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I'm learning how to program a Cisco router with dhcpv6 and i saw that they never configure the default gateway address in the dhcpv6 configuration . Then how can the pc know what it's gateway address is? Do i have to configure it myself ? This doesn't sound like something Cisco would forget to add . What am I missing here ? . Thanks guys

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  • The routers send out RAs (Router Advertisements), so you don't need that. There is also the Router Anycast address (all zeroes address).
    – Ron Maupin ♦
    Apr 27, 2018 at 10:31
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer.
    – Ron Maupin ♦
    Dec 25, 2018 at 8:20

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In IPv6 there are two ways of dynamic address assignment. One is SLAAC (StateLess Address Auto Configuration), the other is DHCPv6. DHCPv6 can be used to only assign additional informations (DNS resolvers, NTP, ...) or only addresses or both.

In IPv6 the router tells the client which method to use via flags in a an mechanism called router advertisement (RA). The origin of the RA is used as the default gateway. This is all done via link local addresses (those addresses starting with fe80::)

Below is an example output of an Cisco router showing possible options. If you don't set the managed-config-flag and the other-config-flag no DHCPv6 will be used.

router(config-if)#ipv6 nd ?
  advertisement-interval  Send an advertisement interval option in RA's
  dad                     Duplicate Address Detection
  managed-config-flag     Hosts should use DHCP for address config
  ns-interval             Set advertised NS retransmission interval
  nud                     Neighbor Unreachability Detection
  other-config-flag       Hosts should use DHCP for non-address config
  prefix                  Configure IPv6 Routing Prefix Advertisement
  ra                      Router Advertisement control
  reachable-time          Set advertised reachability time

Note: Android does not support DHCPv6 at all.

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IPv6 uses the concept of neighbour discovery (ND). I do not use IPv6 on a regular basis in my day to day work, but when I do encounter it and need to brush up, www.tcpipguide.com is a very useful starting point.

Here is a link to the Router discovery section that will help explain ND far more elegantly than I could. Enjoy.

http://www.tcpipguide.com/free/t_IPv6NDHostRouterDiscoveryFunctionsRouterDiscoveryP.htm

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