Part 1: Implementing DHCPv6 - Stateful DHCPv6

Sivagami Narayanan · ‎10-28-2012

Introduction

This is the first document in the document series Implementing DHCPv6. In this we will see how configure Stateful DHCPv6 in Cisco IOS routers.

In Stateful DHCP the adddress assignment is centrally managed and clients must obtain configuration information such as address autoconfiguration and neighbor discovery that is not available through protocols.DHCPv6 can be implemented in two ways : Rapid-Commit and Normal Commit mode.

In Rapid-Commit mode , the DHCP client obtain configuration parameters from the server through a rapid two message exchange (solicit and reply).

In Normal-Commit mode, the DHCP client uses four message exchanges (solicit, advertise, request and reply). By default normal-commit is used.

In order to use the rapid-commit option, it has to be enabled by both client and server so that it uses two-message exchange.

Requirements

Understanding of IPv6 Addressing Scheme
Implementing DHCPv6 - An Introduction

Background

In this configuration example,the router R1 is configured as DHCP server while router R2 is configured as DHCPv6Client.

Note: All configurations are tested in a lab environment involving ASR 1000 series router as DHCP server and Cisco 2800 series router as DHCP Clients.

Topology Diagram

Summary Steps

Server Configuration

In Global Configuration Mode

ipv6 unciast-routing
ipv6 dhcp pool <pool name>
address prefix <specify address prefix> lifetime <infinite> <infinite>
dns-server <specify the dns server address>
domain-name <specify the domain name>
exit

In Interface Configuration Mode

ipv6 address <specify IPv6 Address>
ipv6 dhcp server <server name>rapid-commit

Client Configuration

In Global Configuration Mode

enable
configure terminal
ipv6 unicast-routing

In Interface Configuration Mode

ipv6 address dhcp rapid commit
ipv6 enable
exit

Configuration

DHCPv6 ServerDHCPv6 Client

!
version 3.4S
!
hostname R1
!
ipv6 unicast-routing
ipv6 cef
ipv6 dhcp pool test
address prefix 2010:AA01:10::/64 lifetime infinite infinite
dns-server AAAA:BBBB:10FE:100::15
dns-server 2010:AA01::15
domain-name example.com
!
!
!
interface G0/0/1
no ip address
duplex auto
speed auto
ipv6 address 2010:AA01:10::2/64
ipv6 dhcp server test rapid-commit
!
end

!
version 15.0
!
hostname R2
!
ipv6 unicast-routing
ipv6 cef
!
interface FastEthernet0/1
no ip address
duplex auto
speed auto
ipv6 address dhcp rapid-commit
ipv6 enable
!
end

Note: To configure the DHCPv6 in normal-commit mode, use the same command without "rapid-commit" keyword.Also ensure that you explicitly mention the IPv6 enable command in the client interface because, if it is not not enabled, the client will not send a solicit message.

Verify Commands

show ipv6 dhcp pool

To display the DHCPv6 pool information, in the DHCPv6 server use this command. The output shows that the number of active clients is 1 and also displays other configuration parameter information, such as domain name server address and preferred life time information.

R1#show ipv6 dhcp pool
DHCPv6 pool: test
Address allocation prefix: 2010:AA01:10::/64 valid 4294967295 preferred 4294967295 (1 in use, 0 conflicts)
DNS server: AAAA:BBBB:10FE:100::15
DNS server: 2010:AA01::15
Domain name: example.com
Active clients: 1

show ipv6 dhcp binding

To display the information information about the clients, which includes their DUIDs, IAPDs, prefixes, and preferred and valid lifetimes.

R2#show ipv6 dhcp binding
Client: FE80::C801:88FF:FEAC:1C
DUID: 00030001CA0188AC0000
Username : unassigned
IA NA: IA ID 0x00040001, T1 43200, T2 69120
Address: 2010:AA01:10:0:D16D:EC61:EDCB:8BD1
preferred lifetime INFINITY, , valid lifetime INFINITY,

show ipv6 dhcp interface

This command shows that interface Fa0/1 is configured in client mode and also shows the details of the DNS server address and the domain name that it received from the DHCP server.

R2#show ipv6 dhcp interface
FastEthernet1/0 is in client mode
Prefix State is IDLE
Address State is OPEN
Renew for address will be sent in 11:17:16
List of known servers:
    Reachable via address: FE80::C800:88FF:FEAC:1C
    DUID: 00030001CA0088AC0000
    Preference: 0
    Configuration parameters:
      IA NA: IA ID 0x00040001, T1 43200, T2 69120
        Address: 2010:AA01:10:0:D16D:EC61:EDCB:8BD1/128
                preferred lifetime INFINITY, valid lifetime INFINITY
      DNS server: AAAA:BBBB:10FE:100::15
      DNS server: 2010:AA01::15
      Domain name: example.com
      Information refresh time: 0
Prefix Rapid-Commit: disabled
Address Rapid-Commit: enabled

References

Routing Information Protocol

netops044 · ‎11-03-2012

Hi Sivagami,

This presentation is extremely wonderful. But just one queury:

Let us assume I use DHCPv6 on a LAN and I successfully obtain an IPv6 address, DNS and domain name. But where is the subnet mask?

Without a subnet mask, how would a PC be able to communicate with other PCs in the LAN [or] be able to communicate outside the LAN? I would appreciate a clarification on this.

Thanks

arnie2010 · ‎05-27-2013

netops004

The subnet mask is /64

Harold Ritter · ‎05-27-2013

Hi Arnulfo,

DHCPv6 does not carry a subnet mask, so it is assumed to be a /128 by default. If the prefix is advertised via SLAAC, the host will pick up the mask from the router advertisement (RA), which will be a /64 as you mentioned. If the prefix is not advertised via SLAAC, the subnet mask used the host will remain /128.

Regards

mohsin.lko · ‎03-26-2014

I have a 1841 router with 12.4 (15 T) ios . Is this ios not support Stateful ipv6 dhcp. It not show address option in dhcp pool configuration. & stateless dhcp work properly.

Mohammad Alobaidi · ‎12-16-2017

Thank you it is very useful but I have a question in a different scenario I want to config one DHCPv6 server using Packet Tracer and with multiple routers the router takes IP addresses from a server and distribute to computers .I have done that with IPv4 using IP helper but in IPv6 there is no Ip helper

I created in local pool but it did not work .

Can you help me to understand .?