Forwarding latency for various switches

Question

i want to quantify the latency in my network, as part of an effort to determine whether or not we can deploy a new application
so i've grabbed my Finisar THG box (a hardware packet sniffer with an internal clock accurate to 20ns), a stack of various switches, and some cables. i plug the two ports of the THG box into a switch, send 1,000 pings at a specified internval from one THG NIC through the switch to the other THG NIC, subtract the packet insertion time, average the resulting pile of numbers, and come up with a figure for the forwarding latency (aka decision time) of the device. see my results below
now i want a sanity check. Cisco must perform this same test (possibly with fancier hardware, like SmartBits boxes) routinely on their gear ... where do they post these results? i've been poking around www.cisco.com without success
for interest, here are my numbers:
Catalyst 4003 100BaseT ports: 3170ns
Catalyst 4003 1000BaseSX ports: 705ns
[same forwarding latency for 64 byte and for 1518 byte packets]
Cat 4503 1000BaseSX 64 byte: 3300ns
Cat 4503 1000BaseSX 1518 byte: 7120ns
[why the change in forwarding latency depending on packet size? remember, i've already subtracted packet insertion time]
Cat 6506 1000BaseSX 64 byte: 5000ns
Cat 6506 1000BaseSx 1518 byte: 7120ns
Datacomm Aggregration tap 100Mb: 320ns
In-Line Finisar 100Mb tap: 0ns
NetGear 100Mb hub: 330ns
and finally, we ran a test across our production network (which translates into two access-layer Cat 4506s, two distribution layer Cat 6506s, one core layer Cat 6506, plus ~500m of cabling ... and came out with ~20us of latency, exclusive of packet insertion time. good stuff
-where does Cisco post the numbers they have recorded?
-with whom could i have a conversation about what drives fowarding latency in different Catalyst models? why, for instance, different packet sizes change decision time, in some models but not in others?
i'm wanting both a sanity check and a deeper understanding
--sk
stuart kendrick
fhcrc

Answer 1

I don't believe you'll ever see those numbers published from a vendor (Cisco or any other).
Think marketing: With hard numbers being public information, the Marketing folks have a lot less "wiggle room" when producing their materials. It provides a hard/fixed target for the competitors, for their engineering and marketing.
Think back to ~1984 when Kalpana introduced Ethernet switching. Their product (a cut-through switch) produced latencies in the ~20s when everyone else's store & forward switches were in the 90s.
What happened? The marketing folks from the other companies (including Cisco) re-defined the term "Latency" in their marketing materials such that it favored their product (or at least made it look like a less-significant difference). It was wild .... some wanted it to mean firstbyte in/first byte out, others wanted first byte in / last byte out ...and everything in-between ,,, whatever worked for their product.
Cisco even came up with "Frag-Free switching" to capture the market generated by all the other vendors FUD campaign ("the fast switching of cut-through{sorta}, with the assurance of runt/frag-free switching offered by S&F").
Even numbers from the third party groups can be suspect. Our Lab (I worked at a different place then)had much of the same equipment used by the thrid-party groups ... and we can up (in some cases) with similar numbers (and sometimes different numbers)... without the spin, the products usually didn't produce the same happy results.
When the manufacturer is paying for the third-party to do the testing, the analysis tends to have a much more positive lean to it.
To finish it all off, in most cases, performance (assuming it's within acceptable limits) is only a small part of the purchasing decision. In many/most cases, pre/post sale support, stability and longevity of the manufacturer, implementation costs, interoperability, post-installation management, and other factors take the forefront of "why do I wanna buy this thing from those guys."
Meaning, would you rather buy the hottest box from a new company that may|may not be around next year, that may|may not integrate well with existing infrastructure, that may|ususally doesn't have a product that doesn't intergrate with the existing management platforms and may|may not have a decent support organization .... OR buy from a company that doesn't have the fastest box, but it's reliable, integrates well, has a good support organization, and has proven longevity (it'll be here next year) ... and the other (mostly positive) attributes?
Sorry for the long post, but it's an issue that's more complex than performance stats, and (to get back to the original point) Marketing likely requires some "flexibility" in their marketing materials and generally doesn't want hard specs beyond the usual packets-per-second, backplane throughput, and MTBF.
FWIW
Scott