Setting Up a DHCP Server

Configuring a DHCP Server on a Cisco router is a very common task on SOHO enviroments. In small companies with a small number of hosts, there is no need of a separate DHCP Server, taking advantage of the DHCP Service avaliable on routers.

Verify the service is running:

This service is enabled by the default, but can be disabled for security or if we are leveraging on a third device. We can check the service is running using this command:

R1#sh udp

Proto    Remote      Port      Local       Port  In Out Stat TTY OutputIF

 17 0.0.0.0             0 85.0.12.1          67   0   0 2211   0

As we can see, the router is listening at DHCP Server’s port UDP 67.

Disabling the service:

R1(config)#no service dhcp

R1(config)#do sh udp

Proto    Remote      Port      Local       Port  In Out Stat TTY OutputIF

Procedure for configuring DHCP:

We are going to use R4 as DHCP server and R5 as DHCP client:

First step is specify the pool, together with the excluded addresses:

R4(config)#ip dhcp excluded-address 85.0.145.1 85.0.145.4

R4(config)#ip dhcp pool LAN

R4(dhcp-config)#network 85.0.145.0 255.255.255.0

R4(dhcp-config)#default-router 85.0.145.4

R4(dhcp-config)#dns-server 85.0.145.4

For our example, I have excluded the first 4 addresses, to avoid duplicated address issues with R1 and R4.

 Verify the correct function:

Check that R5 gets an ip:

R5(config)#int fa0/0

R5(config-if)#ip add dhcp

R5(config-if)#do sh ip

*Mar  1 00:02:17.651: %DHCP-6-ADDRESS_ASSIGN: Interface FastEthernet0/0 assigned DHCP address 85.0.145.5, mask 255.255.255.0, hostname R5

See the corresponding binding at R4:

R4(config)#do sh ip dhcp bind

Bindings from all pools not associated with VRF:

IP address          Client-ID/              Lease expiration        Type

                    Hardware address/

                    User name

85.0.145.5          0063.6973.636f.2d63.    Mar 02 2002 12:02 AM    Automatic

                    3230.382e.3066.6330.

                    2e30.3030.302d.4661.

                    302f.30

R4#sh ip dhcp server stat

Memory usage         23799

Address pools        1

Database agents      0

Automatic bindings   1

Manual bindings      0

Expired bindings     0

Malformed messages   0

Secure arp entries   0

Message              Received

BOOTREQUEST          0

DHCPDISCOVER         1

DHCPREQUEST          1

DHCPDECLINE          0

DHCPRELEASE          0

DHCPINFORM           0

Message              Sent

BOOTREPLY            0

DHCPOFFER            1

DHCPACK              1

DHCPNAK              0

R4#sh ip dhcp pool

Pool LAN :

 Utilization mark (high/low)    : 100 / 0

 Subnet size (first/next)       : 0 / 0

 Total addresses                : 254

 Leased addresses               : 1

 Pending event                  : none

 1 subnet is currently in the pool :

 Current index        IP address range                    Leased addresses

 85.0.145.6           85.0.145.1       - 85.0.145.254      1