An IPv6 DHCP offer does not have an IP prefix within the offer, unlike an IPv4 DHCP offer. The IPv6 prefix is usually obtained from the IPv6 Route Advertisement (RA) arriving from the upstream router. For ZTP, SR OS is a host and assigns a /128 prefix to the IPv6 address obtained from the DHCP offer.In addition, SR OS supports the installation of IPv6 default and static routes from upstream routers using the IPv6 RA. Multiple upstream routers can respond to a route solicitation with their own RA. SR OS installs all the routes advertised by the RA. If the same route is advertised by multiple upstream routers (next hops), the SR OS installs the route with the highest preference. The SR OS does not support ECMP when the same route is advertised from multiple next hops by multiple RAs.
To ensure that all the RAs are obtained before the auto-provisioning process is started for IPv6, SR OS follows the RFC 4861 recommendation that the host (in this case SR OS) send a minimum of three route solicitations. This is to ensure that if a route solicitation is lost, at least one of the three would reach the upstream routers. Each route solicitation is followed by a 4 s timeout. If the first route solicitation is sent at T0, the second is sent at T0+4 s and the third is sent at T0+8 s. The upstream routers must respond to the route solicitation with in 0.5 s. This means that the SR OS has all of the RAs and the routes within 8.5 s of the first route solicitation. Therefore, SR OS waits for a maximum of 9 s to receive all RAs.
If the DHCPv6 timeout is less than 9 s, the DHCPv6 timeout is honored even for the RA wait time. If the node has received a single RA and DHCP offer, the process is considered a success. However, it is possible that not all the RAs have arrived on the node because the node has waited less than 9 s.