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My company recently purchased some Dell R740's with 2 Dual Port 25G SFP28 NIC's (Broadcom 57414) and wants to hook them up to an Arista 25G switch. We have two types of SFP adapters from Arista: SFP-25G-SR and SFP-10G-SR. We were hoping to get the servers running at 25G, but none of the 25G transceivers we have will operate at 25G when plugged into the R740.

Arista states in this document that the SFP-25G-SR connectors are the same as SFP28

Also, to keep the most basic configuration, there is a single fiber line running to each server, and each connected interface has only the following command:

switchport mode trunk

These are the configurations I've tried:

Server <--> Switch

Working Connections:

SFP-25G-SR <--> SFP-10G-SR

SFP-10G-SR <--> SFP-10G-SR

Unsuccessful Connections:

SFP-10G-SR <--> SFP-25G-SR

SFP-25G-SR <--> SFP-25G-SR

Just to ensure the switch and SFP adapters were not faulty, I plugged two of the 25G switches together with the 25G SFP's mentioned above and a link was established.

Are there any ideas as to why they aren't working as intended?

  • You may want to share the exact interface configurations. – Teun Vink Jan 4 '18 at 19:54
  • Did the answer help you? You should accept the answer if it helped you to a solution. :-) – user36472 Feb 4 '18 at 10:06
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Feb 21 '18 at 18:20
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Forgot to add an answer..

In the Dell BIOS, they had set the default link speed on the port to be 10G non-negotiable. Once that was changed, I had to set the Link FEC to the same standard on both the switch and the NIC. The two options available were Fire-Code and Reed Solomon. Both were tested before being put in to production but we ended up ultimately choosing Reed Solomon Link FEC

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There's no real 'SFP25' standard, there's only SFP28 (SFF-8402).

Since only the SFP-10G-SR seems to work in the switch: are you sure the port is SFP28-compatible and the rate isn't limited to 10G?

Many vendors limit the choice of transceiver to their own brand. So, the Dell NIC might only work with Dell transceivers (or 'compatible' ones faking the brand). However, this doesn't seem to be the case as both transceiver speeds work in the server but only the 10G in the switch.

Edit: Another thought: you're within spec with the MM fiber, aren't you? OM2 is fine for 80 m with 10G, but probably only 20 m or so with 25G (unspec'ed). OM3 should go 70 m and OM4 100 m.

  • The two Arista switches we have are 7160-48YC6's which support up to 25G on 48 ports, and 100G on 6 others. Arista manufactured both the switch and the transceivers. I understand where you're coming from with the Dell transceivers, but the servers still functioned with Arista's 10G transceivers. – Missing Bit Jan 4 '18 at 20:28
  • NICs aren't usually a problem with non-brand transceivers but I wanted to mention the problem. In your case it seems that 25G in the switches is the main problem. Have you tried using two 25G transceivers but forcing a 10G link? – Zac67 Jan 5 '18 at 7:54

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