2148T-1GE - Latency Tests

The Nexus 5000 is marketed as a low latency switch, specifically 3.2 microseconds between 2 ports.  What happens to the latency when you're not traveling between 2 ports on a Nexus 5000, but between two 2148T fabric extenders that connect to a 5000?  After scouring CCO and finding no mention of FEX latency numbers, I decided to mock up a simple topology and run some tests.  The topology is quite simple, there are two 2148 fabric extenders, each with (1) 10G uplink to a 5020.  A traffic generator is connected to port 1 on each FEX.

Please note:
    1. Latency values are listed in microseconds
    2. Frame sizes do not count the Inter Frame Gap and MAC Preamble

Frame Size	Load %	Min Latency	Max Latency
64	10	8.3	8.6
	20	8.3	9.4
	30	8.3	9.2
	40	8.3	10.2
	50	8.3	9.6
	60	8.3	9.1
	70	8.3	9.0
	80	8.3	10.6
	90	8.3	9.9
250	10	9.8	10.1
	20	9.8	10.1
	30	9.8	10.1
	40	9.8	11.0
	50	9.8	10.3
	60	9.8	10.1
	70	9.8	11.0
	80	9.8	10.4
	90	9.8	10.6
500	10	11.8	12.1
	20	11.8	12.1
	30	11.8	12.1
	40	11.8	12.1
	50	11.8	12.1
	60	11.8	12.1
	70	11.8	12.1
	80	11.8	12.1
	90	11.8	12.1
1000	10	15.8	16.1
	20	15.8	16.1
	30	15.8	16.1
	40	15.8	16.1
	50	15.8	16.1
	60	15.8	16.1
	70	15.8	16.1
	80	15.8	16.1
	90	15.8	16.1
1518	10	19.9	20.2
	20	19.9	20.2
	30	19.9	20.2
	40	19.9	20.2
	50	19.9	20.2
	60	19.9	20.2
	70	19.9	20.2
	80	19.9	20.2
	90	19.9	20.2

As a point of reference, if you go between (2) WS-X6748-GE-TX (DFC3C) cards in a Catalyst 6500, the latency will range between 8 and 14 microseconds when tested at 90% load for similar frame sizes.