Display and Verify DHCPv4

show running-config | section dhcp      # Doesn't work in packet tracer
show ip dhcp binding
show ip dhcp server statistics

DHCPv4: Basic Configuration

Steps:

  1. Exclude IPv4 Addresses: e.g. those assigned to devices that require static addresses - routers, servers, printers, other manually configured devices.
ip dhcp excluded-address <single-ip>
ip dhcp excluded-address <low-address> <high-address>
  1. Configuring a DHCPv4 Pool that will be used for assignment
ip dhcp pool <pool-name>        # Enters DHCPv4 config mode
  1. Configuring Specific Tasks

Required:

network <network-num> [ mask | /prefix-length ]
default-router <address [ address2 ... address8 ]

Optional:

dns-server <address> [ address2 .. address8 ]
domain-name <name>
lease ( days [hours] [minutes] | infinite )
netbios-name-server <address> [ address2 ... address8 ]

Example:

ip dhcp excluded-address 192.168.10.1 192.168.10.9
ip dhcp excluded-address 192.168.10.254
ip dhcp pool LAN-POOL-1
  network 192.168.10.0 255.255.255.0
  default-router 192.168.10.1
  dns-server 192.168.11.5
  domain-name example.com
  exit

Release/Renew IP Address on Windows

ipconfig /release
ipconfig /renew         # PC will broadcast a DHCPDISCOVER message

DHCPv4 Relay Configuration

interface <interface-id>
  ip helper-address <DHCP-IPv4-server-address>
  exit

Example:

  • Router1:
    • G0/0 connected to network1 192.168.10.0/24 with PC1
    • G0/1 connected to network2 192.168.20.0/24 with DHCP server at 192.168.20.5
  • Router1 will need to setup a DHCPv4 relay for PC1 to be able to use DHCP on network2.
interface g0/0
  ip helper-address 192.168.20.5
  end

show ip interface   # Verify configuration

DHCPv4: Troubleshooting

  1. Resolve IPv4 address conflicts
  2. Verify physical connectivity
  3. Test with a static IPv4 address
  4. Verify switch port configuration
  5. Test from the same subnet or VLAN
show ip dhcp conflict

Verify Router DHCPv4 Configuration

If the IPv4 helper address is not configured properly (for when DHCPv4 server is located on a separate LAN from the client), client DHCPv4 requests are not forwarded to the DHCPv4 server.

show running-config | section interface <interface-id>
show running-config | include no service dhcp

DHCPv4 on Routers: Debugging

Verify that the router is receiving the DHCPv4 request from the client:

  • The DHCPv4 process fails if the router is not receiving requests from the client.
  • This involves configuring an ACL for debugging output.
conf t
  access-list 100 permit udp any any eq 78
  access-list 100 permit udp any any eq 68
  end

debug ip packet 100
debug ip dhcp server events

DHCPv6 Router Configuration

IPv6 routing must b enabled before a router can send RA messages:

ipv6 unicast-routing

RA messages are configured on an individual interface of a router. To re-enable an interface for SLAAC that might have been set to another option, the M and O flags need to be reset to their initial values of 0. This is done using the following interface configuration mode commands:

Router(config-if)# no ipv6 nd managed-config-flag 
Router(config-if)# no ipv6 nd other-config-flag

Stateless DHCPv6

For stateless DHCPv6, the O flag is set to 1 and the M flag is left at the default setting of 0. The O flag value of 1 is used to inform the client that additional configuration information is available from a stateless DHCPv6 server.

To modify the RA message sent on the interface of a router to indicate stateless DHCPv6, use the following command:

Router(config-if)# ipv6 nd other-config-flag

Stateful DHCPv6

The M flag indicates whether or not to use stateful DHCPv6. The O flag is not involved. The following command is used to change the M flag from 0 to 1 to signify stateful DHCPv6:

Router(config-if)# ipv6 nd managed-config-flag