Why there is the bond0 interface in RHEL 7 even didn't have any network configuration?

Issue

• Why does bond0 appear in ifconfig or ip a when it is not configured?
• Only bond1 or bond2 is configured, but why is there bond0 interface?

 $ nmcli con show |grep bond
bond1    0826c0fe-f5ac-4194-8fcc-d5e71e2d03b7  bond      bond1  
bond2    47cc8e2d-403d-49ea-b287-a547198fca53  bond      bond2  

 $ ls /etc/sysconfig/network-scripts/ifcfg-bond*
/etc/sysconfig/network-scripts/ifcfg-bond1  /etc/sysconfig/network-scripts/ifcfg-bond2

 $ ip a
11: bond0: <BROADCAST,MULTICAST,MASTER> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ether XX:XX:XX:XX:XX:XX brd ff:ff:ff:ff:ff:ff
12: bond1: <BROADCAST,MULTICAST,MASTER,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
    link/ether XX:XX:XX:XX:XX:XX brd ff:ff:ff:ff:ff:ff
13: bond2: <BROADCAST,MULTICAST,MASTER,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
    link/ether XX:XX:XX:XX:XX:XX brd ff:ff:ff:ff:ff:ff