Allan.

Marwan ALshawi Sun, 04/05/2009 - 03:15

very nice instructions Allan


5+ from me :)


Allan from your experience with Stacked switches if i have a swerver with tow NIC teaming and i wanna connect each port to diffrent switch in the stack as redundancy


this will be etherchannel

and make it as access port and put it in a Vlan

the mode is better to be ON

or LACP or what you suggest ?


thank you

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