@vishalbhandari wrote:
Yes, assigning a priority to the other stack members can be beneficial. If you leave SW2 at the default priority of 1, it will be the last choice for becoming the stack master in case SW1 and SW3 fail. However, setting a priority (like 13) ensures that SW2 takes over in a predictable order rather than leaving it to chance. This helps maintain network stability and faster recovery during failures. If the stack is critical, it's best to define clear priorities rather than relying on defaults.
@vishalbhandari could you expand upon how a predictable order increases network stability and is faster for recovery?
If I understand you correctly, you're saying, for a stack of 3 (and/or more members?), if SW1 is priority 15 (and assuming it's the active/master), SW3 is priority 14 (and assuming it's the standby) then if (both?) SW1 and SW3 fail, SW2 becomes active/master faster from it being priority 13 rather than 1, correct? If so, how much faster? The "network stability", you have in mind, is due to a faster recovery, or something else?
It's certainly possible, since multiple criteria (in the 9K series, only switch priority and MAC are considered) can be considered for stack election, that switch priority is considered first, sort of like the first ACE in an ACL. But, I would presume, much like an ACL, processing a single election criteria, before moving onto the next, would be somewhat done as quickly as processing a single ACE, i.e. a delay under a millisecond, possibly in microseconds.(?)
BTW, I've skimmed the following:
Cisco StackWise Architecture on Catalyst 9200 Series Switches White Paper
Catalyst 9300 Stackwise System Architecture White Paper
Stacking and High Availability Configuration Guide, Cisco IOS XE Amsterdam 17.3.x (Catalyst 9300 Switches)
Cisco Catalyst 9300 Series Switches Hardware Installation Guide
Verify and Troubleshoot Stackwise on Catalyst 9200/930
The only Cisco recommendation I noticed, regarding assigning switch priority was:
Stack Member Priority Values
A higher priority value for a stack member increases the probability of it being elected active switch and retaining its stack member number. The priority value can be 1 to 15. The default priority value is 1. You can display the stack member priority value by using the show switch EXEC command.
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Note |
We recommend assigning the highest priority value to the that you prefer to be the active switch. This ensures that the is reelected as the active switch if a reelection occurs.
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So, I'm missing the importance of setting switch priority. A "preference" doesn't seem, to me, too critical.
In the case where, as above, SW1 and SW3 fail, and SW2 become active/master, if SW1 and SW3, individually, or both, rejoin the stack, SW2 will remain the active/master. That said, I'm unsure about whether SW1, if it rejoins the stack after SW3, whether it would take over the standby role.
