Backup browsers are a subset of potential browsers and announce themselves in the same manner as non-browsers. However, backup browsers participate in browser elections. Backup browsers call the master browser every 15 minutes to obtain updated network-resource browse lists and lists of workgroups and domains. The backup browser caches these lists and returns the browse list to any client that sends out a browse request by making a call, using the NetServerEnum API, to the backup browser. If the backup browser cannot find the master browser, it will force an election.
Because it can take up to 15 minutes for a backup browser to receive an updated browse list, it is possible that a computer will appear in the browse list for as long as 51 minutes after it is no longer an available resource on the network. The time period consists of 36 minutes, which is three 12-minute announcement cycles plus the 15 minutes for the backup browser to receive an updated list.