THE PEACE OFFERING:

In Norse mythology there exist two groups of Gods:  The Vanir & the Aesir.  The Vanir are gods of light; the Aesir, gods of darkness.  The Aesir had more power than the Vanir.

A young Vanir Goddess, with an overly chatty & possibly ditzy personality, named Gullveig was captured & tortured by the Aesir Gods. 

The Vanir demanded revenge & a war was waged among the Gods. 

Ten Suns passed before an agreement was made. 

The Aesir conceeded to give the Vanir equal power & also to apologize for the assualt on Gullveig.

There was however a condition.  The Vanir had to give the Aesir their most prized possession, the Goddess Freya. Freya is considered the most beautiful of all of the Norse goddesses; curvaceous, pale skined (probably so pale it was actually white), blue eyed, & blonde.
Freya was given to the Aesir & the two races of Gods became one. 
Some mythologists believe that Gullveig is another name for Freya.

How does this story back up assumptions that Freya & Frigga are the same deity?

The relation to Persephone