[c-nsp] DWDM optics on 6500s

[Jeff Bacon]

> Don't forget they are absurdly under-buffered (16MB per card, compared
> to 256MB for 6708), and you can easily cause head of line blocking
with
> certain traffic profiles. If you want to run anywhere close to line
rate
> on them you need to monitor for drops or overruns and be prepared to
> play the port shuffle game to find an arrangement that works. Passing
a
> lot of traffic within the same fabric channel (from port 1<->2, or
> 3<->4) is the biggest sin, it will start dropping at 7 Gbps.

Well that's wonderfully comforting. Though I really probably only need
two ports anyway - ring-in and ring-out. Maybe not so bad. I'd consider
a 720-VS-10G head if I had some confidence that those two ports on the
sup were actually connected to the fabric. 

I don't really need to run line rate - this is more about latency and
burst capacity than sustained throughput. I have loads that burst from 0
to 500Mb/sec (then back) in nothing flat, and multiple of those may run
through the wire at the same time. Or not. 

Someone pointed out that the X2 and SFP+ xcvrs don't have much punch,
and I'm going to be shooting 20-30km through passive MUXes. So that
might matter.

(This is a bit of a roll-yer-own local metro NYC ring, which I'm doing
because I can get the wave for not much more than I'd pay for the
switched gig.)