In addition to a role, each ERN has two configured interfaces:
Traffic enters one interface (ingress) and exits the device using the other interface (egress). The right and left interfaces are physically connected.
You must configure these left and right interfaces in the same pattern across all ERNs within a topology. For example you can assign the interfaces as left/right, left/right, left/right, and so on. It is not acceptable, however, to assign interfaces in random order, such as left/right in the configuration of one ERN and then right/left in the configuration of the next ERN.