Switch stack

[kamrannaseem]

Hi All,

Just need to confirm what happens if the master switch boots up after the slave, does it become master or slave ?
Many Thanks.

Sent from Cisco Technical Support iPhone App

[JohnTylerPearce]

This depends. If you left everything to their defaults, and setup 2 3750 switches in a stack, one of them will be elected the master. I forget how the whole election process goes.

You can set individual switches and their priority though to influence the master selection.

For Example:

switch 1 priority 15

switch 2 priority 10

With this configuration, Switch 1 will be the master, and as far as I know, if you remove the master switch, switch 2 will become the maste,r and when switch 1 coms back online, an election will be held and switch 1 will be the master.

Please correct me if I"m wrong guys

[acampbell]

Kamran,

When is the stack master elected?

• When the whole switch stack is reset¹

  
  

• When the stack master is reset or powered off

  Note: If you reset the stack master, it would reset the whole stack.

  
  

• When the stack master is removed from the stack

  
  

• When the stack master switch has failed

  
  

• The switch stack membership is increased by if you add powered-on standalone switches or switch stacks.¹

  
  

¹Within these events, the current stack master has a greater chance to get re-elected.

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

Regards,
Alex.
Please rate useful posts.

[kamrannaseem]

Thanks guys,

So if I have two switches master and slave. Slave has booted up and the master fails and boots up let's say after 2 hours, so would it do re election when master is booting up ? which would mean the slave would reset as well.
Thanks guys much appreciated for your help.

Sent from Cisco Technical Support iPhone App

[JohnTylerPearce]

That would trigger an election, so if you haven't manually configured priority values for any of the two switches, then it will go through the election process.

The stack behaves as a single switching unit that is managed by a master switch elected from one of the member switches. The master switch automatically creates and updates all the switching and optional routing tables. Any member of the stack can become the master switch. Upon installation, or reboot of the entire stack, an election process occurs among the switches in the stack. There is a hierarchy of selection criteria for the election.

1. User priority - The network manager can select a switch to be master.

2. Hardware and software priority - This will default to the unit with the most extensive feature set. The Cisco Catalyst 3750 IP Services (IPS) image has the highest priority, followed by Cisco Catalyst 3750 switches with IP Base Software Image (IPB).

Catalyst 3750-E and Catalyst 3750-X run the Universal Image. The feature set on the universal image is determined by the purchased license. The "show version" command will list operating license level for each switch member in the stack.

3. Default configuration - If a switch has preexisting configuration information, it will take precedence over switches that have not been configured.

4. Uptime - The switch that has been running the longest is selected.

5. MAC address - Each switch reports its MAC address to all its neighbors for comparison. The switch with the lowest MAC address is selected.

The above was taking from the 3750 Configuration Guide.

I hope this helps.

[Joseph W. Doherty · Accepted solution]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

The simple thing to remember, once there's an active master, it stays master until something causes the master to go down.  There's no active preemption like what HSRP offers with preempt.

Specifically to your question, if the "slave" boots first, and has become active master, the preferred later booting "master" will not take over.  If both "slave" and "master" are still in the election time period, i.e. "slave" has not yet become active master, then the "master" will become master.

There are multiple rules for which switch becomes master when there's a stack election.  Setting priority influcenses the election such that when correctly set, your choice of switch will become master.  Again, though, this is during a stack election.

BTW, as far as I know, there's normally no technical advantage for which switch becomes master except perhaps with a mixed generation stack (i.e. the later E/X series might have a faster CPU).  One of the default master selection rules, during the election, is the IOS feature set on the switch.  This so the whole stack will have access to the "better" IOS features as the master's IOS becomes the stack's IOS.

[vaibhavSingh86650]

Hi Joseph,

Why slaves serail numbers are empty though all are coming up but few are not coming what could be the reason for this..?

Thanks,
Vaibhav

[Joseph W. Doherty]

Possibly a bug. Contact TAC.