Cisco 3850 Stack SPAN port limitation

[ctgill]

Hello,

     I have read the documentation on the 3850 SPAN port limitations which states that you are limited to 4 SPAN ports.

"On each switch, you can configure a maximum of 4 (2 if switch is stacked with Catalyst 2960-S switches) source sessions and 64 RSPAN destination sessions. A source session is either a local SPAN session or an RSPAN source session"

I do not currently have access to the hardware and was curious if this would be limited to 4 per stack or if in a stack of 5 switches we'd have access to 20 SPANs.

We are designing for 3 SPANs and curious if we could build in redundancy to 6 on the stack.

thanks,

[Peter Paluch]

Hi,

If I am reading the quotation correctly, it does not refer to the limit the number of SPAN ports, either ingress or egress, but rather to the total number of simultaneously running monitoring sessions. There can be at most 4 of them but each one can have as many source and destination ports as necessary.

Would this answer your question?

Best regards,
Peter

[ctgill]

Yes, I was referring to monitor session.  Intuitively it would seem that if the individual switches could each handle 4 monitor sessions, then the stack would support more than 4 but unsure without equipment access.

Thanks for the response,

[Peter Paluch]

Hi,

I am also unsure about this, as I do not have access to this kind of equipment, either. I would suppose, though, that having multiple swiches in a stack is not going to increase the number of simultaneous SPAN sessions, simply because the switch stack operates as a single switch, and there is a distinct possibility of doing a local SPAN even though the source port is on one stack member while the destination port is on another. If a single switch cannot handle more than 4 SPAN sessions itself, it can hardly handle more SPAN session if being a stack member where multiple other stack members may direct their SPANned traffic to it.

This is of course just a hypothesis and I cannot say nothing for certain but this would be my logical conclusion. As every so often, I get "logical" things very wrong, though, so take this just as an attempt for an educated guess.

Best regards,
Peter