100g PCS Errors

We've got a 100g qsfp in an mx204 that has 1207 bit errors and 29666 errored blocks after 24 hours of just being linked up...
I would assume this is not normal behavior, but I haven't used 100g before. Do others see high error rates on their 100g optics?

Id hope by this point you’ve already reseated not only the card but the connection to the card as well ?.

Possibly a faulty card.

Matt Harris​

Infrastructure Lead Engineer



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It’s not normal, no.

we have been making 100G packet capture systems for 5 years now ( fmad.io ). In the early days vendor qualified transceivers really do make a difference, its 25Gbps signaling per differential pair which is anything but easy. Back then (4-5Y ago) the cheap QSFP28 vendors had some really marginal parts… we had to tune the fpga alot to get the QSFP28s to to work correctly, and even then some just wouldnt work at all / have alot of errors.

If your using latests Finisar or Avago level transceivers should be fine, currently (last 12 months) the cheap transceivers dont need any tuning too. Guess depends if your using old HW / old transceivers or new HW with new transceivers.


What is the device on the other side of the MX204 100G link. We've had some incrementing PCS errors on 100G links when the other side was a Juniper PTX1000 using port et-0/0/25. Using a different port on the PTX1000 resolved the incrementing PCS errors. We opened JTAC cases for two incidents and a root cause was never found.

Good to know, we are just about to start deploying a bunch of PTX1000's.


On QSFP28 devices I would recommend always when possible run RS-FEC.
By default LR4 doesn't run it, but the added value is fantastic. You
will immediately during turn-up know if circuit works or not, without
any ping testing or live traffic. You will know if the circuit doesn't
work, before it impacts customers. Combine preFEC with DDM and you
have fantastic predictive power over failures and you can
reroute/schedule maintenance to fix issues before they become

Unfortunately no SNMP counters for RS-FEC. No for Juniper, not for
Nokia, not for Arista, so screenscraping you go. I have an ER-079886
for JNPR, if someone wants to chip in.

Yes, thanks. I recall you recommended this to someone earlier in the
year, so we have it in our library for deployment when we do the turn-up.

All our 100Gbps deployments are currently within the data centre, so SR4
optics; which Juniper enables RS-FEC on by default. But yes, we shall
definitely do the same (manually) for LR4.

IOS XR appears to suggest that FEC configuration is not explicitly
needed, unless the optic is a non-Cisco qualified non-LR4 unit.