Indicates the source priority of a service instance.
It is used to set the I-TAG priority field when packets are
encapsulated for transmission on the tunnel.
The source priority has eight levels that range from 0 to 7.
By default, the source priority is copied to the mac-tunnel.
When the source priority is set to 8,
it indicates that the service instance is not configured with a priority,
and the priority does not exist.