The line for a secondary server is similar to the primary except that it lists addresses of other servers (usually primary servers) from which the zone data will be obtained.
secondary Berkeley.Edu 128.32.0.10 128.32.0.4 ucbhosts.bak
The first field specifies that the server is a secondary server for the zone stated in the second field. The two network addresses specify the name servers which have data for the zone. Note that at least one of these will be a primary, and, unless you are using some protocol other than IP/DNS for your zone transfer mechanism, the others will all be other secondary servers. Having your secondary server pull data from other secondary servers is usually unwise, since you can add delay to the propagation of zone updates if your network's connectivity varies in pathological but common ways. The intended use for multiple addresses on a secondary declaration is when the primary server has multiple network interfaces and therefore multiple host addresses. The secondary server gets its data across the network from one of the listed servers. The server addresses are tried in the order listed. If a filename is present after the list of primary servers, data for the zone will be dumped into that file as a backup. When the server is first started, the data is loaded from the backup file if possible, and a primary server is then consulted to check that the zone is still up-to-date. Note that listing your server as a secondary server does not necessarily make it one -- the parent zone must delegate authority to your server as well as the primary and the other secondaries, or you will be transferring a zone over for no reason; no other server will have a reason to query you for that zone unless the parent zone lists you as a server for the zone.
As with primary you may specify a secondary server for a class other than IN by appending /class to the secondary keyword, e.g., secondary/HS.