Cisco Stackwise


(Lazaros Agapides) #61

Hello Samer

The master switch is responsible for performing the management tasks of the stack. Which switch you choose to use for the uplinks, whether etherchannel or not, has no affect on the choice of master switch or choice of the backup master switch (with the second highest priority). So if the scenario you described happened, connectivity would still be maintained via the second link from the 5th switch.

Remember that the stacking of switches will take place (ideally) using a closed loop stack. This is where SW1 is connected to SW2, SW2 to SW3, SW3 to SW4, SW4 to SW5 and SW back to SW1 thus creating a loop. If any single switch goes down or if any single stack link between switches goes down, all switches are still connected because of the loop. The only way some switches will become isolated and subsequently disconnected is if two switches, such as SW2 and SW5 fail.

The uplinks however can be on any switch in the stack and even a failure of the master switch will still allow the uplink to function.

I hope this has been helpful!

Laz


(Andy K) #62

If I have 5 switches in a stacked arrangement and have access to each of the members via OOB access (e.g. a terminal server), how can I tell which switch I’m logged into? That is, when I log into each switch in the stack, is there a way to see if I’m logged into the active switch, the standby switch or a member? I can see they are stacked and I can see the stack MAC address, but I just can’t find a command that tells me WHICH switch I’m logged into (the active, standby or member). I’m guessing there’s some way to do this, but just haven’t found it yet.


(Lazaros Agapides) #63

Hello Andy

When connecting to the CLI of an already configured stack of switches, you are always connecting to the master switch. If you use an IP address, you are connected to the master switch. Even if you connect to the console port of a member switch, you are still given access to the CLI of the master, and being provided with configuration abilities for the stack as a whole. If you do want to access the CLI of a particular member, you can do this via the master switch using the remote command.

You can find out more information about this command on page 25 of the following Cisco documentation:

Also, according to page 17 of the above document, it is good practice to use only one CLI session at a time:

Be careful when using multiple CLI sessions to the stack master. Commands that you enter in one session are not displayed in the other sessions. Therefore, it is possible that you might not be able to identify the session from which you entered a command.
We recommend using only one CLI session when managing the switch stack.

I hope this has been helpful!

Laz


(Andy K) #64

Thanks for that info. It’s still kind of odd how you always have access to the master CLI regardless of which switch you’re consoled into. There’s some voodoo going on there.


(Lakshmikant W) #65

Thanks for the explanation Laz, I have few doubts

  1. As per Cisco below is the sequence for Master selection. Can you please explain 2nd point “The switch that uses a non-default interface-level configuration” ?

a.The switch with the highest priority value
b.The switch that uses a non-default interface-level configuration.
c.The switch with the higher Hardware/Software priority. This priority is based on the software
d.The switch with the longest system up-time.
e.The switch with the lowest MAC address

  1. Does Matser re-election occur if we connect one more switch in existing stack ?

  2. Consider, in 4 Switch stack topology 1st switch is Master with priority 15 and rest 3 switches have default priority 1. If we connect 5th switch in stack which already has priority 15, which switch will be Master? Switch1 or Switch 5 ?


(Trust_the P) #66

Good day my friend,

It means the switch that has been pre-configured vs a switch that hasn’t been pre-configured. What do I mean by this? it means the switch which had some configurations before joining the stack will be selected ahead of the switch that had zero configs in its nvram.

Hope you understand it now :slight_smile:


(Lazaros Agapides) #67

Hello Lakshmikant

@sales2161 has got it right. And more specifically, the switches look at the configuration found within the interfaces themselves. If even one interface does not have the default IOS configuration, then it is considered a non-default interface-level configuration and it takes prescience.

If a new switch is added to the stack, an election does not take place. The current stack master is retained. An election will only take place when stacked switches are initially powered up or if the master fails.

Since the addition of a switch does not trigger an election, Switch 1 will retain its position as master. If the switches are powered down and re-powered up, then the election will take place normally using the master selection process.

I hope this has been helpful!

Laz


(Lakshmikant W) #68

Thanks a lot Laz,

Regarding my doubt no 3, can you please check last point listed below, as per Cisco guideline stack master might be re-elected during below actions. I am still doubtful on whether or not master reelection will occur when we add new switch in existing stack


A stack master keeps its role unless one of these events occurs:
•The stack is reset.*
•The master is removed from the stack.
•The master is reset or powered off.
•The master fails.
•The stack membership is increased by adding powered-on standalone switches or switch stacks.*
In the events marked by an asterisk (*), the current stack master might be re-elected based on the listed factors.


(Trust_the P) #69

@wlakshmikant,
I think I understand what you are saying. Yes, master re-election will occur (upon powering on the stack) if you add a new switch while the stack was powered off. However, there will be no re-election if you add that switch while the stack is up and running.

Think of this as OSPF DR/BDR/DROTHER election/re-election on multi-access network.

Please @lagapides, correct me if I am wrong, I want to master these concepts. Thanks


(Lazaros Agapides) #70

Hello Lakshmikant

It seems that @sales2161 has got it. The documentation that you are referring to continues and states the following:

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 become stack members.

Upon power up, elections occur within the first 20 seconds. If you have a stack of 3 switches functioning and you add an already powered on switch to an already powered on stack, that 20 second window has already passed, so no election will take place.

I hope this has been helpful!

Laz