What is a use case for the interface sub-command ip address ip-address subnet mask secondary?

[continuedprogress101]

I am currently reading the ENCOR 350-401 Official Cert Guide. I am wondering what is the use case for adding a secondary IP to an interface? 

[Martin L]

During some sort of network migration, consolidation, or expansion like a company merger, etc. anyway, it is useful for short period of time.

also, good trick during CCIE lab Troubleshooting section or other exams

Regards, ML
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[Richard Burts]

@Martin L describes a common use case in which you want to migrate a network from one addressing plan (x.x.x.n) to a different addressing plan (y.y.y.n). You can configure y.y.y.0 as a secondary address on the interface. Gradually you migrate hosts from x.x.x to y.y.y. This allows hosts in both address spaces to coexist and to communicate with each other and communicate with outside resources. When all hosts are off of x.x.x you change the interface configuration to make y.y.y.0 the primary IP and remove any secondary IP.

Another use case is that you work for an organization that is growing. You have an existing subnet of some size and are running out of addresses in that subnet. Sometimes you might solve this by changing the subnet mask and making the subnet larger. But frequently this is not an option because the adjoining address space is already allocated somewhere else. So you solve this issue by configuring a secondary subnet which provides the additional address space that you need. 

[luis_cordova]

Hi @continuedprogress101 

Check this link:

http://cisco2960.over-blog.com/2014/03/assigning-multiple-ip-addresses-to-network-interfaces.html

Regards