Welcome to the Intel® Embedded Community.
I want to make you aware that the IXP425 was End-of-Life by Intel in April 2010. It is therefore problematic to start a new product development based on this discontinued part. Perhaps you are planning only to update the software on an existing deployed hardware? If that is the case, you might want to check the links below for answer to your question:
IXP400 Software - VLAN & QoS Application v2.2
This pairs with the IXP400 Software Access Library and NPE Microcode v2.4.
The Programmer’s Guide for it is:
I hope this helps.
J. Felix McNulty
Community Moderator (Intel contractor)