The value of tBgpPeerNgAdvLabelAddrFamily specifies the
advertisement label address family to support 6PE feature.
The value of ipv6 can be set only for an IPv6 peer. The value
of ipv4 can be set only for an IPv4 peer.
If set to 'ipv6', explicit null will be used as the label.
The default value 'none' disables the 6PE feature.