Distributing DNS load via DHCP

[Marc Perea]

Just a heads' up because I discovered this information just a few weeks ago:
 
If your clients are Broadcom chipset based residential gateways, and you use multiple DHCP servers, beware!
 
If the Broadcom RG gets a DHCP OFFER with DNS server order A,B and then gets a ACK with server order B,A, from the 1/2 lease time REQUEST, the firmware is likely to force a software reboot! I believe the IP stack must be unconfigured in order to process a DNS change of order, leaving an unnumbered interface until the DHCP process occurs again. This is the result of nearly a year of troubleshooting various weird issues finally resulting in this discovery. We relay to 3 DHCP servers for redundancy and tried to do some DNS balancing in this way by offering DNS servers in the order:
 
server 1: A, B, C
server 2: B, C, A
server 3: C, A, B
 
Every time a different server responded faster to a client REQUEST than the previous request, the DNS order would change and our customer would suffer a 30 second - 3 minute outage while the RG reconciled the DNS changes by dumping it's DHCP IP interface. Very weird and painful.
 
Additional FYI: The Broadcom chipset can also only handle 2 DNS servers in its internal table, so don't hand back a tertiary. We now hand back B, C from all servers.
 
--Marc
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