Stackwise Data plane forwarding

[gcube]

Hi All,

I'm wondering how does stackwise (for 3850, 3650) features handles data traffic. As an example, if there's a traffic sourced from a port on switch 1 and destined to switch 2, how will the traffic/packet being forwarded to Switch 2? Does the packet leave the stacked or it will be forwarded through the stacked links and the packet will just stayed on the stacked switches? 

Also, how does local switching works in a stacked switches. If someone can give an example to get a clear picture will be greatly appreciated. 

Thanks in advanced. Looking forward to your inputs. 

[Joseph W. Doherty · Accepted solution]

If switches 1 and 2 are part of the same functional stack, then the packet will be transferred using a stack link. However, unsure whether it would take the shortest stack path, or the stack port with the least load, or it just hashed to one or the other.

I don't recall seeing documentation for how a 3650/3850 stack functions in all cases. For the earlier 3750 StackWise, all packets were placed on the stack ring, and removed by the source switch. For the subsequent 3750 StackWisePlus, only unicast that had to go to another switch was placed on stack ring, and such were removed by destination switch; multicast, though was all placed on stack ring (don't recall mention of which switch removed, but suspect multicast removed by source switch).

[gcube]

Joseph, 

Thanks for the reply and effort. Appreciate it. 

So it means that as long as the data traffic/packets sourced from one of the Switch and destined to another Switch of the same Stacked, those packets are forwarded on the Stack links (stacking cable) between the switches? 

Coz I have an impression that only Management and control traffic are only packets being forwarded on the Stack links.

[Joseph W. Doherty]

"Coz I have an impression that only Management and control traffic are only packets being forwarded on the Stack links."

No, the stack links are also used for data traffic. The stack links are sort of the "fabric" for the stacked switches. This is also why those stack links support the bandwidth they do, i.e. to provide the high bandwidth for inter-switch data transfers.

What might have caused some confusion, for VSS pairs, their cross links only pass data traffic is there's no other path.