Introduction to oper group#

One of the most common use cases that can be covered with the Event Handler framework is known as "Operational group" or "Oper-group" for short. An oper-group feature covers several use cases, but in essence, it creates a relationship between logical elements of a network node so that they become aware of each other - forming a logical group.

In the data center space oper-group feature can tackle the problem of traffic black-holing when leaves lose all connectivity to the spine layer. Consider the following simplified Clos topology where clients are multi-homed to leaves:

With EVPN all-active multihoming enabled in fabric traffic from client1 is load-balanced over the links attached to the upstream leaves and propagates via fabric to its destination.

Since all links of a client' bond interface are active, traffic is hashed to each of the constituent links and thus utilizes all available bandwidth. A problem occurs when a leaf looses connectivity to all upstream spines, as illustrated below:

When leaf1 loses its uplinks, traffic from client1 still gets sent to it since the client is not aware of any link loss problems happening on the leaf. This results in traffic blackholing on leaf1.

To remedy this particular failure scenario an oper-group can be used. The idea here is to make a logical grouping between certain uplink and downlink interfaces on the leaves so that downlinks would share fate with uplink status. In our example, oper-group can be configured in such a way that leaves will shutdown their downlink interfaces should they detect that uplinks went down. This operational group's workflow depicted below:

When a leaf loses its uplinks, the oper-group gets notified about that fact and reacts accordingly by operationally disabling the access link towards the client. Once the leaf's downlink transitions to a down state, the client's bond interface stops using that particular interface for hashing, and traffic moves over to healthy links. In our example, the client stops sending to leaf1 and everything gets sent over to leaf2.

In this tutorial, we will see how SR Linux's Event Handler framework enables oper-group capability.

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