Most IPV6 BGP configuration i can see both of these commands under router BGP
no bgp fast-external-fallover
no bgp default ipv4-unicast
From my understanding "no bgp default ipv4-unicast" is to make bgp advertise ipv4 and ipv6 at the same time if you are running dual stack.
For "no bgp fast-external-fallover" command. I do not know if this is necessary since you want BGp to session to go down immediately so that if you have iBGP with another router if will failover quickly.
Actually the 2nd command is widely used in SP environnment where you want your PE to advertise only VPNv4 address-family. This command means the PE will not advertise any address-family (by default it will announce IPv4 AF) and you have to specifically activate your neighbor under the AF you are interested in.
From what I understand disabling default ipv4-unicast - will enable you to configure other address families under the MBGP process, could be multicast, IPv4, IPv6, VPNv4, etc... it is not only used to enable or disable IPv6 capability under the MBGP process.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...