When iSeries NetServer™ is configured as a Logon Server, it tries to become the Primary Domain Controller (PDC) for the domain. Part of that responsibility is the role of the Master Browser (MB). This includes being the Domain Master Browser (DMB) for the domain and a Local Master Browser (LMB) for the subnet.
Browsers maintain the list of computers for their respective domain and a list of reachable domains. Computers that have SMB resources to share, announce themselves to the local subnet (typically every 12 minutes). The LMB for that domain and subnet listens for these announcements and adds these computers to their browse list. Backup Browsers on the subnet periodically contact the LMB for the most recent list. If the LMB knows who the DMB is, it will periodically announce itself to the DMB, which in turn asks the LMB for its most recent local (same subnet) list to merge with the DMB's own. The LMB will periodically ask the DMB for the complete primary list. In this way, each browser will eventually have a complete list of computers sharing resources for their domain, and the list will be at most 45 minutes old.