Hello again Abhishek.
Concerning your questions about the stack design, I will attempt you answer here below:
1. since other 3 switches also have their default priority as 15 how will they automatically become members ?
First of all, the default priority value for switches in a stack is 1. If all of the switches have a priority of 1, then the election takes place using the following criteria. In case of a tie, the next criterion is used:
- The switch with the highest priority value
- The switch that uses a non-default interface-level configuration.
- The switch with the higher Hardware/Software priority. This priority is based on the software version of IOS that is being run by the Switch. These switch software versions are listed from highest to lowest priority:
a)Cryptographic IP services image software
b)Noncryptographic IP services image software
c)Cryptographic IP base image software
d)Noncryptographic IP base image software
- The switch with the longest system up-time.
- The switch with the lowest MAC address."
To find out more, take a look at: http://www.cisco.com/c/en/us/support/docs/switches/catalyst-3750-series-switches/71925-cat3750-create-switch-stks.html?referring_site=RE&pos=1&page=http://www.cisco.com/c/en/us/products/collateral/switches/catalyst-3750-series-switches/prod_white_paper09186a00801b096a.html#anc8
2. If we want one of the switch to become hot standby what needs to be done?
Stacking does not allow for the logic of a hot standby. If a master goes down, the very next switch in line becomes master based on the above criteria. If you want a specific switch to be “second in line” then just make it the second highest priority.
3. once master goes down then hot standby takes over , but if master comes back will it become master again ?
If the original master comes back up, it will not become the master again unless a new election takes place. An election is triggered only under the following circumstances:
- When the whole switch stack is reset
- When the stack master is reset or powered off
- When the stack master is removed from the stack
- When the current stack master switch has failed
- When the switch stack membership is increased by adding an additional switch
So, for question 3, it is not possible to have the original master come back up unless a new election is triggered.
I hope this has been helpful!