Change default DHCP timeout to 60 seconds instead of 20 to avoid STP issues

Change default DHCP timeout to 60 seconds instead of 20 to avoid issues with STP (Spanning Tree Protocol). STP defaults to 15 + 15 seconds on a lot of popular devices for listening and learning states. Which if not disabled (which you should on access switches, sure..) most netboots will fail intermittently due to the port access on switches performing port learning while the machine is trying to get DHCP leases.

More information can be found here:

• https://www.ciscopress.com/articles/article.asp?p=2832407&seqNum=6
• https://techhub.hpe.com/eginfolib/networking/docs/switches/5980/5200-3921_l2-lan_cg/content/499036677.htm