The primary use for secondary addressing is for address migration, for example:
Your company wants to re-do their addressing, perhaps to make it more hierarchical for the purpose of proper summarization, or to expand the node count.
Both address blocks can be assigned *** TEMPORARILY *** to one interface, so that the clients can be migrated in phases and still have be be able to access all of the network resources. Once the clients are migrated, the SECONDARY addressing should be removed from the interface.
SECONDARY addressing is considered a security risk, it can screw up some routing protocols, and unnecessarily complicates the operation and administration.
SECONDARY addressing should not be used for anything other than temporary, "band aid" workarounds until the ultimate solution can be implemented.
To use it, just add an additional address command to an existing interface:
ip address 172.16.1.0 255.255.255.0 <---existing
ip address 192.168.1.0 255.255.255.0 SECONDARY <---adds additional address block
ip address 10.1.1.1 255.255.255.0 SECONDARY <-----Adds a third address block
You can have any number of additional SECONDARYs, but remember, it's not meant to be anything but a temporary solution.
Good Luck
Scott
