Part 1: Implementing DHCPv6 - Stateful DHCPv6

[Sivagami Narayanan]

 

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

1. ipv6 unciast-routing
2. ipv6 dhcp pool <pool name>
3. address prefix <specify address prefix> lifetime <infinite> <infinite>
4. dns-server <specify the dns server address>
5. domain-name <specify the domain name>
6. exit

In Interface Configuration Mode

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

Client Configuration

In Global Configuration Mode

1. enable
2. configure terminal
3. ipv6 unicast-routing

In Interface Configuration Mode

1. ipv6 address dhcp rapid commit
2. ipv6 enable
3. 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

• Part 2: Implementing DHCPv6 - Stateless DHCPv6
• Implementing DHCP for IPv6
• IP Version 6 (IPv6) - Cisco Systems
• Cisco IOS IPv6 Command Reference
• IPv6 - Frequently Asked Questions

Routing Information Protocol

[netops044]

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]

netops004

The subnet mask is /64

[Harold Ritter]

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]

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]

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 .?

[blorkovic]

Thank you @Sivagami Narayanan 

I know I am coming in late to this, and I should really update my IOS, BUT, I have IOS 15.5 on ISR4321 routers, and the address prefix command is an unrecognized command.  In your example, it seems you are using IOS 15.0.  I don't know why I cannot configure the address prefix in my stateful DHCPv6 pool.  Any ideas?

I've seen some examples online where people used the prefix-delegation command to essentially do the same thing, but that too is unavailable to me.

To add to my confusion, in Packet Tracer 8.2.2, the ISR4321 router running version 15.5 of the IOS has both of these commands.

The IOS file on my routers is isr4300-universalk9.03.16.04b.S.155-3.S4b-ext.SPA.bin

The IOS file on the 4321 routers in packet tracer is isr4300-universalk9.03.16.05.S.155-3.S5-ext.SPA.bin

Seems they "corrected" the problem, but I cannot find any documentation to support that.  Any help will be welcomed.

Thank you.

[Harold Ritter]

Hi @blorkovic ,

Stateful DHCPv6 as been supported for a while. You run a very old version on your ISR4321. I would definitely recommend you upgrade to a more recent level of IOS-XE.

[blorkovic]

Thank you, @Harold Ritter 

I am aware that DHCPv6 has been supported for a long time and that the version on my ISR4321 is old, but I do not understand why my particular version of the IOS does not recognize the commands I mentioned in my previous post.  I was looking for some confirmation that I was not crazy.

Regardless, I will be updating to the most recent, stable version of IOS-XE.  Thanks again!