You may notice the following error that is present in the event viewer of your Windows DNS server after configuring your forwarders to use Cisco Anycast resolvers.
After you deploy a Windows-based DNS server, DNS queries to some domains may not be resolved successfully and you may see the Event ID 5501 occuring repeatedly in your event viewer logs.
The DNS server encountered a bad packet from X.X.X.X. Packet processing leads beyond packet length. The event data contains the DNS packet.
Where X.X.X.X might list our external resolvers 188.8.131.52 and 184.108.40.206
EDNS0 allows larger User Datagram Protocol (UDP) packet sizes. However, some firewall programs may not allow UDP packets that are larger than 512 bytes. Therefore, these DNS packets may be blocked by the firewall.
Windows Server 2008 R2
Windows Server 2003