How to decide which Stack Member becomes the Master?

[thilo schueler-mach]

The answer to my question is very easy if we talk about a lot of switches with the same software image and feature set. Because different software images in the stack are not supported and mostly do not run cause of different stacking protocols, but I have read that different feature sets in the same stack are official supported.

Now I have seen a switchstack with different feature sets, two with IP-Service and one with IP-Base Image.

After a electrical power outage all the vrf config on this switchstack was gone. What has happend?

My first idea was that only the two IP-Service Switches are rebooted by the power outage. But that was not true, all three Switches have nearly the same uptime.

The problem is that IP-Base Images are much smaller than IP-Services and during the boot process of the IP-Service switches the IP-Base switch is in the Master Election State und after the wait time the switch run in the final state and get the master role, dropping all the unknown vrf commands from the IP-Service Image.

Is it possibel to prevent a switch become the master of a stack? What I am looking for is a switch master priority "0", tell him that he never could become a master and that he should wait in the master election state until a other switch is online.

Has anybody else seen this problem before and what was the solution if you could not upgrade all switches to the same feature set?

Regards,

Thilo

[Reza Sharifi · Accepted solution]

According to this document when switches boot at the same time and there is no priority assigned, the switch that runs IP Services should become the master.  Now, there is also documentation that says you can run switches in the same stack with different feature sets. This usually works fine until some thing  happens (power outage, software issue, etc) that is when the entire stack will have seizure and things go wrong as it happened to you.

My opinion, you should run the same exact software version and feature sets on all your devices to prevent this from happening. It initially cost more money to go with the same feature sets, but incidents like this can case more financial damages.

Master Switch Election

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

http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps5023/prod_white_paper09186a00801b096a.html

HTH

[andtoth · Accepted solution]

Hi,

I would recommend you to read through the following documentation for more details and examples about 3750 stack management.

Creation and Management of Catalyst 3750 Switch Stacks

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

Best regards,

Andras

[smogra · Accepted solution]

If you want a particular switch not to become a master, change the switch priority of the switch to be added to "1".

switch priority >

Generally, the master id choosen on based rules:

1. The switch that is currently the stack master

2. The switch with the highest stack member priority value

   Note: Cisco recommends that you assign the highest priority  value to the switch that you prefer to be the stack master. This ensures  that the switch is re-elected as stack master if a re-election occurs.

3. The switch that uses the non-default interface-level configuration

4. The switch with the higher Hardware/Software priority. These switch  software versions are listed from highest to lowest priority:

   1. Cryptographic IP services image software

   2. Noncryptographic IP services image software

   3. Cryptographic IP base image software

   4. Noncryptographic IP base image software

   Note: Switches that run Cryptographic or IP services image  will take a longer time to load than Non-Cryptographic or IP base image.  When you power on or reset an entire switch stack, some stack members  will not participate in the stack master election. This is because stack  members that are powered on within the same 20-second time frame  participate in the stack master election and have a chance to become the  stack master. Stack members that are powered on after the 20-second  time frame do not participate in this initial election and only become  stack members. At times, switches with lower software priority can  become the stack master, but all stack members will participate in the  stack master re-election.

5. The switch with the longest system up-time.

  6. The switch with the lowest MAC address

But if you hardcode certain parameters like priority, you can make a particular switch master or slave as you need.

Hope that helps.

Cheers

Sweta

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[Latchum Naidu · Accepted solution]

Hi Thilo,

1. The switch that is currently the stack master

   Note: When stacks merge, the elected stack master would have been the master of one of the merged stacks.

   Note: When stacks partition, the stack master of the original stack will be the master of its partition.

   
   
2. The switch with the highest stack member priority value

   Note: Cisco recommends that you assign the highest priority value to the switch that you prefer to be the stack master. This ensures that the switch is re-elected as stack master if a re-election occurs.

   
   
3. The switch that uses the non-default interface-level configuration
   
4. The switch with the higher Hardware/Software priority. These switch software versions are listed from highest to lowest priority:
   1. Cryptographic IP services image software
      
   2. Noncryptographic IP services image software
      
   3. Cryptographic IP base image software
      
   4. Noncryptographic IP base image software
      

   Note: Switches that run Cryptographic or IP services image will take a longer time to load than Non-Cryptographic or IP base image. When you power on or reset an entire switch stack, some stack members will not participate in the stack master election. This is because stack members that are powered on within the same 20-second time frame participate in the stack master election and have a chance to become the stack master. Stack members that are powered on after the 20-second time frame do not participate in this initial election and only become stack members. At times, switches with lower software priority can become the stack master, but all stack members will participate in the stack master re-election.

   
   
5. The switch with the longest system up-time
   
6. The switch with the lowest MAC address
   

See the below link for more informaiton.

http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps5023/prod_white_paper09186a00801b096a.html

Please rate the helpfull posts.

Regards,

Naidu.