The support site for the Dell S4820T claims the switch supports QSFP to 4x SFP+ breakout cables (the supported ones at least), but I had trouble tracking down how to enable this for a given port (by default, only one physical interface appears in the OS for the 40G ports).

The answer is on page 464 of the FTOS Configuration Guide for the S4820T (FTOS 8.3.19.0) System.

40G QSFP+ to (4) SFP+ breakout cable

Dell sells supported cables, but it appears cheaper “non-qualfied” cables work as well (YMMV). The cable I’m using is sold as a “40G QSFP+ to 4xSFP+ Breakout DAC - 40GBASE-CR4 Passive Direct Attach Copper Twinax QSFP to SFP Cable for Mellanox MC2609130-002 Devices, 2-Meter(6.5ft)”

My switch sees the vendor as “Mellanox” and the part number as “MC3309130-002”.

Initial configuration

I’ll be using QSFP port 52 in this example, but you could also use one of the others (48, 56, or 60).

Dell(conf)#stack-unit 0 port 52 portmode quad
Please save and reset unit 0 for the changes to take effect.
Dell(conf)#end
Dell#Aug  3 10:54:36 %STKUNIT0-M:CP %SYS-5-CONFIG_I: Configured from  console
write
!
Aug  3 10:54:37 %STKUNIT0-M:CP %FILEMGR-5-FILESAVED: Copied running-config to startup-config in flash by default


Dell#reload

Proceed with reload [confirm yes/no]: yes
Aug  3 10:54:42 %STKUNIT0-M:CP %CHMGR-5-RELOAD: User request to reload the chassis

Using the ports

After rebooting, I got a warning about non-qualified optics:

Aug  3 10:57:43 %S4820T:0 %IFAGT-5-UNSUP_OPTICS: Non-qualified optics in slot 0 port 52
Aug  3 10:57:43 %S4820T:0 %IFAGT-5-UNSUP_OPTICS: Non-qualified optics in slot 0 port 53
Aug  3 10:57:43 %S4820T:0 %IFAGT-5-UNSUP_OPTICS: Non-qualified optics in slot 0 port 54
Aug  3 10:57:43 %S4820T:0 %IFAGT-5-UNSUP_OPTICS: Non-qualified optics in slot 0 port 55

Nonetheless, ports 52-55 are all available within the OS to configure (previously, only 52), now under the TenGigabitEthernet identifier (instead of FortyGigE):

Dell(conf)#interface TenGigabitEthernet 0/52
Dell(conf-if-te-0/52)#show config
!
interface TenGigabitEthernet 0/52
 no ip address
 shutdown

Putting ports in L2 (switchport) mode

Dell(conf)#interface tengigabitethernet 0/52
Dell(conf-if-te-0/52)#switchport
Dell(conf-if-te-0/52)#no shutdown
Dell(conf-if-te-0/52)#Aug  3 11:01:29 %STKUNIT0-M:CP %IFMGR-5-ASTATE_UP: Changed interface Admin state to up: Te 0/52
exit
Dell(conf)#interface tengigabitethernet 0/53
Dell(conf-if-te-0/53)#switchport
Dell(conf-if-te-0/53)#no shutdown
Dell(conf-if-te-0/53)#Aug  3 11:01:51 %STKUNIT0-M:CP %IFMGR-5-ASTATE_UP: Changed interface Admin state to up: Te 0/53
exit
Dell(conf)#interface tengigabitethernet 0/54
Dell(conf-if-te-0/54)#switchport
Dell(conf-if-te-0/54)#no shutdown
Dell(conf-if-te-0/54)#Aug  3 11:02:03 %STKUNIT0-M:CP %IFMGR-5-ASTATE_UP: Changed interface Admin state to up: Te 0/54
exit
Dell(conf)#interface tengigabitethernet 0/55
Dell(conf-if-te-0/55)#switchport
Dell(conf-if-te-0/55)#no shutdown
Dell(conf-if-te-0/55)#Aug  3 11:02:13 %STKUNIT0-M:CP %IFMGR-5-ASTATE_UP: Changed interface Admin state to up: Te 0/55
exit

Interface status

With this configuration in place, the switch successfully passes traffic between the SFP+ ports that the breakout cable is connected to.

Dell#show interfaces tengigabitethernet 0/52
TenGigabitEthernet 0/52 is up, line protocol is up
Hardware is DellEth, address is 90:b1:1c:f4:39:14
    Current address is 90:b1:1c:f4:39:14
Non-qualified pluggable media present, QSFP type is 40GBASE-CR4
Interface index is 1055236
Internet address is not set
Mode of IPv4 Address Assignment : NONE
DHCP Client-ID :90b11cf43914
MTU 1554 bytes, IP MTU 1500 bytes
LineSpeed 10000 Mbit
Flowcontrol rx off tx off
Energy Efficient Ethernet: Not Applicable
ARP type: ARPA, ARP Timeout 04:00:00
Last clearing of "show interface" counters 00:09:11
Queueing strategy: fifo
Input Statistics:
     348 packets, 28632 bytes
     300 64-byte pkts, 32 over 64-byte pkts, 0 over 127-byte pkts
     16 over 255-byte pkts, 0 over 511-byte pkts, 0 over 1023-byte pkts
     173 Multicasts, 155 Broadcasts, 20 Unicasts
     0 runts, 0 giants, 0 throttles
     0 CRC, 0 overrun, 0 discarded
Output Statistics:
     8 packets, 572 bytes, 0 underruns
     6 64-byte pkts, 2 over 64-byte pkts, 0 over 127-byte pkts
     0 over 255-byte pkts, 0 over 511-byte pkts, 0 over 1023-byte pkts
     4 Multicasts, 0 Broadcasts, 4 Unicasts
     0 throttles, 0 discarded, 0 collisions, 0 wreddrops
Rate info (interval 299 seconds):
     Input 00.00 Mbits/sec,          1 packets/sec, 0.00% of line-rate
     Output 00.00 Mbits/sec,          0 packets/sec, 0.00% of line-rate
Time since last interface status change: 00:01:02


Dell#show interfaces tengigabitethernet 0/53
TenGigabitEthernet 0/53 is up, line protocol is up
Hardware is DellEth, address is 90:b1:1c:f4:39:14
    Current address is 90:b1:1c:f4:39:14
Non-qualified pluggable media present, QSFP type is 40GBASE-CR4
Interface index is 1055364
Internet address is not set
Mode of IPv4 Address Assignment : NONE
DHCP Client-ID :90b11cf43914
MTU 1554 bytes, IP MTU 1500 bytes
LineSpeed 10000 Mbit
Flowcontrol rx off tx off
Energy Efficient Ethernet: Not Applicable
ARP type: ARPA, ARP Timeout 04:00:00
Last clearing of "show interface" counters 00:09:34
Queueing strategy: fifo
Input Statistics:
     18 packets, 2354 bytes
     3 64-byte pkts, 9 over 64-byte pkts, 6 over 127-byte pkts
     0 over 255-byte pkts, 0 over 511-byte pkts, 0 over 1023-byte pkts
     0 Multicasts, 1 Broadcasts, 17 Unicasts
     0 runts, 0 giants, 0 throttles
     0 CRC, 0 overrun, 0 discarded
Output Statistics:
     96 packets, 7712 bytes, 0 underruns
     83 64-byte pkts, 7 over 64-byte pkts, 0 over 127-byte pkts
     6 over 255-byte pkts, 0 over 511-byte pkts, 0 over 1023-byte pkts
     87 Multicasts, 0 Broadcasts, 9 Unicasts
     0 throttles, 0 discarded, 0 collisions, 0 wreddrops
Rate info (interval 299 seconds):
     Input 00.00 Mbits/sec,          0 packets/sec, 0.00% of line-rate
     Output 00.00 Mbits/sec,          0 packets/sec, 0.00% of line-rate
Time since last interface status change: 00:01:18