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Dynamic host configuration

Configuring dynamic host configuration (DHCP and SLAAC) on a Cisco device.

DHCP​

DHCP stands for Dynamic Host Configuration Protocol.

DHCP messages​

  • DHCPDISCOVER: trigger available servers to send an offer (broadcast)
  • DHCPOFFER: respond with an offer holding the config (uni-/broadcast)
  • DHCPREQUEST: request to use the offer / renew the lease / inform other servers (broadcast)
  • DHCPACK: acknowledge the lease (uni-/broadcast)

Configuration​

! Exclude ranges from all pools (low- and high address)
ip dhcp excluded-address 192.168.0.1
ip dhcp excluded-address 192.168.0.200 192.168.0.255

! Create a pool
ip dhcp pool PRODUCTION
  network 192.168.0.0 255.255.255.0
  default-router 192.168.0.1
  dns-server 192.168.11.5
  domain-name example.com
  ! lease <days>
  lease 7

Evaluation​

#
show running-config | section dhcp
show ip dhcp binding
show ip dhcp server statistics

ICMPv6 (SLAAC / DHCPv6)​

SLAAC stands for StateLess Address Auto Configuration.

It can optionally work together with a DHCPv6 server.

If both SLAAC and DHCPv6 address config are enabled, a host can have multiple IPv6 addresses.

Terminology​

  • link-local address: an IPv6 address used for local communication
  • network prefix: usually the first 64 bits of a 128 bit IPv6 address
  • GUA: a globally unique address
  • DAD: duplicate address detection (is performed after GUA configuration)

Common ICMPv6 messages​

ICMPv6 stands for Internet Control Message Protocol and is also used for IPv6 address configuration.

  • Router Solicitation (RS): ask routers to send RA (multicast to all routers using ff02::2)
  • Router Advertisement (RA): carries the following data (multicast to all nodes using ff02::1)
    • router's link local
    • network prefix information
    • config flags
      • A (autonomous): hosts should use the network prefix to create their own GUA (stateless)
      • O (other): hosts should use DHCPv6 for non-address configuration (DNS, NTP, etc.)
      • M (managed): hosts should use DHCPv6 for address configuration (stateful) AND config from the O flag
  • Neighbor Solicitation (NS): find MAC addresses of other hosts (like ARP)
  • Neighbor Solicitation (NA): respond to NS messages or notify about address changes

DHCPv6 messages​

  • SOLICIT: trigger DHCPv6 servers to send an advertisement (multicast to all routers)
  • ADVERTISE: inform the host about the server
  • REQUEST: request a lease from the server (INFORMATION-REQUEST when stateless)
  • REPLY: server replying with the data

Configuration​

Minimal router config​

! Make sure routing is enabled to emit RA messages
ipv6 unicast-routing

interface gi0/0
  no shutdown

  ! enables SLAAC by default
  ipv6 address 2001:db8:acad:1::1/64

  ! this should work too to enable SLAAC, but sometimes just doesn't on Cisco Packet Tracer
  ipv6 address autoconfig

  ! Typically not needed on newer devices
  ipv6 enable

More router config​

! Create a DHCPv6 pool
ip dhcp pool LA_PISCINA
  ! O and M flags
  dns-server 2001:db8:acad:ffff::254
  domain-name example.com

  ! only M flag
  address prefix 2001:db8:acad:2::/64

interface gi0/0
  ! change the link-local address
  ipv6 fe80::1 link-local

  ! Configure dhcp pool to retrieve the info specified by flags
  ipv6 dhcp server LA_PISCINA

  ! set the O flag (stateless)
  ipv6 nd other-config-flag

  ! set the M flag (stateful)
  ipv6 nd managed-config-flag

Host configuration​

interface gi0/0
  description LAN interface
  no shutdown

  ! Stateless
  ipv6 address autoconfig

  ! Stateful
  ipv6 address dhcp
  ! Disable SLAAC or both are used
  ipv6 nd prefix default no-autoconfig

Evaluation​

#
show ipv6 interface brief
show ipv6 neighbors

! DHCPv6
show ipv6 dhcp pool
show ipv6 dhcp binding

DHCPv6 relay agents​

config-if#
! Relays DHCPv6 traffic for this server to another
! Does not work in Cisco Packet Tracer yet
ipv6 dhcp relay destination 2001:db8:acad:adb::1