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3750 Backplane Speed - Is it fixed regardless of stack size?

Hi All,

Just been watching the following QOS video from Cisco Live -

He mentions the 3750 has a 64Gbps backplane (2 x 32Gbps dual counter rotating rings). It is then said that if a switch was added to the stack, the backplane would stay the same. Would the stack backplane not increase if another switch was added or is does the stack as a whole share one backplane of 64Gbs? It's at 1:00:20 in the video that this is all mentioned.


Super Bronze

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Much depends on whether the 3750 series in question has StackWise or StackWisePlus.  The former is "documented" as 32 Gbps, the latter as 64 Gbps.  But, besides the difference in ring bandwidths, the former uses the source device to take the frame off the ring that it placed on the ring while the latter, for unicast, uses the destination device to take the frame off the ring (Cisco calls this spacial reuse).  So, for the former, the ring bandwidth is shared with all stack members.  With the latter, bandwidth sharing depends on where the traffic flows are going.  Best case, for the latter, would be source and destinations are only on neighbors on the ring.  For such a case, each member to member ring adds 64 Gbps.  Worst case, would be multiple switch members needing to transit other switch members but (again for unicast) this is better than source stripping.