Retaining the AS Path Attribute in a Service Chain

Service chaining allows two virtual networks to communicate with each other using a service policy or network policy. The VNs communicate through services instances defined in the network policy. Service instances can be physical network functions or virtual network functions.

For data to traverse between VNs, the BGP route attributes are modified according to the network policy. One such BGP attribute, is the AS path attribute. AS path is a sequence of autonomous systems that network packets traverse. By default, the AS path is nullified while leaking routes from the source to the destination network in a service chain. Starting with Contrail Networking Release 2011, you can configure the AS path to be retained in the routes re-originated from the destination VN to the source VN in a service chain. You also have the ability to enable or disable the path retention for selected service chains.

You can enable or disable the Retain AS Path option while configuring the network policy. A network policy is unique to a service chain and configuring the knob at the policy level will apply that feature to that service chain and its component service instances defined by the policy. Even when service instances are shared between multiple service chains, the same service instance can behave differently in different service chains based on the Retain AS Path knob in the policy configuration.