Switch Port Attributes

Cumulus Linux exposes network interfaces for several types of physical and logical devices:

  • lo is the network loopback device
  • eth is a switch management port (for out of band management only)
  • swp is a switch front panel port
  • (optional) br is a bridge (IEEE 802.1Q VLAN)
  • (optional) bond is a bond (IEEE 802.3ad link aggregation trunk, or port channel)

Each physical network interface (port) has several settings:

  • Auto-negotiation
  • Duplex Mode
  • Link speed
  • MTU
  • FEC

For NVIDIA Spectrum ASICs, MTU is the only port attribute you can directly configure. The Spectrum firmware configures FEC, link speed, duplex mode and auto-negotiation automatically, following a predefined list of parameter settings until the link comes up. However, you can disable FEC if necessary, which forces the firmware to not try any FEC options.

MTU

Interface MTU applies to traffic traversing the management port, front panel or switch ports, bridge, VLAN subinterfaces, and bonds (both physical and logical interfaces). MTU is the only interface setting that you must set manually.

In Cumulus Linux, ifupdown2 assigns 9216 as the default MTU setting. The initial MTU value set by the driver is 9238. After you configure the interface, the default MTU setting is 9216.

To change the MTU setting, run the following commands. The example command sets the MTU to 1500 for the swp1 interface.

cumulus@switch:~$ nv set interface swp1 link mtu 1500
cumulus@switch:~$ nv config apply

Edit the /etc/network/interfaces file, then run the ifreload -a command.

cumulus@switch:~$ sudo nano /etc/network/interfaces

auto swp1
iface swp1
    mtu 1500
cumulus@switch:~$ sudo ifreload -a

Runtime Configuration (Advanced)

Run the ip link set command. The following example command sets the swp1 interface MTU to 1500.

cumulus@switch:~$ sudo ip link set dev swp1 mtu 1500

A runtime configuration is non-persistent; the configuration you create does not persist after you reboot the switch.

Set a Global Policy

To set a global MTU policy, create a policy document (called mtu.json). For example:

cumulus@switch:~$ sudo cat /etc/network/ifupdown2/policy.d/mtu.json
{
  "address": {"defaults": { "mtu": "9216" }
            }
}

The policies and attributes in any file in /etc/network/ifupdown2/policy.d/ override the default policies and attributes in /var/lib/ifupdown2/policy.d/.

Bridge MTU

The MTU setting is the lowest MTU of any interface that is a member of the bridge (every interface specified in bridge-ports in the bridge configuration of the /etc/network/interfaces file). You are not required to specify an MTU on the bridge. Consider this bridge configuration:

auto bridge
iface bridge
    bridge-ports bond1 bond2 bond3 bond4 peer5
    bridge-vids 100-110
    bridge-vlan-aware yes

For a bridge to have an MTU of 9000, set the MTU for each of the member interfaces (bond1 to bond 4, and peer5) to 9000 at minimum.

When configuring MTU for a bond, configure the MTU value directly under the bond interface; the member links or slave interfaces inherit the configured value. If you need a different MTU on the bond, set it on the bond interface, as this ensures the slave interfaces pick it up. You do not have to specify an MTU on the slave interfaces.

VLAN interfaces inherit their MTU settings from their physical devices or their lower interface; for example, swp1.100 inherits its MTU setting from swp1. Therefore, specifying an MTU on swp1 ensures that swp1.100 inherits the MTU setting for swp1.

If you are working with VXLANs, the MTU for a virtual network interface (VNI must be 50 bytes smaller than the MTU of the physical interfaces on the switch, as various headers and other data require those 50 bytes. Also, consider setting the MTU much higher than 1500.

To show the MTU setting for an interface:

cumulus@switch:~$ nv show interface swp1
cumulus@switch:~$ ip link show dev swp1
3: swp1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9216 qdisc pfifo_fast state UP mode DEFAULT qlen 500
   link/ether 44:38:39:00:03:c1 brd ff:ff:ff:ff:ff:ff

Drop Packets that Exceed the Egress Layer 3 MTU

The switch forwards all packets that are within the MTU value set for the egress layer 3 interface. However, when packets are larger in size than the MTU value, the switch fragments the packets that do not have the DF bit set and drops the packets that do have the DF bit set.

Run the following command to drop all IP packets that are larger in size than the MTU value for the egress layer 3 interface instead of fragmenting packets:

cumulus@switch:~$ nv set system control-plane trap l3-mtu-err state off
cumulus@switch:~$ nv config apply
cumulus@switch:~$ echo "0 >" /cumulus/switchd/config/trap/l3-mtu-err/enable

FEC

FEC is an encoding and decoding layer that enables the switch to detect and correct bit errors introduced over the cable between two interfaces. The target IEEE BER on high speed Ethernet links is 10-12. Because 25G transmission speeds can introduce a higher than acceptable BER on a link, FEC is often required to correct errors to achieve the target BER at 25G, 4x25G, 100G, and higher link speeds. The type and grade of a cable or module and the medium of transmission determine which FEC setting is necessary.

For the link to come up, the two interfaces on each end must use the same FEC setting.

FEC requires small latency overhead. For most applications, this small amount of latency is preferable to error packet retransmission latency.

The two FEC types are:

  • Reed Solomon (RS), IEEE 802.3 Clause 108 (CL108) on individual 25G channels and Clause 91 on 100G (4channels). This is the highest FEC algorithm, providing the best bit-error correction.
  • Base-R (BaseR), Fire Code (FC), IEEE 802.3 Clause 74 (CL74). Base-R provides less protection from bit errors than RS FEC but adds less latency.

Cumulus Linux includes additional FEC options:

  • Auto FEC instructs the hardware to select the best FEC. For copper DAC, the remote end can negotiate FEC. However, optical modules do not have auto-negotiation capability; if the device chooses a preferred mode, it might not match the remote end. This is the current default on the NVIDIA Spectrum switch.
  • No FEC (no error correction).

While Auto FEC is the default setting on the NVIDIA Spectrum switch, do not explicitly configure the fec auto option on the switch as this leads to a link flap whenever you run net commit or ifreload -a.

For 25G DAC, 4x25G Breakouts DAC and 100G DAC cables, the IEEE 802.3by specification creates 3 classes:

  • CA-25G-L (Long cable) - Requires RS FEC - Achievable cable length of at least 5m. dB loss less or equal to 22.48. Expected BER of 10-5 or better without RS FEC enabled.
  • CA-25G-S (Short cable) - Requires Base-R FEC - Achievable cable length of at least 3m. dB loss less or equal to 16.48. Expected BER of 10-8 or better without Base-R FEC enabled.
  • CA-25G-N (No FEC) - Does not require FEC - Achievable cable length of at least 3m. dB loss less or equal to 12.98. Expected BER 10-12 or better with no FEC enabled.

The IEEE classification specifies various dB loss measurements and minimum achievable cable length. You can build longer and shorter cables if they comply to the dB loss and BER requirements.

If a cable has a CA-25G-S classification and FEC is not on, the BER might be unacceptable in a production network. It is important to set the FEC according to the cable class (or better) to have acceptable bit error rates. See Determining Cable Class below.

You can check bit errors using cl-netstat (RX_ERR column) or ethtool -S (HwIfInErrors counter) after a large amount of traffic passes through the link. A non-zero value indicates bit errors. Expect error packets to be zero or extremely low compared to good packets. If a cable has an unacceptable rate of errors with FEC enabled, replace the cable.

For 25G, 4x25G Breakout, and 100G Fiber modules and AOCs, there is no classification of 25G cable types for dB loss, BER or length. Use FEC if the BER is low enough.

Cable Class of 100G and 25G DACs

You can determine the cable class for 100G and 25G DACs from the Extended Specification Compliance Code field (SFP28: 0Ah, byte 35, QSFP28: Page 0, byte 192) in the cable EEPROM programming.

For 100G DACs, most manufacturers use the 0x0Bh 100GBASE-CR4 or 25GBASE-CR CA-L value (the 100G DAC specification predates the IEEE 802.3by 25G DAC specification). Use RS FEC for 100G DAC; shorter or better cables might not need this setting.

A manufacturer’s EEPROM setting might not match the dB loss on a cable or the actual bit error rates that a particular cable introduces. Use the designation as a guide, but set FEC according to the bit error rate tolerance in the design criteria for the network. For most applications, the highest mutual FEC ability of both end devices is the best choice.

You can determine for which grade the manufacturer has designated the cable as follows.

For the SFP28 DAC, run the following command:

cumulus@switch:~$ sudo ethtool -m swp1 hex on | grep 0020 | awk '{ print $6}'
0c

The values at location 0x0024 are:

  • 0x0b : CA-L (long cable - RS FEC required)
  • 0x0c : CA-S (short cable - Base-R or better FEC required)
  • 0x0d : CA-N (no FEC required)

For the QSFP28 DAC, run the following command:

cumulus@switch:~$ sudo ethtool -m swp1s0 hex on | grep 00c0 | awk '{print $2}'
0b

The values at 0x00c0 are:

  • 0x0b : CA-L (long cable - RS FEC required) or 100G CR4
  • 0x0c : CA-S (short cable - Base-R or better FEC required)
  • 0x0d : CA-N (no FEC required)

In each example below, the Compliance field comes from the method described above; the ethool -m output does not show it.

3meter cable that does not require FEC
(CA-N)
Cost: More expensive
Cable size: 26AWG (Note that AWG does not necessarily correspond to overall dB loss or BER performance)
Compliance Code: 25GBASE-CR CA-N

3meter cable that requires Base-R FEC
(CA-S)
Cost: Less expensive
Cable size: 26AWG
Compliance Code: 25GBASE-CR CA-S

When in doubt, consult the manufacturer directly to determine the cable classification.

Spectrum ASIC FEC Behavior

The firmware in a Spectrum ASIC applies an FEC configuration to 25G and 100G cables based on the cable type and whether the peer switch also has a Spectrum ASIC.

When the link is between two switches with Spectrum ASICs:

  • For 25G optical modules, the Spectrum ASIC firmware chooses Base-R/FC-FEC.
  • For 25G DAC cables with attenuation less or equal to 16db, the firmware chooses Base-R/FC-FEC.
  • For 25G DAC cables with attenuation higher than 16db, the firmware chooses RS-FEC.
  • For 100G cables/modules, the firmware chooses RS-FEC.
Cable Type
FEC Mode
25G optical cablesBase-R/FC-FEC
25G 1,2 meters: CA-N, loss <13dbBase-R/FC-FEC
25G 2.5,3 meters: CA-S, loss <16dbBase-R/FC-FEC
25G 2.5,3,4,5 meters: CA-L, loss > 16dbRS-FEC
100G DAC or opticalRS-FEC

When linking to a non-Spectrum peer, the firmware lets the peer decide. The Spectrum ASIC supports RS-FEC (for both 100G and 25G), Base-R/FC-FEC (25G only), or no-FEC (for both 100G and 25G).

Cable Type
FEC Mode
25G optical cablesLet peer decide
25G 1,2 meters: CA-N, loss <13dbLet peer decide
25G 2.5,3 meters: CA-S, loss <16dbLet peer decide
25G 2.5,3,4,5 meters: CA-L, loss > 16dbLet peer decide
100GLet peer decide: RS-FEC or No FEC

How Does Cumulus Linux use FEC?

A Spectrum switch enables FEC automatically when it powers up. The port firmware tests and determines the correct FEC mode to bring the link up with the neighbor. It is possible to get a link up to a switch without enabling FEC on the remote device as the switch eventually finds a working combination to the neighbor without FEC.

The following sections describe how to show the current FEC mode, and how to enable and disable FEC.

Show the Current FEC Mode

On a Spectrum switch, the --show-fec output shows you the current active state of FEC only if the link is up; if the FEC modes matches that of the neighbor. If the link is not up, the value displays None, which is not valid.

To show the FEC mode on a switch port, run the ethtool --show-fec <interface> command.

cumulus@switch:~$ sudo ethtool --show-fec swp1
FEC parameters for swp1:
Configured FEC encodings: Auto
Active FEC encoding: Off

Enable or Disable FEC

To enable Reed Solomon (RS) FEC on a link:

cumulus@switch:~$ nv set interface swp1 link fec rs
cumulus@switch:~$ nv config apply

Edit the /etc/network/interfaces file, then run the ifreload -a command. The following example enables RS FEC for the swp1 interface (link-fec rs):

cumulus@switch:~$ sudo nano /etc/network/interfaces

auto swp1
iface swp1
    link-autoneg off
    link-speed 100000
    link-fec rs
cumulus@switch:~$ sudo ifreload -a

Runtime Configuration (Advanced)

Run the ethtool --set-fec <interface> encoding RS command. For example:

cumulus@switch:~$ sudo ethtool --set-fec swp1 encoding RS

A runtime configuration is non-persistent. The configuration you create does not persist after you reboot the switch.

To enable Base-R/FireCode FEC on a link:

cumulus@switch:~$ nv set interface swp1 link fec baser
cumulus@switch:~$ nv config apply

Edit the /etc/network/interfaces file, then run the ifreload -a command. The following example enables Base-R FEC for the swp1 interface (link-fec baser):

cumulus@switch:~$ sudo nano /etc/network/interfaces

auto swp1
iface swp1
    link-autoneg off
    link-speed 100000
    link-fec baser
cumulus@switch:~$ sudo ifreload -a

Runtime Configuration (Advanced)

Run the ethtool --set-fec <interface> encoding baser command. For example:

cumulus@switch:~$ sudo ethtool --set-fec swp1 encoding BaseR

A runtime configuration is non-persistent. The configuration you create does not persist after you reboot the switch.

To enable FEC with Auto-negotiation:

You can use FEC with auto-negotiation on DACs only.

cumulus@switch:~$ nv set interface swp1 link auto-negotiate on
cumulus@switch:~$ nv config apply

Edit the /etc/network/interfaces file to set auto-negotiation to on, then run the ifreload -a command:

cumulus@switch:~$ sudo nano /etc/network/interfaces

auto swp1
iface swp1
link-autoneg on
cumulus@switch:~$ sudo ifreload -a

Runtime Configuration (Advanced)

You can use ethtool to enable FEC with auto-negotiation. For example:

ethtool -s swp1 speed 10000 duplex full autoneg on

A runtime configuration is non-persistent. The configuration you create does not persist after you reboot the switch.

To show the FEC and auto-negotiation settings for an interface:

cumulus@switch:~$ sudo ethtool swp1 | egrep 'FEC|auto'
Supports auto-negotiation: Yes
Supported FEC modes: RS
Advertised auto-negotiation: Yes
Advertised FEC modes: RS
Link partner advertised auto-negotiation: Yes
Link partner advertised FEC modes: Not reported

To disable FEC on a link:

cumulus@switch:~$ nv set interface swp1 link fec off
cumulus@switch:~$ nv config apply

To configure FEC to the default value, run the nv unset interface swp1 link fec command.

Edit the /etc/network/interfaces file, then run the ifreload -a command. The following example disables Base-R FEC for the swp1 interface (link-fec baser):

cumulus@switch:~$ sudo nano /etc/network/interfaces

auto swp1
iface swp1
link-fec off
cumulus@switch:~$ sudo ifreload -a

Runtime Configuration (Advanced)

Run the ethtool --set-fec <interface> encoding off command. For example:

cumulus@switch:~$ sudo ethtool --set-fec swp1 encoding off

A runtime configuration is non-persistent. The configuration you create does not persist after you reboot the switch.

Default Policies for Interface Settings

Instead of configuring settings for each individual interface, you can specify a policy for all interfaces on a switch or tailor custom settings for each interface. Create a file in /etc/network/ifupdown2/policy.d/ and populate the settings accordingly. The following example shows a file called address.json.

cumulus@switch:~$ cat /etc/network/ifupdown2/policy.d/address.json
{
    "ethtool": {
        "defaults": {
            "link-duplex": "full"
        },
        "iface_defaults": {
            "swp1": {
                "link-autoneg": "on",
                "link-speed": "1000"
          },
            "swp16": {
                "link-autoneg": "off",
                "link-speed": "10000"
            },
            "swp50": {
                "link-autoneg": "off",
                "link-speed": "100000",
                "link-fec": "rs"
            }
        }
    },
    "address": {
        "defaults": { "mtu": "9000" },
        "iface_defaults": {
            "eth0": {"mtu": "1500"}
        }
    }
}

Setting the default MTU also applies to the management interface. Be sure to add the iface_defaults to override the MTU for eth0, to remain at 9216.

Breakout Ports

Cumulus Linux supports the following ports breakout options:

18x SFP+ 25G and 4x QSFP28 100G interfaces only support NRZ encoding. You can set all speeds down to 1G.

All 4x QSFP28 ports can break out into 4x SFP28 or 2x QSFP28.

  • 18x 10G - 18x SFP28 set to 10G
  • 16x 10G - 4x QSFP28 configured as 4x25G breakouts and set to 10G

Maximum 10G ports: 34

  • 18x 25G - 18x SFP28 (native speed)
  • 16x 25G - 4x QSFP28 breakouts to 4x25G

Maximum 25G ports: 34

4x 40G - 4x QSFP28 set to 40G

Maximum 40G ports: 4

8x 50G - 4x QSFP28 break out into 2x 50G

Maximum 50G ports: 8

4x 100G - 4x QSFP28 (native speed)

Maximum 100G ports: 4

16x QSFP28 100G interfaces only support NRZ encoding. You can set all speeds down to 1G.

All QSFP28 ports can break out into 4x SFP28 or 2x QSFP28.

64x 10G - 16x QSFP28 break out into 4x 25G and set to 10G

Maximum 10G ports: 64

64x 25G - 16x QSFP28 break out into 4x 25G

Maximum 25G ports: 64

16x 40G - 4x QSFP28 set to 40G

Maximum 40G ports: 16

32x 50G - 16x QSFP28 break out into 2x 50G

Maximum 50G ports: 32

16x 100G - 16x QSFP28 (native speed)

Maximum 100G ports: 16

48x SFP28 25G and 8x QSFP28 100G interfaces only support NRZ encoding. You can set all speeds down to 1G.

The top 4x QSFP28 ports can break out into 4x SFP28. You cannot use the lower 4x QSFP28 disabled ports.

All 8x QSFP28 ports can break out into 2x QSFP28 without disabling ports.

  • 48x 10G - 48x SFP28 set to 10G
  • 16x 10G - 4x QSPF28 break out into 4x25G and set to 10G

Maximum 10G ports: 64

  • 48x 25G - 48x SFP28 (native speed)
  • 16x 25G - Top 4x QSFP28 break out into 4x25G (bottom 4x QSFP28 disabled)

Maximum 25G ports: 64

8x 40G - 8x QSFP28 set to 40G

Maximum 40G ports: 8

16x 50G - 8x QSFP28 break out into 2x 50G

Maximum 50G ports: 16

8x 100G - 16x QSFP28 (native speed)

Maximum 100G ports: 8

32x QSFP28 100G interfaces only support NRZ encoding. You can set all speeds down to 1G.

The top 16x QSFP28 ports can break out into 4x SFP28. You cannot use the lower 4x QSFP28 disabled ports.

All 32x QSFP28 ports can break out into 2x QSFP28 without disabling ports.

64x 10G - Top 16x QSFP28 break out into 4x 25G and set to 10G (bottom 16x QSFP28 disabled)

Maximum 10G ports: 64

64x 25G - Top 16x QSFP28 break out into 4x25G (bottom 16x QSFP28 disabled)

Maximum 25G ports: 64

32x 40G - 32x QSFP28 set to 40G

Maximum 40G ports: 32

64x 50G - 64x QSFP28 break out into 2x 50G

Maximum 50G ports: 64

32x 100G - 32x QSFP28 (native speed)

Maximum 100G ports: 32

48x SFP28 25G and 12x QSFP28 100G interfaces only support NRZ encoding.

All 12x QSFP28 ports can break out into 4x SFP28 or 2x QSFP28.

  • 48x 10G - 48x SFP28 set to 10G
  • 48x 10G - 12x QSPF28 break out into 4x 25G and set to 10G

Maximum 10G ports: 96

  • 48x 25G - 48x SFP28 (native speed)
  • 48x 25G - 12x QSPF28 break out into 4x 25G

Maximum 25G ports: 96

12x 40G - 12x QSFP28 set to 40G

Maximum 40G ports: 12

24x 50G - 12x QSFP28 break out into 2x 50G

Maximum 50G ports: 24

12x 100G - 12x QSFP28 (native speed)

Maximum 100G ports: 12

32x QSFP28 100G interfaces only support NRZ encoding.

All 32x QSFP28 ports can break out into 4x SFP28 or 2x QSFP28.

128x 10G - 32x QSFP28 break out into 4x 25G and set to 10G

Maximum 10G ports: 128

128x 25G - 32x QSFP28 break out into 4x 25G

Maximum 25G ports: 128

32x 40G - 32x QSFP28 set to 40G

Maximum 40G ports: 32

64x 50G - 32x QSFP28 break out into 2x 50G

Maximum 50G ports: 64

32x 100G - 32x QSFP28 (native speed)

Maximum 100G ports: 32

32x QSFP56 200G interfaces support both PAM4 and NRZ encodings.

For lower speed interface configurations, PAM4 is automatically converted to NRZ encoding.

All 32x QSFP56 ports can break out into 4xSFP56 or 2x QSFP56.

128x 10G - 32x QSFP56 break out into 4x 50G and set to 10G

Maximum 10G ports: 128

128x 25G - 32x QSFP56 break out into 4x 50G and set to 25G

Maximum 25G ports: 128

32x 40G - 32x QSFP56 set to 40G

Maximum 40G ports: 32

128x 50G - 32x QSFP56 break out into 4x 50G

Maximum 50G ports: 128

64x 100G - 32x QSFP56 break out into 2x 100G

Maximum 100G ports: 64

32x 200G - 32x QSFP56 (native speed)

Maximum 200G ports: 32

SN4410 24xQSFP28-DD (100GbE) interfaces [ports 1-24] only support NRZ encoding and wll speeds down to 1G.

The 8xQSFP-DD (400GbE) interfaces [ports 25-32] support both PAM4 and NRZ encodings with all speeds down to 1G.

For lower speeds, PAM4 is automatically converted to NRZ encoding.

The 24xQSFP28-DD ports can break out into 2xQSFP28 (2x100GbE) using special 2x100GbE breakout cable, or 4xSFP28 (4x25GbE).

The top 4xQSFP-DD ports can break out into 8xSFP56 (8x50GbE). But, in this case, the adjacent 4xQSFP-DD ports are blocked.

All the 8xQSFP-DD ports can break out into 4xQSFP56 (4x100GbE), or 2xQSFP56 (2x200GbE) without blocking ports.

  • 96x10G - 24xQSFP28-DD break out into 4x25G and set to 10G
  • 32x10G - 4 top QSFP-DD break out into 8x50G and set to 10G (bottom 4xQSFP-DD blocked*)

Maximum 10G ports: 128

*Other QSFP-DD breakout combinations are available up to maximum of 128x10G ports.

  • 96x25G - 24xQSFP28-DD break out into 4x25G
  • 32x25G - 4 top QSFP-DD break out into 8x50G and set to 25G (bottom 4xQSFP-DD blocked*)

Maximum 25G ports: 128

*Other QSFP-DD breakout combinations are available up to maximum of 128x25G ports.

32x40G - 24xQSFP28-DD and 8xQSFP-DD set to 40G

Maximum 40G ports: 32

  • 48x50G - 24xQSFP28-DD break out into 2x50G
  • 32x50G - 4 top QSFP-DD break out into 8x50G (bottom 4xQSFP-DD blocked*)

Maximum 50G ports: 80

*Other QSFP-DD breakout combinations are available up to maximum of 80x50G ports.

  • 48x100G - 24xQSFP28-DD break out into 2x100G (using special 2xQSFP28-DD breakout cable)
  • 32x100G - 8xQSFP-DD break out into 4x100G

Maximum 100G ports: 80

16x200G - 8xQSFP-DD break out into 2x200G

Maximum 200G ports: 16

8x400G - 8xQSFP-DD (native speed)

Maximum 400G ports: 8

64x QSFP28 100G interfaces only support NRZ encoding.

Only 32x QSFP28 ports can break out into 4x SFP28. You must disable the adjacent QSFP28 port. Only the first and third or second and forth rows can break out into 4xSFP28.

All 64x QSFP28 ports can break out into 2x QSFP28 without disabling ports.

128x 10G - 32x QSFP28 break out into 4x 25G and set to 10G

Maximum 10G ports: 128

128x 25G - 32x QSFP28 break out into 4x 25G

Maximum 25G ports: 128

64x 40G - 64x QSFP28 set to 40G

Maximum 40G ports: 64

128x 50G - 64x QSFP28 break out into 2x 50G

Maximum 50G ports: 128

64x 100G - 64x QSFP28 (native speed)

Maximum 100G ports: 80

SN4600 64xQSFP56 (200GbE) interfaces support both PAM4 and NRZ encodings with all speeds down to 1G.

For lower speeds, PAM4 is automatically converted to NRZ encoding.

Only 32xQSFP56 ports can brea kout into 4xSFP56 (4x50GbE). But, in this case, the adjacent QSFP56 port are blocked (only the 1st/3rd or 2nd/4th rows can break out into 4xSFP56).

All 64xQSFP56 ports can break out into 2xQSFP56 (2x100GbE) without blocking ports.

128x10G - 64xQSFP56 break out into 4x50G and set to 10G

Maximum 10G ports: 128

128x25G - 64xQSFP56 break out into 4x50G and set to 25G

Maximum 25G ports: 128

64x40G - 64xQSFP56 set to 40G

Maximum 40G ports: 64

128x50G - 32xQSFP56 break out into 4x50G

Maximum 50G ports: 128

  • 128x100G - 64xQSFP56 break out into 2x100G
  • 64x100G - 64xQSFP28 set to 100G

Maximum 100G ports: 128

{< /tab >}} {< tab “200G “>}}

64x200G - 64xQSFP56 (native speed)

Maximum 200G ports: 64

SN4700 32x QSFP-DD 400GbE interfaces support both PAM4 and NRZ encodings.

For lower speed interface configurations, PAM4 is automatically converted to NRZ encoding.

Only the top or the bottom 16x QSFP-DD ports can break out into 8x SFP56. You must disable the adjacent QSFP-DD port.

All 32x QSFP-DD ports can break out into 2x QSFP56 at 2x200G or 4x QSFP56 at 4x 100G without disabling ports.

128x 10G - 16x QSFP-DD break out into 8x 50G and set to 10G

Maximum 10G ports: 128

*Cumulus Linux supports other QSFP-DD breakout combinations up to maximum of 128x 10G ports.

128x 25G - 16x QSFP-DD break out into 8x 50G and set to 25G

Maximum 25G ports: 128

*Cumulus Linux supports other QSFP-DD breakout combinations up to maximum of 128x 25G ports.

32x 40G - 32x QSFP-DD set to 40G

Maximum 40G ports: 32

128x 50G - 16x QSFP-DD break out into 8x 50G

Maximum 50G ports: 128

*Cumulus Linux supports other QSFP-DD breakout combinations up to maximum of 128x 50G ports.

128x 100G - 32x QSFP-DD break out into 4x 100G

Maximum 100G ports: 128

64x 200G - 64x QSFP-DD break out into 2x 200G

Maximum 200G ports: 64

32x 400G - 32x QSFP-DD (native speed)

Maximum 400G ports: 32

SN4800 is a modular chassis with up to 8 line cards. Each line card has 16x QSFP28 NRZ encoding 100G interfaces and supports all speeds down to 1G.

This platform is avaliable only with 16x QSFP28 line cards without an option to breakout ports. Thus, you can install a maximum of 128x100G ports.

  • You can use a single SFP (10/25/50G) transceiver in a QSFP (100/200/400G) port with QSFP-to-SFP Adapter (QSA). Set the port speed to the SFP speed with the nv set interface <interface> link speed <speed> command. Do not configure this port as a breakout port.

  • If you break out a port, then reload the switchd service on a switch running in nonatomic ACL mode, temporary disruption to traffic occurs while the ACLs reinstall.

  • Cumulus Linux does not support port ganging.

  • Switches with the Spectrum 1 ASIC have a limit of 64 logical ports. If you want to break ports out to 4x25G or 4x10G:

    • You can only break out odd-numbered ports into four logical ports.
    • You must disable the next even numbered port. For example, if you break out port 11 into four logical ports, you must disable port 12. These restrictions do not apply to a 2x50G breakout configuration or to the NVIDIA SN2100 and SN2010 switch.
  • Switches with the Spectrum 2 and Spectrum 3 ASIC have a limit of 128 logical ports. To ensure that the number of total logical interfaces does not exceed the limit, if you split ports into four interfaces on Spectrum 2 and Spectrum 3 switches with 64 interfaces, you must disable the adjacent port. For example, when splitting port 1 into four 25G interfaces, you must disable port 2 in the /etc/cumulus/ports.conf file:

    1=4x25G
    2=disabled
    

    When you split a port into two interfaces, such as 2x50G, you do not have to disable the adjacent port.

For valid port configuration and breakout guidance, see the /etc/cumulus/ports.conf file.

Configure a Breakout Port

To configure a breakout port:

This example command breaks out the 100G port on swp1 into four 25G ports:

cumulus@switch:~$ nv set interface swp1 link breakout 4x25G 
cumulus@switch:~$ nv config apply

To break out a port into four 10G ports, you must also disable the next port.

cumulus@switch:~$ nv set interface swp1 link breakout 4x10G
cumulus@switch:~$ nv set interface swp2 link breakout disabled
cumulus@switch:~$ nv config apply
  1. Edit the /etc/cumulus/ports.conf file to configure the port breakout. The following example breaks out the 100G port on swp1 into four 25G ports. (To break out swp1 into four 10G ports, use 1=4x10G.) You also need to disable the next port. The example also disables swp2.

    cumulus@switch:~$ sudo cat /etc/cumulus/ports.conf
    ...
    1=4x25G
    2=disabled
    3=100G
    4=100G
    ...
    
  2. Configure the breakout ports in the /etc/network/interfaces file. The following example shows the swp1 breakout ports (swp1s0, swp1s1, swp1s2, and swp1s3).

cumulus@switch:~$ sudo cat /etc/network/interfaces
...
auto swp1s0
iface swp1s0

auto swp1s1
iface swp1s1

auto swp1s2
iface swp1s2

auto swp1s3
iface swp1s3
...

Reload switchd with the sudo systemctl reload switchd.service command. The reload does not interrupt network services.

cumulus@switch:~$ sudo systemctl reload switchd.service

Remove a Breakout Port

To remove a breakout port:

  1. Run the nv unset interface <interface> command. For example:

    cumulus@switch:~$ nv unset interface swp1s0
    cumulus@switch:~$ nv unset interface swp1s1
    cumulus@switch:~$ nv unset interface swp1s2
    cumulus@switch:~$ nv unset interface swp1s3
    cumulus@switch:~$ nv config apply
    
  2. Run the nv unset interface <interface> link breakout command to configure the interface for the original speed. For example:

    cumulus@switch:~$ nv unset interface swp1 link breakout
    cumulus@switch:~$ nv config apply
    
  1. Edit the /etc/cumulus/ports.conf file to configure the interface for the original speed.

    cumulus@switch:~$ sudo nano /etc/cumulus/ports.conf
    ...
    
    1=100G
    2=100G
    3=100G
    4=100G
    ...
    
  2. Reload switchd. The reload does not interrupt network services.

    cumulus@switch:~$ sudo systemctl reload switchd.service
    

Logical Switch Port Limitations

100G and 40G switches can support a certain number of logical ports depending on the switch. Before you configure any logical ports on a switch, check the limitations listed in the /etc/cumulus/ports.conffile.

ports.conf File Validator

Cumulus Linux includes a ports.conf validator that switchd runs automatically before the switch starts up to confirm that the file syntax is correct. You can run the validator manually to verify the syntax of the file whenever you make changes. The validator is useful if you want to copy a new ports.conf file to the switch with automation tools, then validate that it has the correct syntax.

To run the validator manually, run the /usr/cumulus/bin/validate-ports -f <file> command. For example:

cumulus@switch:~$ /usr/cumulus/bin/validate-ports -f /etc/cumulus/ports.conf

Troubleshooting

This section shows basic commands for troubleshooting switch ports. For a more comprehensive troubleshooting guide, see Troubleshoot Layer 1.

Statistics

To show high-level interface statistics, run the nv show interface <interface> command.

cumulus@switch:~$ nv show interface swp1

    Name    MAC                Speed      MTU  Mode
--  ------  -----------------  -------  -----  ---------
UP  swp1    44:38:39:00:00:04  1G        1500  Access/L2

Vlans in disabled State
-------------------------
br0

Counters      TX    RX
----------  ----  ----
errors         0     0
unicast        0     0
broadcast      0     0
multicast      0     0

LLDP
------  ----  ---------------------------
swp1    ====  44:38:39:00:00:03(server01)

To show low-level interface statistics, run the following ethtool command:

cumulus@switch:~$ sudo ethtool -S swp1
NIC statistics:
      HwIfInOctets: 21870
      HwIfInUcastPkts: 0
      HwIfInBcastPkts: 0
      HwIfInMcastPkts: 243
      HwIfOutOctets: 1148217
      HwIfOutUcastPkts: 0
      HwIfOutMcastPkts: 11353
      HwIfOutBcastPkts: 0
      HwIfInDiscards: 0
      HwIfInL3Drops: 0
      HwIfInBufferDrops: 0
      HwIfInAclDrops: 0
      HwIfInBlackholeDrops: 0
      HwIfInDot3LengthErrors: 0
      HwIfInErrors: 0
      SoftInErrors: 0
      HwIfOutErrors: 0
      HwIfOutQDrops: 0
      HwIfOutNonQDrops: 0
      SoftOutErrors: 0
      SoftOutDrops: 0
      SoftOutTxFifoFull: 0
      HwIfOutQLen: 0

Query SFP Port Information

To verify SFP settings, run the ethtool -m command. The following example shows the vendor, type and power output for the swp1 interface.

cumulus@switch:~$ sudo ethtool -m swp1 | egrep 'Vendor|type|power\s+:'
        Transceiver type                          : 10G Ethernet: 10G Base-LR
        Vendor name                               : FINISAR CORP.
        Vendor OUI                                : 00:90:65
        Vendor PN                                 : FTLX2071D327
        Vendor rev                                : A
        Vendor SN                                 : UY30DTX
        Laser output power                        : 0.5230 mW / -2.81 dBm
        Receiver signal average optical power     : 0.7285 mW / -1.38 dBm

Considerations

Auto-negotiation and FEC

If auto-negotiation is off on 100G and 25G interfaces, you must set FEC to OFF, RS, or BaseR to match the neighbor. The FEC default setting of auto does not link up when auto-negotiation is off.

If auto-negotiation is on and the link speed is set for a port, auto-negotiation takes precedence over the link speed setting.

Port Speed and the ifreload -a Command

When you configure port speed or break outs in the /etc/cumulus/ports.conf file, you must run the ifreload -a command to reload the configuration after restarting switchd if:

  • You configure or configure then remove the port speed in the /etc/cumulus/ports.conf file and you also set or remove the speed on the same physical port or breakouts of that port in the /etc/network/interfaces file after the last time you restarted switchd.
  • You break out a switch port or remove a break out port, and you set the port speed in both the /etc/cumulus/ports.conf file and the /etc/network/interfaces file.

1000BASE-T SFP Modules Supported Only on Certain 25G Platforms

The following 25G switches support 1000BASE-T SFP modules:

  • NVIDIA SN2410
  • NVIDIA SN2010
  • NVIDIA SN3420

100G or faster switches do not support 1000BASE-T SFP modules.

After rebooting the NVIDIA SN2100 switch, eth0 always has a speed of 100MB per second. If you bring the interface down and then back up again, the interface negotiates 1000MB. This only occurs the first time the interface comes up.

To work around this issue, add the following commands to the /etc/rc.local file to flap the interface automatically when the switch boots:

modprobe -r igb
sleep 20
modprobe igb

Delay in Reporting Interface as Operational Down

When you remove two transceivers simultaneously from a switch, both interfaces show the carrier down status immediately. However, it takes one second for the second interface to show the operational down status. In addition, the services on this interface also take an extra second to come down.

NVIDIA Spectrum-2 Switches and FEC Mode

The NVIDIA Spectrum-2 (25G) switch only supports RS FEC.