You're seeing CRC and framing errors and general input errors. If this happened while setting up the port this could be caused by people still fiddling with the fiber.
If this happens during normal operation most of the time it indicates a low light level or some other error with the fiber(s) or optics.
You can check the light levels with
show interfaces transceiver
Beware that this might report incorrect readings for 3rd party optics. You would then have to look up the power budget / limits of the optic to see if you're in range with the specifications.
As Mike suggested, try cleaning all the fiber terminations. If that doesn't help try replacing the optics.
At the moment the errors are too small to be noticed but that can change very quickly. Better fix it now than being woken at 3am because there is suddenly more loss on the line.
Also for interface (error) counters it sometimes pays off to use the Cisco output interpreter to analyse what you're seeing:
https://www.cisco.com/pcgi-bin/Support/OutputInterpreter/home.pl
Take the analysis with a grain of salt, sometimes they're missing the point but it can help in getting a quick view about what's wrong.
UPDATE:
When using a XENPAK/SFP+ Adapter and DAC cables the problem could be with either of them. Try replacing the Adapter(s) and/or cables. As DAC has no optics in it (it's copper), the interface transceiver command will not show anything useful.
Also the cable length and possible electromagnetic interferences could cause problems with DAC. If everything fails try switching to optics and optical cabling and see if that helps.
Yes, you'd use two VLANs:
- EXTERNAL : cablemodem
- INTERNAL : LAN (10.0.0./24)
The TRUNK port to the Linux router will carry both VLANs (tagging one or both.) The linux router would then have a physical interface (eth0) and one or two vlan interfaces (eth0.X and eth0.Y -- depending on the numbers you choose for your vlans) I suggest two tagged VLANs to avoid any confusion between eth0 (untagged) and eth0.#. From there, networking is the same as having multiple physical interfaces.
(NOTE: if you use wireshark/tcdump, there are many buggy nic drivers that continue to eat vlan tags in promisc mode. As a result, capturing eth0 may not include any dot1q tags making the traffic look like it's all on the same network. Also, some management adapters will remove tags, even if the system management board isn't there. Broadcom is well known for this.)
Best Answer
I usually check the Cisco Gigabit Ethernet Transceiver Modules Compatibility Matrix and the 10-Gigabit Ethernet Transceiver Modules Compatibility Matrix. However, none of them contains any reference to SG500X
Anyway: regarding the switch, SFP-H10GB-CUxM is listed in the datasheet of the 500 Series as an option for 10G connectivity, so you should have no problem to attach it to the switch. Regarding the NIC, its specification states it supports standard SFF-8431 and according to 10-Gigabit Ethernet Transceiver Modules Compatibility Matrix the SFP-H10GB-CUxM does also support SFF-8431 (as you can see next), so they should be able to work toghether.