40Gb LR over short lengths - any issues

[wintermute000]

Hey guys


Has anyone run into issues with 40Gb LR over short distances burning out or damaging the optics after a sustained period of time?


This definitely used to happen with 1Gb LX, but with 40Gb I'm not sure the issue is there. Can't find any literature via a quick goog.

[icecream-guy]

Cisco QSFP to Four SFP+ Copper Breakout Cables

Cisco QSFP to four SFP+ copper direct-attach breakout cables (Figure 2) are suitable for very short distances and offer a very cost-effective way to connect within racks and across adjacent racks. These breakout cables connect to a 40G QSFP port of a Cisco switch on one end and to four 10G SFP+ ports of a Cisco switch on the other end. Cisco currently offers passive cables in lengths of 0.5, 1, 2, 3, 4 and 5 meters and active cables in lengths of 7 and 10 meters.
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40GB attenuator ?

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thats all i could find quickly

[mmcgurty]

For short runs, you might want to consider something like this.  We had some 9K's that we wanted to hook up to each other with 40Gb so we decided to go this route:

(6) QSFP-H40G-AOC5M= - Cisco 40Gbase Active Optical Cables, 5 meters

Now we paid about $3300 for those 6 cables but you can get the same thing from FS.com for $780.  We are one of those places that it has to be Cisco end-to-end because of some problem a decade ago that caused us not to be able to utilize TAC during a P1 because of some grey market gear.  We did use FS.com for one of these on a Palo Alto HCSI cable though.  Palo Alto said they didn't care what kind of cable we used for the HCSI connection.

[Otanx]

Looking at the Cisco data sheet for 40G optics (link below) table 7 shows max transmit power, and max receive power. The max transmit power is 2.3dBm, and the max receive power is also 2.3dBm so in theory the transmitter will never transmit more power than the receiver can accept, and it shouldn't burn out. However, if you get a good transmitter, and a bad receiver I could see it happening maybe.

https://www.cisco.com/c/en/us/products/collateral/interfaces-modules/transceiver-modules/data_sheet_c78-660083.html

-Otanx

[wintermute000]

Thanks all, but I"m not asking about twinaxs or breakouts. I'm asking specifically about running SM fibre and then 40Gb LR over short distances (that are beyond the technical min spec of 2m but still short, like 3m or 10m). To simplify a lot of cabling, having only MM LC (bidi) and SM LC is attractive, esp. at scale, and esp when working out lots and lots of xconnects (thanks leaf-spine), and now you have a ton of them just over 150M (MM bidi limit). Would simplify a lot if all inter-rack cabling was LC and leave it at that. Just assessing ATM whether its worth a complicated mix of MM (within room) and SM (between rooms).