Tuesday, September 8, 2020

Cumulus Linux 4.2

Cumulus Linux is a network operating system for open networking hardware. Cumulus VX is a free virtual appliance that allows network engineers to experiment with Cumulus Linux and verify configurations before deploying into production. 
The Cumulus VX documentation describes how to build network topologies in KVM, VirtualBox, using VMWare hypervisors. If you want to run virtual machines locally, Cumulus in the Cloud is a free service that will allow you to access pre-built networks in the public cloud.

A key feature of Cumulus Linux is the use of the Linux kernel as the authoritative repository of network state. A result of this approach is that the behavior of a Cumulus Linux VX virtual appliance is the same as Cumulus Linux running on a hardware switch. For example, the open source FRR routing daemon shipped with Cumulus Linux uses the Linux netlink API to push routes to the kernel, which forwards packets in the virtual appliance. On a physical switch, routes are still pushed to the kernel, but kernel routing configuration is then offloaded to the switch ASIC so that packets bypass the kernel and are routed by hardware.

Cumulus Linux includes the open source Host sFlow agent. Here again, standard Linux APIs are used to implement sFlow packet sampling (see Linux 4.11 kernel extends packet sampling support). On the virtual appliance, packet sampling is performed by the Linux kernel. On a hardware switch packet sampling is offloaded to the switch ASIC. In both cases streaming sFlow telemetry provides visibility into packet forwarding.

This article demonstrates how to configure and enable sFlow Cumulus Linux (see Monitoring System Statistics and Network Traffic with sFlow). If you don't have access to a switch, download Cumulus VX to follow the example.

First log into the switch.
ssh cumulus@leaf01
Next, edit configuration file:
sudo vi /etc/hsflowd.conf
Change the following configuration setting to send sFlow to a collector at address
sflow {
  collector { ip= }
Start the hsflowd daemon:
sudo systemctl enable hsflowd@mgmt.service
sudo systemctl start hsflowd@mgmt.service
In this case the collector is on the out of band management network and so the daemon needs to run in the management VRF (see Management VRF).

Use the following commands instead to send sFlow to an in-band collector on the default VRF:
sudo systemctl enable hsflowd.service
sudo systemctl start hsflowd.service
Note: Automating sFlow configuration is straightforward since the configurations are not switch specific so that every switch in the network can be given the same configuration.

Docker Desktop provides a convenient method of running sFlow analytics software on the collector machine (
docker run --rm -p 6343:6343/udp sflow/sflowtool
Use the sflow/sflowtool image to verify that sFlow telemetry is being received.
docker run --rm -p 8008:8008 -p 6343:6343/udp sflow/prometheus
Use the sflow/prometheus image to run the sFlow-RT real-time analyzer packaged with tools to browse flows and metrics and export data to Prometheus and Grafana (see sFlow-RT Network Interfaces, sFlow-RT Countries and Networks, and sFlow-RT Health dashboards for examples). Open the URL http://localhost:8008/ to access the web interface.

RESTful control of Cumulus Linux ACLs describes an open source extension to the Cumulus Linux REST API that has been used to automate control actions based on real-time traffic analytics: Triggered remote packet capture using filtered ERSPAN and DDoS mitigation with Cumulus Linux.

Cumulus VX provides a convenient platform for developing and testing monitoring and control strategies before deploying them into a production network. The consistency between the behavior of the Cumulus VX virtual appliance and Cumulus Linux running on a physical switch assures a seamless transition.