Why it is not recommended to use "dhcpv6-stateful & ipv6-address-mode=not-set" addressing scheme combination for provider networks in RHOSP10 ?

Solution In Progress - Updated -

Environment

  • Red Hat OpenStack Platform 10

Issue

  • Why it is not recommended to use dhcpv6-stateful & ipv6-address-mode=not-set addressing scheme combination for provider networks in RHOSP10 ?
  • Multiple DHPCPV6 servers exist in the network while using "ipv6_ra_mode=dhcpv6-stateful & ipv6-address-mode=not-set" addressing mode combinations for provider networks.

Resolution

It is not recommended to use ipv6_ra_mode=dhcpv6-stateful & ipv6-address-mode=not-set mode combinations for provider networks. This specific address mode combination on provider networks makes the instances to receive an IPv6 address and optional information from an external DHCPv6 server.

The primary issue is that, while creating IPv6 subnets, we have to enable dhcp on the subnet (this is a prerequisite for IPv6 Subnets). When using dhcpv6-stateful subnets (on the provider network), Neutron would spawn a DHCP agent on the external network. So, we would have two DHCPv6 Servers on the network (i.e., one from OpenStack Neutron and the external non-openstack DHCPv6 Server). This could create issues in the network.

The other part of the problem with external DHCP server is that Neutron doesn't know the address got assigned to the VM, so it can't program security groups to allow traffic.

Due to the above reasons, it is not generally recommended to use those RA Mode & Address Mode combination on provider networks.
Important : Please do note that other address mode combinations ( listed as supported ) in the following documentation should work for provider networks, as IP assignment is always managed by OpenStack

https://docs.openstack.org/mitaka/networking-guide/config-ipv6.html

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