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ipv6 address autoconfig vs ipv6 address dhcp

tresdodi
Beginner
Beginner

Hello everybody. Please pardon my silly questions, I'm a newby in this field. I have two questions about IPv6 address configuration:

1) If an interface is set with ipv6 address autoconfig, will it use any of the three auto-configurations options (RA info only, RA info and DHCPv6 server, DHCPv6 server only) as indicated in the RA message?

2) If an interface is set with ipv6 address dhcp, will it look for DHCPv6 servers only? If so, what if no servers are found?

Thanks!

1 Accepted Solution

Accepted Solutions

Peter Paluch
Hall of Fame Cisco Employee Hall of Fame Cisco Employee
Hall of Fame Cisco Employee

Hi,

Don't apologize, you are welcome.

1) If an interface is set with ipv6 address autoconfig, will it use any of the three auto-configurations options (RA info only, RA info and DHCPv6 server, DHCPv6 server only) as indicated in the RA message?

With this configuration, the interface should only obtain its configuration using RAs and the stateless address autoconfiguration (SLAAC). If RAs are received then the device has everything it needs: It can derive its own prefix and its default gateway. DHCPv6 would be providing another settings that are not that relevant to a router (DNS address, domain name, etc.). With ipv6 address autoconfig, a router will not attempt to contact DHCPv6.

2) If an interface is set with ipv6 address dhcp, will it look for DHCPv6 servers only? If so, what if no servers are found?

It will be looking for DHCPv6 and try to obtain its configuration from it, with a notable exception of the default gateway. DHCPv6 does not have an option to advertise a default gateway, and the default gateway still has to be discovered through RAs. So in this case, it will depend on DHCPv6 to discover its address and prefix, and on RAs to discover its gateway. If no DHCPv6 is available, the interface will not obtain any global IPv6 address.

Best regards,
Peter

View solution in original post

4 Replies 4

Peter Paluch
Hall of Fame Cisco Employee Hall of Fame Cisco Employee
Hall of Fame Cisco Employee

Hi,

Don't apologize, you are welcome.

1) If an interface is set with ipv6 address autoconfig, will it use any of the three auto-configurations options (RA info only, RA info and DHCPv6 server, DHCPv6 server only) as indicated in the RA message?

With this configuration, the interface should only obtain its configuration using RAs and the stateless address autoconfiguration (SLAAC). If RAs are received then the device has everything it needs: It can derive its own prefix and its default gateway. DHCPv6 would be providing another settings that are not that relevant to a router (DNS address, domain name, etc.). With ipv6 address autoconfig, a router will not attempt to contact DHCPv6.

2) If an interface is set with ipv6 address dhcp, will it look for DHCPv6 servers only? If so, what if no servers are found?

It will be looking for DHCPv6 and try to obtain its configuration from it, with a notable exception of the default gateway. DHCPv6 does not have an option to advertise a default gateway, and the default gateway still has to be discovered through RAs. So in this case, it will depend on DHCPv6 to discover its address and prefix, and on RAs to discover its gateway. If no DHCPv6 is available, the interface will not obtain any global IPv6 address.

Best regards,
Peter

I see. Thanks a lot!

Note that there is a extra "static DHCP" case for v6 not found in v4.  If the RA has the "Managed" flag off but the "Other config" flag on, and there is an advertised prefix flagged for autoconfiguration, the expected client behavior is to do SLAAC for the address but do non-address DHCP for other network parameters such as DNS, syslog, NTP servers and the like.  I'm not sure what Cisco devices do with this as clients; they definitely support it from the server/router side.

LAN behaviors are actually the place where IPv6 differs the most from IPv4; from a WAN architecture point of view they are both packet switched, next hop routed, best effort delivery.  Cisco Sweden put up a nice L2/L3 presentation a couple years ago; try a search for "Janne Östling IPv6 dummies cisco"

-- Jim Leinweber, WI State Lab of Hygiene

hi there,

im still trying to undertand in depth about autoconfiguration and dhcp

what are the advanatges of autoconfiguration over dhcp

and if an ipv4 network uses autoconfiguration, but may still using dhcp at the same time, why?

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