This is a number representing the service in the rack. This id can be used to reference the service elsewhere in the MIB. There are several conventions used to allocate a service Id. Firstly if a card forms a service by itself, for example a bridge card, which uses none of the backplane 802.3 or token ring lines, then the service id is the same as the slot position of that card within the rack.

If a service is constructed using one of the backplane 802.3 facilities then the service id is 101, 102 or 103 for the three backplane 802.3 paths.

If a service is a token ring fragment then the service id for that service is 200 plus the ring fragment id. For example if Ring Builder builder builds a ring from MAU cards with ring fragment id 7, the corresponding service will be 207.