If the system is listening on port 514 on all interfaces, then you may be hitting a feature in CentOS 6 where incoming traffic arrives on the interface but is dropped by the kernel. This is dropped by the reverse path filter where the routing suggests the traffic should arrive on a different interface.
Add the following lines in /etc/sysctl.conf to ensure that any dropped traffic is being logged. The line net.ipv4.conf.all.rp_filter=0 disables the reverse path filter on all interfaces.
Add the following line to your /etc/sysctl.conf file