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Dual stackwise cables for 2 x 3750 switch stack - Any benefit?

David Anderson
Level 1
Level 1

Is there any increase in throughput or redundancy when adding a secondary stackwise cable to a 2 x 3750 switch stack? The "sh switch det" appears to show both up / active but I'm not sure if they are both being used, or if it will even provide redundancy if one fails, any input appreciated - Thanks!

1 Accepted Solution

Accepted Solutions

Joseph W. Doherty
Hall of Fame
Hall of Fame
Yes to both your questions.

Leo mentions when you have both stack cables active, you have "full duplex". Understand, as Bradley information notes, each stack cable is actually already "full duplex", but when both are in operation, the stack uses both, effectively doubling the stack bandwidth.

Bradley also notes in the "real world" you're unlikely to overrun the capacity of a stack ring. Well, if you're talking about the original StackWise, I understand every stack member places all its traffic on the stack ring, even for traffic between ports on the same stack member, and further, the stack member that places traffic on the stack ring is also the stack member that removes it.

StackWisePlus (which came out with the 3750E), both doubles the physical bandwidth on the stack ports, and is also much smarter in how it manages traffic on the ring. For unicast traffic, same switch member traffic is not placed on the ring and traffic to another stack member is removed by that stack member.

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4 Replies 4

brselzer
Cisco Employee
Cisco Employee

Hello David,

 

https://www.cisco.com/c/en/us/products/collateral/switches/catalyst-3750-series-switches/prod_white_paper09186a00801b096a.html

 

"Bidirectional Flow

To efficiently load balance the traffic, packets are allocated between two logical counter-rotating paths. Each counter-rotating path supports 16 Gbps in both directions, yielding a traffic total of 32 Gbps bidirectionally. The egress queues calculate path usage to help ensure that the traffic load is equally partitioned."
 
"Physical Sequential Linkage
The switches are physically connected sequentially, as shown in Figure 3. A break in any one of the cables will result in the stack bandwidth being reduced to half of its full capacity. Subsecond timing mechanisms detect traffic problems and immediately institute failover. This mechanism restores dual path flow when the timing mechanisms detect renewed activity on the cable."
 
So yes it increase throughput and provides redundancy. In the real world it is rare to overrun the stack interface. You are more likely to overrun the front panel interfaces first. 
 
Hope that helps!
-Bradley Selzer
CCIE# 60833

Leo Laohoo
Hall of Fame
Hall of Fame
Yes. The stackwise ring becomes full duplex if using two cables (instead of 1).

Joseph W. Doherty
Hall of Fame
Hall of Fame
Yes to both your questions.

Leo mentions when you have both stack cables active, you have "full duplex". Understand, as Bradley information notes, each stack cable is actually already "full duplex", but when both are in operation, the stack uses both, effectively doubling the stack bandwidth.

Bradley also notes in the "real world" you're unlikely to overrun the capacity of a stack ring. Well, if you're talking about the original StackWise, I understand every stack member places all its traffic on the stack ring, even for traffic between ports on the same stack member, and further, the stack member that places traffic on the stack ring is also the stack member that removes it.

StackWisePlus (which came out with the 3750E), both doubles the physical bandwidth on the stack ports, and is also much smarter in how it manages traffic on the ring. For unicast traffic, same switch member traffic is not placed on the ring and traffic to another stack member is removed by that stack member.

Thank you all for your responses!

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