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Layer 1 and Switch Port Attributes

This chapter discusses the various network interfaces on a switch running Cumulus RMP, how to configure various interface-level settings (if needed) and some troubleshooting commands.

Interface Types

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

  • lo, network loopback device

  • ethN, switch management port(s), for out of band management only

  • swpN, switch front panel ports

  • (optional) brN, bridges (IEEE 802.1Q VLANs)

  • (optional) bondN, bonds (IEEE 802.3ad link aggregation trunks, or port channels)

Interface Settings

Each physical network interface has a number of configurable settings:

Almost all of these settings are configured automatically for you, depending upon your switch ASIC, although you must always set MTU manually.

You can only set MTU for logical interfaces. If you try to set auto-negotiation, duplex mode or link speed for a logical interface, an unsupported error gets returned.

Enabling Auto-negotiation

To configure auto-negotiation, set link-autoneg to on for all the switch ports. For example, to enable auto-negotiation for swp1 through swp52:

cumulus@switch:~$ net add interface swp1-52 link autoneg on
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

Any time you enable auto-negotiation, Cumulus RMP restores the default configuration settings specified in the table below.

By default, auto-negotiation is disabled - except on 10G and 1G BASE-T switches, where it’s required for links to work at all. And for RJ45-SFP converters, you need to manually configure the settings as described in the default settings table below.

If you disable it later or never enable it, then you have to configure the duplex and link speed settings manually using NCLU

  • see the relevant sections below. The default speed if you disable auto-negotiation depends on the type of connector used with the port. For example, SFP+ optics default to 10G.

You cannot or should not disable auto-negotiation off for any type of copper cable, including:

  • 10G BASE-T

  • 10G DAC

However, RJ-45 (10/100/1000 BASE-T) adapters do not work with auto-negotiation enabled. You must manually configure these ports using the settings below (link-autoneg=off, link-speed=1000|100|10, link-duplex=full|half).

Default Interface Configuration Settings

The configuration for each type of interface is described in the following table. Except as noted below, the settings for both sides of the link are expected to be the same.

If the other side of the link is running a version of Cumulus RMP or Cumulus Linux earlier than 3.2, depending up on the interface type, auto-negotiation may not work on that switch. Cumulus Networks recommends you use the default settings on this switch in this case.



Manual Configuration Steps


1000BASE-T (RJ45)


$ net add interface swp1 link speed 1000
$ net add interface swp1 link autoneg off
auto swp1
iface swp1
  link-autoneg off
  link-speed 1000
  • The module has two sets of electronics - the port side, which communicates to the switch ASIC, and the RJ45 side.

  • Auto-negotiation is always used on the RJ45 side of the link by the PHY built into the module. This is independent of the switch setting. Set link-autoneg to off.

  • Auto-negotiation needs to be enabled on the server side in this scenario.



$ net add interface swp1 link speed 10000
$ net add interface swp1 link autoneg off
auto swp1
iface swp1
  link-autoneg off
  link-speed 10000

Port Speed and Duplexing

Cumulus RMP supports both half- and full-duplex configurations. Supported port speeds include 100M, 1G and 10G. Set the speeds in terms of Mbps, where the setting for 1G is 1000 and 10G is 10000.

The duplex mode setting defaults to full. You only need to specify link duplex if you want half-duplex mode.

Example Port Speed and Duplexing Configuration

The following NCLU commands configure the port speed for the swp1 interface:

cumulus@switch:~$ net add interface swp1 link speed 10000
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

The above commands create the following /etc/network/interfaces code snippet:

auto swp1
iface swp1
   link-speed 10000

Port Speed Limitations

Ports can be configured to one speed less than their maximum speed.

Switch port TypeLowest Configurable Speed
1G100 Mb
10G1 Gigabit (1000 Mb)


Interface MTU (maximum transmission unit) applies to traffic traversing the management port, front panel/switch ports, bridge, VLAN subinterfaces and bonds - in other words, both physical and logical interfaces.

MTU is the only interface setting that must be set manually.

In Cumulus Linux, ifupdown2 assigns 1500 as the default MTU setting. You can override this default value by specifying a policy file in /etc/network/ifupdown2/policy.d/, like in the following example:

cumulus@switch:~$ cat /etc/network/ifupdown2/policy.d/address.json
    "address": {
        "defaults": { "mtu": "9000" }

MTU for a Bridge

The MTU setting is the lowest MTU setting of any interface that is a member of that bridge (that is, every interface specified in bridge-ports in the bridge configuration in the interfaces file), even if another bridge member has a higher MTU value. There is no need 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

In order for 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.

Use MTU 9216 for a bridge

Two common MTUs for jumbo frames are 9216 and 9000 bytes. The corresponding MTUs for the VNIs would be 9166 and 8950.

When configuring MTU for a bond, configure the MTU value directly under the bond interface; the configured value is inherited by member links/slave interfaces. If you need a different MTU on the bond, set it on the bond interface, as this ensures the slave interfaces pick it up. There is no need to specify 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. Hence, specifying an MTU on swp1 ensures that swp1.100 inherits swp1’s MTU setting.

Example MTU Configuration

In general, the policy file specified above handles default MTU settings for all interfaces on the switch. If you need to configure a different MTU setting for a subset of interfaces, use NCLU.

The following commands configure an MTU minimum value of 9000 on swp1:

cumulus@switch:~$ net add interface swp1 mtu 9000
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

These commands create the following code snippet:

auto swp1
iface swp1
   mtu 9000

You must take care to ensure there are no MTU mismatches in the conversation path. MTU mismatches will result in dropped or truncated packets, degrading or blocking network performance.

To view the MTU setting, use net show interface <interface>:

cumulus@switch:~$ net show interface swp1
    Name    MAC                Speed      MTU  Mode
--  ------  -----------------  -------  -----  ---------
UP  swp1    44:38:39:00:00:04  1G        1500  Access/L2

Verification and Troubleshooting Commands


High-level interface statistics are available with the net show interface command:

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
Counters      TX    RX
----------  ----  ----
errors         0     0
unicast        0     0
broadcast      0     0
multicast      0     0
------  ----  ---------------------------
swp1    ====  44:38:39:00:00:03(server01)

Low-level interface statistics are available with ethtool:

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
     SoftInDrops: 0
     SoftInFrameErrors: 0
     HwIfOutDiscards: 0
     HwIfOutErrors: 0
     HwIfOutQDrops: 0
     HwIfOutNonQDrops: 0
     SoftOutErrors: 0
     SoftOutDrops: 0
     SoftOutTxFifoFull: 0
     HwIfOutQLen: 0

Querying SFP Port Information

You can verify SFP settings using ethtool -m . The following example shows the output for 1G and 10G modules:

cumulus@switch:~# sudo ethtool -m | egrep '(swp|RXPower :|TXPower :|EthernetComplianceCode)'
swp1: SFP detected
              EthernetComplianceCodes : 1000BASE-LX
              RXPower : -10.4479dBm
              TXPower : 18.0409dBm
swp3: SFP detected
              10GEthernetComplianceCode : 10G Base-LR
              RXPower : -3.2532dBm
              TXPower : -2.0817dBm