[RndTbl] Looking for PTP aware switch
Adam Thompson
athompso at athompso.net
Mon Dec 2 12:39:18 CST 2019
On 2019-12-02 11:42, Gerald Brandt wrote:
> Hi,
>
> I'm looking for an IEEE 1588v2/PTP aware switch. Looking around, I've
> found several ruggadized ones and a couple in office types. I only
> need something for use in office, and something with 16 or 24 GigE
> ports would work.
>
> The ones I've found are pretty expensive, considering we would need 4
> of them.
Yes, PTP support is generally limited to Enterprise / Industrial / ISP
grade equipment. Some of which can be found used, if you're willing to
go that route.
Otherwise, have a look at:
https://www.nist.gov/el/intelligent-systems-division-73500/ieee-1588-products-implementations
https://en.wikipedia.org/wiki/List_of_PTP_implementations
and this fellow's experience, which seems to mirror yours:
https://www.reddit.com/r/networking/comments/568xax/new_to_ptp_ieee_1588_have_questions_on_how_to_get/
I can tell you from direct experience that the two lists I linked above
are FAR from comprehensive. You can, with a bit of work, find pages
like this:
https://gtacknowledge.extremenetworks.com/articles/Q_A/What-Timing-Protocols-are-supported-on-Extreme-Switches
from each major vendor.
An alternative is to use a cheap server (Atom-based is fine) with two
server-class NICs in it, and run a PTP bridge, see below.
Also, if there's a straight layer-2 path from the end station that
consumes PTP time, to the Grandmaster, the intermediate switches DO NOT
have to be PTP boundary clocks. The PTP protocol can compensate for
non-PTP transparent switches/bridges between the consumer and the
Grandmaster. You can lose a small amount of accuracy (sub-millisecond)
but that's still good enough for many/most scenarios. NOTE: this is not
a guarantee; YMMV; circumstances may vary.
Where you would need a Boundary Clock is, for example, to retransmit PTP
time onto a different VLAN than you received it on. This is, AFAICT,
why PTP-capable switches exist... you'll note that most PTP-capable
products are high-end because they're actually *routing* switches, and
the L2 path to the Grandmaster is broken when you cross a router hop.
That's where a cheap server can come in handy, it can rebroadcast PTP
time received on one VLAN onto others. Again, you'll lose accuracy &
precision simultaneously (no more 5ns sync!) but you'll get something.
Finally, the $64,000 question would be why you need to use PTP in the
first place? Properly configured NTP can keep (non-Windows) systems in
sync to within about 3msec on a LAN, including local router hops. The
only
I'm pushing to get PTP capability at MBIX, but that's because I already
happen to run mostly PTP-capable gear, it doesn't add much to MBIX's
cost, so hey why not! MBIX members wouldn't be required to get
PTP-capable equipment: if this happens, I think it would just be a thing
that's magically available on the IX fabric if your gear knows how to
listen for it. I think.
-Adam
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