If user does not configure the circuit-id of pppoe-ia explicitly on a port, then the user can config Tr-101 format to generate the circuit-id, the format like follow 'identifier-string STRING option (sp|sv|pv|spv) delimiter WORD', the STRING can be any ascii except space, and it max length is 47, the WORD must be one in (#|.|,|;|:|/|space). If user input NULL, then delete the last configuration. For example, if STRING is switch , option is spv and WORD is '/', the user input the string like as 'identifier-string switch option spv delimiter /'.