My setup: 2 Active Directory domains running in the same building. Both fully are trusted by each other. Each AD domain has one DC, running AD-integrated DNS for its domain. For *some* of the devices in one domain (not all), they can be pinged from a PC in the other domain without the domain suffix, while others cannot. I can't figure out what the difference is.
E.g. on PC pc1.domain.local, I can ping pc11.domain2.local just by typing 'ping pc11' but if I try 'ping pc12' I get no response. I do get a successful ping by using 'ping pc12.domain2.local'. (Pinging the FQDN of any device always works.)
I have conditional forwarders set up on both DNS servers to point to the other. All desktop PCs have both DNS servers set in their IPv4 settings. All device names/IPs are unique across both domains. One server is Server 2003 x32 Std SP2. The other is Server 2008 R2 Std. Any help here is appreciated.