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I apologize in advance because I suspect this is already covered. I've read some of the posts and I suspect the answer is "it depends on the vendors involved" or just "no".

I'm looking at the Dell S4128T-ON and I'm new to SFP/SFP+/SFP28/QSFP/QSFP28 tech. The reseller is saying that it's a no - the quad breakout is intended to all to go the same host.

I have two switches, each with two QSFP28 ports. I was going to dedicate one port to link the switches and the other to connect to two ports on a Fortigate 100F firewall via SPF+ (so two of the quad cable connections would be unused - switchA to FortiGateA & switchB to FortiGateB - active/passive HA for firewalls).

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On most devices, that is possible. Check the manual for splitting a QSFP28 port into four SFP28 or SFP+ interfaces.

The datasheet specifically shows 28 10GbT and 8 SFP+ for 10G and lists a 40GbE, 4x10GbE, QSFP+ to 4xSFP+, passive DAC.

According to the user guide, using feature auto-breakout should automatically detect a breakout cable and use split ports:

Breakout auto-configuration

You can globally enable front-panel Ethernet ports to automatically detect SFP pluggable media in a QSFP+ or QSFP28 port. The port autoconfigures breakout interfaces for media type and speed. For example, if you plug a 40G direct attach cable (DAC) with 4x10G far-side transceivers into a QSFP28 port, the port autoconfigures in 10g-4x Interface-breakout mode.

RJ-45 ports and ports that are members of a port group do not support breakout auto-configuration. Breakout auto-configuration is disabled by default.

Enable breakout auto-configuration

OS10(config)# feature auto-breakout

Display breakout auto-configuration

Before you plug a cable in Ethernet port 1/1/25:

OS10# show interface status

                ------------------------------------------------------------------
                Port       Description  Status Speed Duplex Mode Vlan Tagged-Vlans
                ------------------------------------------------------------------
                Eth 1/1/1               down   0     auto    -
                Eth 1/1/2               down   0     auto    A    1    -
                Eth 1/1/25              down   0     auto    A    1    -
                Eth 1/1/29              down   0     auto    A    1    -

After you enter feature auto-breakout and plug a breakout cable in Ethernet port 1/1/25:

OS10# show interface status

                ------------------------------------------------------------------
                Port       Description  Status Speed Duplex Mode Vlan Tagged-Vlans
                ------------------------------------------------------------------
                Eth 1/1/1               down   0     auto    -
                Eth 1/1/2               down   0     auto    A    1    -
                Eth 1/1/25:1            down   0     auto    A    1    -
                Eth 1/1/25:2            down   0     auto    A    1    -
                Eth 1/1/25:3            down   0     auto    A    1    -
                Eth 1/1/25:4            down   0     auto    A    1    -
                Eth 1/1/29              down   0     auto    A    1    -

There is no mention that all breakout ports need to link to the same host nor would that make much sense - it would imply a LAG trunk, largely negating the reason for a port breakout.

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  • Yeah, I call BS on the reseller. Once the port is broken out, it's 4 ports like any other.
    – Jörg W Mittag
    Commented Aug 31, 2020 at 21:02

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