http://serverfault.com/questions/672513/why-are-eno1-and-eno2-in-this-order
For NICs embedded on the motherboard, rather than in PCI/PCI-x/PCIe slots, the "consistent" network device names are actually obtained from information provided by the system BIOS.
To quote Dell, who helped develop this feature:
The system BIOS indicates the order of Onboard Network Devices to the OS via SMBIOS type 41 records. The system BIOS provides "system slot information" to the OS via type 9 records. The biosdevname makes use of SMBIOS type 41 to suggest names to the onboard network devices and type 9 records to suggest new names for PCI add-in adapters.
In other words, the BIOS decides which onboard NIC is NIC 1 and which is NIC 2.
Thus, I suspect that you had a system BIOS update at some point between your installation of CentOS 6 and your installation of Fedora 21.
Also note that the structure of the names themselves has changed in RHEL/CentOS 7 and Fedora, compared to RHEL/CentOS 6.
In EL6, embedded NICs begin with
em and a number, and NICs on expansion cards start with p followed by their bus, slot and function. This was the original biosdevname feature.
In Fedora and RHEL 7 biosdevname has been replaced
with
native systemd support, and the device naming scheme has
changed. All wired NICs begin with en, and onboard NICs continue with
o and a number, while NICs on expansion cards continue with p, the bus number,
s, the slot number, and optionally f and the function number.
(Though, if you upgrade in place from EL6 to EL7, the old-style names will be kept.)
An example of what you'll see from my own systems:
Onboard NICs (in a Dell PowerEdge):
2: eno1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP mode DEFAULT group default qlen 1000
link/ether 54:9f:35:17:f4:32 brd ff:ff:ff:ff:ff:ff
3: eno2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9000 qdisc mq state UP mode DEFAULT group default qlen 1000
link/ether 54:9f:35:17:f4:34 brd ff:ff:ff:ff:ff:ff
NICs on an expansion card (in a SuperMicro piece of crap):
2: enp3s0f0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP mode DEFAULT group default qlen 1000
link/ether 0c:c4:7a:45:b8:d2 brd ff:ff:ff:ff:ff:ff
3: enp3s0f1: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000
link/ether 0c:c4:7a:45:b8:d3 brd ff:ff:ff:ff:ff:ff