Switch Port Attributes

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

• lo is the network loopback device
• ethN are switch management ports (for out of band management only)
• swpN are switch front panel ports
• (optional) brN are bridges (IEEE 802.1Q VLANs)
• (optional) bondN are bonds (IEEE 802.3ad link aggregation trunks, or port channels)

Each physical network interface (port) has several settings:

• Auto-negotiation
• Duplex Mode
• Link speed
• MTU (maximum transmission unit)
• FEC (forward error correction)

For NVIDIA Spectrum ASICs, the firmware configures FEC, link speed, duplex mode and auto-negotiation automatically, following a predefined list of parameter settings until the link comes up. 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:~$ net add interface swp1 mtu 1500
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

auto swp1
iface swp1
    mtu 1500

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

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

auto swp1
iface swp1
    mtu 1500

cumulus@switch:~$ sudo ifreload -a

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

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" }
            }
}

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:~$ net show interface swp1
    Name    MAC                Speed      MTU  Mode
--  ------  -----------------  -------  -----  ---------
UP  swp1    44:38:39:00:00:04  1G        9216  Access/L2

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.

In Cumulus Linux 4.4.2 and later, 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:~$ net add trap l3-mtu-err action off
cumulus@switch:~$ net commit

cumulus@switch:~$ echo "0 >" /cumulus/switchd/config/trap/l3-mtu-err/enable

FEC

Forward Error Correction (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 bit error rate (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.

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).

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.

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 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.

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).

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:~$ sudo net add interface swp1 link fec rs
cumulus@switch:~$ sudo net pending
cumulus@switch:~$ sudo net commit

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

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

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

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

cumulus@switch:~$ sudo net add interface swp1 link fec baser
cumulus@switch:~$ sudo net pending
cumulus@switch:~$ sudo net commit

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

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

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

To enable FEC with Auto-negotiation:

cumulus@switch:~$ sudo net add interface swp1 link autoneg on
cumulus@switch:~$ sudo net pending
cumulus@switch:~$ sudo net commit

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

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

auto swp1
iface swp1
link-autoneg on

cumulus@switch:~$ sudo ifreload -a

ethtool -s swp1 speed 10000 duplex full autoneg on

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:~$ sudo net add interface swp1 link fec off
cumulus@switch:~$ sudo net pending
cumulus@switch:~$ sudo net commit

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

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

auto swp1
iface swp1
link-fec off

cumulus@switch:~$ sudo ifreload -a

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

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"}
        }
    }
}

Breakout Ports

Cumulus Linux supports the following ports breakout options:

Configure a Breakout Port

To configure a breakout port:

cumulus@switch:~$ net add interface swp1 breakout 4x25G
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

cumulus@switch:~$ net add interface swp2 breakout disabled
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

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

auto swp1s1
iface swp1s1

auto swp1s2
iface swp1s2

auto swp1s3
iface swp1s3
...

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

cumulus@switch:~$ nv set interface swp1 link breakout 4x10G
cumulus@switch:~$ nv set interface swp2 link breakout disabled
cumulus@switch:~$ nv config apply

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

auto swp1s1
iface swp1s1

auto swp1s2
iface swp1s2

auto swp1s3
iface swp1s3
...

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

Remove a Breakout Port

To remove a breakout port:

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 NCLU net show interface command. The NVUE Command is nv show interface.

cumulus@switch:~$ net 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.

Auto-negotiation and Link Speed

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 configuring port speed or break outs in the /etc/cumulus/ports.conf file, you need to run the ifreload -a command to reload the configuration after restarting switchd in the following cases:

• 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 since the last time you restarted switchd.

• If 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.

Port Speed Configuration

If you change the port speed in the /etc/cumulus/ports.conf file but the speed for that port is also in the /etc/network/interfaces file, after you edit the /etc/cumulus/ports.conf file and restart switchd, you must also run the ifreload -a command.

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

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

• NVIDIA SN2410
• NVIDIA SN2010

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

NVIDIA SN2100 Switch and eth0 Link Speed

After rebooting the NVIDIA SN2100 switch, eth0 always has a speed of 100Mb/s. 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.

Related Information

• Debian - Network Configuration