Nonblocking versus Noncontending
“We use a nonblocking fabric…”
Probably not. Nonblocking is a word that is thrown around a lot, particularly in the world of spine and leaf fabric design—but, just like calling a Clos a spine and leaf, we tend to misuse the word nonblocking in ways that are unhelpful. Hence, it is time for a short explanation of the two concepts that might help clear up the confusion. To get there, we need a network—preferably a spine and leaf like the one shown below.
Based on the design of this fabric, is it nonblocking? It would certainly seem so at first blush. Assume every link is 10g, just to make the math easy, and ignore the ToR to server links, as these are not technically a part of the fabric itself. Assume the following four 10g flows are set up—
- B through [X1,Y1,Z2] towards A
- C through [X1,Y2,Z2] towards A
- D through [X1,Y3,Z2] towards A
- E through [X1,Y4,Z2] towards A
As there are four different paths between these four servers (B through E) and Z2, which serves as the ToR for A, all 40g of traffic can be delivered through the fabric without dropping or queuing a single packet (assuming, of Continue reading
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