Using QSFP-SFP+ adapters / break out cables with Cumulus Linux

As the two SFP+ ports of EdgeCore AS7726-32X aren’t supported by Cumulus Linux I had to use QSFP+-SFP+ adapters to make the 10G links work. As expected the port has be put in 10G mode to make this work, setting the link-speed isn’t enough though.

Configure the ports mode

Each port has it’s mode set in /etc/cumulus/ports.conf. This file looks like something like this:

# ports.conf --
# The Accton AS7726 has:
# 32 QSFP28 ports numbered 1-32
# These ports are configurable as 100G, 40G, or split into
# 4x25G, or 4x10G ports.

# QSFP28 ports
# <port label 1-32> = [4x10G|4x25G|40G|100G]


Using a QSFP+-SFP+ adapter now warrants 4x10G mode as this is the only mode breaking the port down in 10G ports although there is only one port.

To configure ports 29 and 30 accordingly the corresponding lines have to be set to


For these changes to take effect the swichtd service has to be restarted to program this into hardware. This is done by restarting the systemd service

systemctl restart switchd.service

Broken down interfaces will show up as swp<num>s<instance> (e.g. swp29s0) and should now be visible in the interface list

# ip -br l
swp29s0 UP   68:21:5f:39:19:b4 <BROADCAST,MULTICAST,UP,LOWER_UP> 
swp29s1 DOWN 68:21:5f:39:19:b5 <BROADCAST,MULTICAST> 
swp29s2 DOWN 68:21:5f:39:19:b6 <BROADCAST,MULTICAST> 
swp29s3 DOWN 68:21:5f:39:19:b7 <BROADCAST,MULTICAST> 
swp30s0 UP   68:21:5f:39:19:b8 <BROADCAST,MULTICAST,UP,LOWER_UP> 
swp30s1 DOWN 68:21:5f:39:19:b9 <BROADCAST,MULTICAST> 
swp30s2 DOWN 68:21:5f:39:19:ba <BROADCAST,MULTICAST> 
swp30s3 DOWN 68:21:5f:39:19:bb <BROADCAST,MULTICAST>

Configure the interface(s)

The break out interfaces no can be configured as usually by /etc/network/interfaces 

auto swp29s0
iface swp29s0
    address 2001:db8:2342:F000::1/126

auto swp30s0
iface swp30s0
    address 2001:db8:2342:F001::1/126

Those interface can be brought up with

# ifup swp29s0
# ifup swp30s0


# ifup -a


# ifreload -a

as you like 🙂

Initial setup for Cumulus Linux for networkers

I just had to set up Cumulus Linux on some EdgeCore AS7726-32X boxes and were a little underwhelmed by the available documentation. So this is my take at an initial setup guide.

Access to the switch

Assuming Cumulus is preinstalled on the box you have two ways to access the switch

by serial console

Fire up your minicom/screen/putty/whatever at 115200 and you will be greeted with a login prompt

by ssh via the mgmt interface

By default Cumulus Linux does dhcp on the mgmt interface(s), so you can ssh into the box by the IP given by DHCP.

The default credentials are

user: cumulus
pass: CumulusLinux!

Root access can be gained via sudo -i using the password from above.

Changing the password

cumulus@sw-01:~$ passwd
Changing password for cumulus.
(current) UNIX password: 
Enter new UNIX password: 
Retype new UNIX password:

Changing the hostname

As Cumulus Linux basically is a Debian Linux changing the hostname is easy and done in two steps:

# hostnamectl set-hostname <new-hostname>

Edit /etc/hosts and change the following line cumulus

to <fqdn> <hostname>

Setting up management in VRF

Edit /etc/network/interfaces with you favorite editor (vi and nano are present) and change the stanza for eth0 like the following

# The primary network interface
auto eth0
iface eth0
    vrf vrf_mgmt

auto vrf_mgmt
iface vrf_mgmt
    vrf-table auto

This will move interface eth0 into VRF vrf_mgmt which seperates routing from the main routing table. vrf-table auto let’s ifupdown2 decide on which Linux routing table it will use for the VRF. If you want to have a fixed table id you can specify that instead of auto. Note ifupdown2 by default requires VRF table ids to be over 1000.

ifup eth0 will bring up the new config on eth0 as well as the VRF. Another way to reload the interface configuration would be ifreload -a which would reload the configuration of all interfaces.

Setting up routed interfaces

Setting up IPs on interfaces happens in /etc/network/interfaces, too. The ports are numbered swp<num> with no leading zeros. Setting IPs on switchport 1 would look like this:

auto swp1
iface swp1
    address 2001:db8:2342:F000::1/126