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Largelos
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CRS518-16XS-2XQ buffer size

Wed Oct 05, 2022 12:54 am

Does anyone have info about switch chip buffer size on CRS518-16XS-2XQ and other CRS5xx series?
Already asked support and sales without any result.
 
guipoletto
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Re: CRS518-16XS-2XQ buffer size

Wed Oct 05, 2022 1:55 am

The closest i ever got (for the CRS317 line) was this russian table:
https://isp-tech.ru/en/switch-asic/

that puts the 32port version of the 8xxx chipset at 4MB/ASIC, translating to probably 2MB on the CRS317 (16-port 8000 series Marvell SoC)

in the before times, the drivers used HoL buffering, which killed the whole switch whenever there was a speed mismatch between ports

somewhere (certainly on 6.49.x and 7.x.x) they fixed this (now it's FIFO, "drop-on-busy"), so that only the "low speed" ports get crappy performance
say i have a ring topology of 6*CRS317, with 30GB pipes between those, and 20GB active troughput
in one of the switches, there is a local feed of 1GB to Joe-Customer
that one port will drop packets like there's no tomorrow in the "downlink" direction (TX-Drop counter will increase, TCP will misbehave)
but the rest of the topology will remain unaffected.

Moral of the stoty:
So long as there are no speed mismatches, CRS will work beautifully

If there are speed mismatches (somehow using LACP makes it less bad), performance will be degraded, but only on the lowest-speed interface, in the "down" direction.
(some of this was discussed in this topic: viewtopic.php?t=185231)

These Marvell chips are great, cheap, and power-efficient.
The way they do that is by not having giant on-die buffers. It's best to understand that, and work around this constraint, avoiding mismatches if at all possible.
 
Largelos
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Re: CRS518-16XS-2XQ buffer size

Wed Oct 05, 2022 7:20 pm

So long as there are no speed mismatches, CRS will work beautifully
Thanks, but this is obvious ;)
I'm asking, because want to understand what to expect from this switches when using 10 and 25Gbps port speed at same time. This is quite common.
This series devices from MikroTik already costs a lot, comparable with other manufacturers (e.g. NVIDIA 25/100GbE lossless switch costs starts only ~2 times more, or you can buy even cheaper used/refurbished switch), but at the same time, distributors does not provide them for test, and Mikrotik does not provide detailed information, even MTBF was absent for a long time and was not disclosed although it has been asked many times on forum and direct request, and appeared very recently for some models. I don't think that many devices will be sold in this segment with this approach. This is no longer the price at which you can buy a device for a test and throw it away after.

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