Overview

You can use a Cisco router as a DHCP server to distribute IP addresses to DHCP clients.This section describes the configuration and verification commands to make a Cisco router (Cisco IOS) act as a DHCP server.

Cisco IOS DHCP Server Configuration Commands

DHCP Server Configuration Flow

The basic flow for making a Cisco router act as a DHCP server is as follows

  1. Enable DHCP server
  2. DHCP Pool Configuration
  3. Configure Excluded Addresses

Enable DHCP server

To configure a Cisco router as a DHCP server, enter the following command in global configuration mode

Enable DHCP server

(config)#service dhcp

This command is enabled by default, so there is no need to enter it again.

DHCP Pool Configuration

Create a DHCP pool and define the TCP/IP configurations to be distributed to DHCP clients; the DHCP pool is configured in global configuration mode as follows

DHCP Pool Configuration

(config)#ip dhcp pool <pool-name>
(dhcp-config)#network <network>  [ <mask> | / <prefix-length> ]
(dhcp-config)#default-router <address>
(dhcp-config)#domain-name <domain>
(dhcp-config)#dns-server <address>
(dhcp-config)#lease { <days> [<hours>] [<minutes>] | infinite}

<pool-name> : DHCP pool name
<network> : Network address to be distributed to DHCP clients
default-router <address> : Default gateway IP address
<domain> : Domain name
dns-server <address> : DNS server IP address
lease { <days> [<hours>] [<minutes>] | infinite} : Lease period for TCP/IP configurations to be distributed

Distributes IP addresses of the network addresses specified <network> in to DHCP clients. IP addresses in the range <network> are distributed in the order of available IP addresses. To distribute only a specific range of addresses, configure the excluded address as well. Also, <default-router> is the IP address of the default gateway. <domain> and <address> configure the domain name and DNS server IP address to be distributed. “lease” is the lease period. The default is 1 day; infinite means no expiration.

Configure Excluded Addresses

IP addresses that you do not want to distribute to DHCP clients are configured as excluded addresses as follows

Configure Excluded Addresses

(config)#ip dhcp excluded-address <low-ip-address> [<high-ip-address>]

With this configuration, IP addresses in the range <low-ip-address> to <high-ip-addres> are not distributed to DHCP clients. Use the ip dhcp excluded-address command to configure as many excluded addresses as necessary.

Inheritance of DNS server/domain name configuration

To distribute configurations with DHCP to DHCP clients on multiple networks, multiple DHCP pools are configured. Within each pool, DNS servers and domain names are also configured, but they are often the same, not different for each pool. Common configurations such as DNS servers and domain names can be combined into one pool, rather than one pool for each.

Create a pool of aggregated network addresses and configure DNS servers and domain names. The DNS server/domain name configuration will then be inherited by the pool of s within the aggregation. For example, create a DHCP pool in the 192.168.0.0/16 range and specify the DNS server/domain name. Then create DHCP pools for 192.168.1.0/24 and 192.168.2.0/24. These individual pools will inherit the DNS server/domain name and can be distributed to clients.

Figure: Cisco IOS DHCP Server Inheriting DNS Server/Domain Name Configuration
Figure: Cisco IOS DHCP Server Inheriting DNS Server/Domain Name Configuration

Cisco IOS DHCP Server Verification Commands

After configuring a Cisco router as a DHCP server, the main commands to verify its operation are as follows

CommandsOverview
On DHCP server 
#show ip dhcp poolDisplays the address range of the DHCP pool.
#show ip dhcp bindingsDisplays IP addresses assigned to DHCP clients.
On DHCP client 
#show ip interface [brief]Verify the IP address assigned to the interface.
#show dhcp lease
#show dhcp server		Verify the information assigned by DHCP.
Table Cisco IOS DHCP Server Verification Commands

show ip dhcp pool

The show ip dhcp pool command displays the address range of the configured DHCP pool.

show ip dhcp pool

R1#show ip dhcp pool

Pool 192.168.3.0 :
 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
 192.168.3.2          192.168.3.1      - 192.168.3.254     1

Pool HOST1 :
 Utilization mark (high/low)    : 100 / 0
 Subnet size (first/next)       : 0 / 0
 Total addresses                : 1
 Leased addresses               : 1
 Pending event                  : none
 0 subnet is currently in the pool :
 Current index        IP address range                    Leased addresses
 192.168.3.101        192.168.3.101    - 192.168.3.101     1

Pool R2 :
 Utilization mark (high/low)    : 100 / 0
 Subnet size (first/next)       : 0 / 0
 Total addresses                : 1
 Leased addresses               : 1
 Pending event                  : none
 0 subnet is currently in the pool :
 Current index        IP address range                    Leased addresses
 192.168.12.2         192.168.12.2     - 192.168.12.2      1

Pool DNS_DOMAIN :
 Utilization mark (high/low)    : 100 / 0
 Subnet size (first/next)       : 0 / 0
 Total addresses                : 65534
 Leased addresses               : 0
 Pending event                  : none
 1 subnet is currently in the pool :
 Current index        IP address range                    Leased addresses
 192.168.0.1          192.168.0.1      - 192.168.255.254   0

show ip dhcp bindings

Use the show ip dhcp bindings command to verify the IP addresses assigned to DHCP clients as a DHCP server.

show ip dhcp bindings 

R1#show ip dhcp binding
Bindings from all pools not associated with VRF:
IP address          Client-ID/              Lease expiration        Type
                    Hardware address/
                    User name
192.168.3.1         0063.6973.636f.2d63.    Mar 02 2002 12:01 AM    Automatic
                    6330.352e.3564.6230.
                    2e30.3030.302d.4574.
                    302f.30
192.168.3.101       0100.0000.0011.11       Infinite                Manual
192.168.12.2        0100.0000.0022.22       Infinite                Manual

show ip interface [brief]

If you have a Cisco router as a DHCP client, you can check the IP address assigned by DHCP with the show ip interface command.

show ip interface [brief] 

HOST1#show ip interface brief
Interface                  IP-Address      OK? Method Status                Protocol
Ethernet0/0                192.168.3.101   YES DHCP   up                    up
Ethernet0/1                unassigned      YES NVRAM  administratively down down
Ethernet0/2                unassigned      YES NVRAM  administratively down down
Ethernet0/3                unassigned      YES NVRAM  administratively down down
HOST1#show ip interface Ethernet 0/0
Ethernet0/0 is up, line protocol is up
  Internet address is 192.168.3.101/24
  Broadcast address is 255.255.255.255
  Address determined by DHCP
  MTU is 1500 bytes
  Helper address is not set
-- omitted --

show dhcp lease/show dhcp server

The show dhcp lease/show dhcp server command is used to verify the configuration information distributed by the DHCP server.

show dhcp lease/show dhcp server 

HOST1#show dhcp lease
Temp IP addr: 192.168.3.101  for peer on Interface: Ethernet0/0
Temp  sub net mask: 255.255.255.0
   DHCP Lease server: 192.168.13.1, state: 3 Bound
   DHCP transaction id: CC
   Lease: 86400 secs,  Renewal: 43200 secs,  Rebind: 75600 secs
Temp default-gateway addr: 192.168.3.3
   Next timer fires after: 11:54:09
   Retry count: 0   Client-ID: 0000.0000.1111
   Client-ID hex dump: 000000001111
   Hostname: HOST1
HOST1#show dhcp server
   DHCP server: ANY (255.255.255.255)
    Leases:   1
    Offers:   1      Requests: 1     Acks : 1     Naks: 0
    Declines: 0      Releases: 0     Query: 0     Bad: 0
    DNS0:   1.1.1.1,   DNS1:  0.0.0.0
    Subnet: 255.255.255.0   DNS Domain: n-study.com

Summary

Points

  • The basic configuration flow for a Cisco router (Cisco IOS) to act as a DHCP server is as follows
    • Enable DHCP server
      (config)#service dhcp
    • DHCP Pool Configuration
      (config)#ip dhcp pool
    • Configure Excluded Addresses
      (config)#ip dhcp excluded-address
  • DNS server and domain name information configured for an aggregated network address pool can be inherited by the pool of network addresses within the scope of the aggregation.

Related articles

The following article is a specific configuration example of a Cisco router as a DHCP server.

N-Study
Cisco Router DHCP Server Configuration Example | N-Study
https://www.n-study.com/en/tcp-ip/cisco-dhcp-server-configuration-example/
Configure the Cisco router as a DHCP server; configure a DHCP relay agent and manual binding to assign a specific IP address.

The following article is summary of how to place a DHCP server.

N-Study
Summary of DHCP Server Placement | N-Study
https://www.n-study.com/en/tcp-ip/dhcp-server-placement/
In principle, the DHCP server and client must be on the same network; this section summarizes how to place the DHCP server.

TCP/IP