Why does the IPv6 route validation succeed when the ip address and gateway are the same?

Issue

• Using one of the locally assigned IPv6 addresses as gateway for a remote network should not be allowed
• Assigning the machine's own ip6 address as its default gateway results in the error below, which is expected:

# ip -6 r s dev eth0
fdde::1  proto kernel  metric 256  mtu 1500 advmss 1440 hoplimit 4294967295
fe80::/64  proto kernel  metric 256  mtu 1500 advmss 1440 hoplimit 4294967295

# ip -6 r a default via fdde::1
RTNETLINK answers: Invalid argument

However, the same when defined in the ifcfg configuration file doesn't throw any error, when in fact, the script uses the same 'ip route' command:

# cat /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE="eth0"
IPV6INIT="yes"
ONBOOT="yes"
TYPE="Ethernet"
IPV6ADDR=fdde::1/128
IPV6_DEFAULTGW=fdde::1

# service network restart
Shutting down interface eth0:                              [  OK  ]
Shutting down loopback interface:                          [  OK  ]
Bringing up loopback interface:                            [  OK  ]
Bringing up interface eth0:  
Determining IP information for eth0... done.
                                                           [  OK  ]

 # ip -6 route show dev eth0
fdde::1  proto kernel  metric 256  mtu 1500 advmss 1440 hoplimit 4294967295
fe80::/64  proto kernel  metric 256  mtu 1500 advmss 1440 hoplimit 4294967295
default via fdde::1  metric 1  mtu 1500 advmss 1440 hoplimit 4294967295