DHCP

General Notes

  • Used to assign IP addresses to hosts without requiring static assignments locally
  • Can operate outside of subnet with ip helper-address on interface to forward DHCP requests to the server
    • This also exists in v6 as ipv6 dhcp relay destination <v6-IP>
  • With IPv6, both stateful and stateless DHCP exist
    • Stateful advertises IPv6 addresses, prefixes, gateways etc like a typical DHCP server in v4
    • Stateless allows hosts to configure their own address, gateway etc locally with SLAAC and only advertises other information such as DNS, TFTP server, etc
    • Stateless DHCP is configured with ipv6 nd other-config-flag under interface on segment that will advertise the information
  • DORA
    • Discover - Broadcast by client discover DHCP server on segment using port 67
    • Offer - Server responds offering IP, SN Mask, gateway, DNS using port 68
    • Request - Client requests to use offered information
    • Acknowledge - Server acknowledges and agrees, records information
  • SARR - IPv6 DORA equivalent
    • Solicit - Client sends v6 DHCP request to all-DHCPv6-server solicit node multicast address FF02::1:2
    • Advertise - Server responds with unicast ADVERTISE message to client
    • Request - Client requests to use offered information
    • Reply - Server acknowledges and agrees, records information (if stateful)

Useful show commands/debugs

  • show ip dhcp binding - When using a local DHCP server on a router, see DHCP binding info
  • show ip dhcp conflict - Show any IPs conflicting with DHCP
  • debug ip dhcp server packet - See DHCP traffic in flow
  • show ipv6 dhcp binding - Same but for v6
  • show ipv6 dhcp interface - Show interfaces advertising v6 address information

Standard DHCP Config

IPv4

ip dhcp excluded-address 10.8.8.1 10.8.8.10
!
ip dhcp pool POOL-A
network 10.8.8.0 255.255.255.0
default-router 10.8.8.1
dns-server 192.168.1.1

IPv6

ipv6 dhcp pool DHCPV6POOL
address prefix 2001:DB8:A:A::/64
dns-server 2001:DB8:B:B::1
domain-name cisco.com
!
interface GigabitEthernet0/0
ipv6 dhcp server DHCPV6POOL ! required to advertise IPs in v6 unlike v4