This article provides information and commands concerning the following topics:
Configuring a DHCP Server on an IOS Router
Router(config)# ip dhcp pool INTERNAL | Creates a DHCP pool named INTERNAL. The name can be anything of your choosing |
Router(dhcp-config)# network 172.16.10.0 255.255.255.0 | Defines the range of addresses to be leased |
Router(dhcp-config)# default-router 172.16.10.1 | Defines the address of the default router for the client One IP address is required; however you can specify up to eight IP addresses in the command line. These are listed in order of precedence |
Router(dhcp-config)# dns-server 172.16.10.10 | Defines the address of the Domain Name System (DNS) server for the client. |
Router(dhcp-config)# netbios-name-server 172.16.10.10 | Defines the address of the NetBIOS server for the client |
Router(dhcp-config)# domain-name fakedomainname.com | Defines the domain name for the client |
Router(dhcp-config)# lease 14 12 23 | Defines the lease time to be 14 days, 12 hours, 23 minutes |
Router(dhcp-config)# lease infinite | Sets the lease time to infinity; the default time is 1 day |
Router(dhcp-config)# exit | Returns to global configuration mode |
Router(config)# ip dhcp excluded-address 172.16.10.1 172.16.10.10 | Specifies the range of addresses not to be leased out to clients |
Router(config)# service dhcp | Enables the DHCP service and relay features on a Cisco IOS router |
Router(config)# no service dhcp | Turns the DHCP service off. The DHCP service is on by default in Cisco IOS Software |
Using Cisco IP Phones with a DHCP Server
Note
This section is not part of the CCNA vendor exam topics.
Enterprises with small branch offices that implement a VoIP solution may choose to implement Cisco Unified Communications Manager, often referred to as CallManager, at a central office to control Cisco IP Phones at small branch offices. This design allows for centralized call processing and reduces equipment and administration required (especially at the branch office).
Cisco IP Phones download their configuration from a TFTP server. When a Cisco IP Phone starts, if it does not have its IP address and TFTP server IP address preconfigured, it sends a request with option 150 or 66 to the DHCP server to obtain this information.
DHCP option 150 provides the IP address of a list of TFTP servers.
DHCP option 66 gives the IP address of a single TFTP server.
Note
Cisco IP Phones may also include DHCP option 3 in their requests, which sets a default route.
Router(dhcp-config)# option 66 ip 10.1.1.250 | Provides the IP address of a TFTP server for option 66 |
Router(dhcp-config)# option 150 ip 10.1.1.250 | Provides the name of a TFTP server for option 150 |
Router(dhcp-config)# option 150 ip 10.1.1.250 10.1.1.251 | Provides the names of two TFTP servers for option 150 |
Router(dhcp-config)# option 3 ip 10.1.1.1 | Sets the default route |
Verifying and Troubleshooting DHCP Configuration
Router# show ip dhcp binding | Displays a list of all bindings created |
Router# show ip dhcp binding w.x.y.z | Displays the bindings for a specific DHCP client with an IP address of w.x.y.z |
Router# clear ip dhcp binding a.b.c.d | Clears an automatic address binding from the DHCP server database |
Router# clear ip dhcp binding * | Clears all automatic DHCP bindings |
Router# show ip dhcp conflict | Displays a list of all address conflicts that the DHCP server recorded |
Router# clear ip dhcp conflict a.b.c.d | Clears an address conflict from the database |
Router# clear ip dhcp conflict * | Clears conflicts for all addresses |
Router# show ip dhcp database | Displays recent activity on the DHCP database |
Router# show ip dhcp pool | Displays information about DHCP address pools |
Router# show ip dhcp pool name | Displays information about the DHCP pool named name |
Router# show ip dhcp server statistics | Displays a list of the number of messages sent and received by the DHCP server |
Router# clear ip dhcp server statistics | Resets all DHCP server counters to 0 |
Router# debug ip dhcp server {events | packet | linkage | class} | Displays the DHCP process of addresses being leased and returned |
Configuring a DHCP Helper Address
Router(config)# interface gigabitethernet 0/0 | Moves to interface configuration mode |
Router(config-if)# ip helper-address 172.16.20.2 | Forwards DHCP broadcast messages as unicast messages to this specific address instead of having them be dropped by the router |
Note
The ip helper-address command forwards broadcast packets as a unicast to eight different UDP ports by default:
TFTP (port 69)
DNS (port 53)
Time service (port 37)
NetBIOS name server (port 137)
NetBIOS datagram server (port 138)
Boot Protocol (BOOTP) client and server datagrams (ports 67 and 68)
TACACS service (port 49)
If you want to close some of these ports, use the no ip forward-protocol udp x command at the global configuration prompt, where x is the port number you want to close. The following command stops the forwarding of broadcasts to port 49:
Router(config)# no ip forward-protocol udp 49
If you want to open other UDP ports, use the ip forward-helper udp x command, where x is the port number you want to open:
Router(config)# ip forward-protocol udp 517
Configuring a DHCP Client on a Cisco IOS Software Ethernet Interface
Router(config)# interface gigabitethernet 0/0 | Moves to interface configuration mode |
Router(config-if)# ip address dhcp | Specifies that the interface acquire an IP address through DHCP |
Configuration Example: DHCP
Figure 17-1 illustrates the network topology for the configuration that follows, which shows how to configure DHCP services on a Cisco IOS router using the commands covered in this post.
Figure 17-1 Network Topology for DHCP Configuration
Edmonton Router
Router> enable | Moves to privileged EXEC mode |
Router# configure terminal | Moves to global configuration mode |
Router(config)# hostname Edmonton | Sets the host name |
Edmonton(config)# interface gigabitethernet 0/0 | Moves to interface configuration mode |
Edmonton(config-if)# description LAN Interface | Sets the local description of the interface |
Edmonton(config-if)# ip address 10.0.0.1 255.0.0.0 | Assigns an IP address and netmask |
Edmonton(config-if)# no shutdown | Enables the interface |
Edmonton(config-if)# interface serial 0/0/0 | Moves to interface configuration mode |
Edmonton(config-if)# description Link to Gibbons Router | Sets the local description of the interface |
Edmonton(config-if)# ip address 192.168.1.2 255.255.255.252 | Assigns an IP address and netmask |
Edmonton(config-if)# clock rate 2000000 | Assigns the clock rate to the DCE cable on this side of link |
Edmonton(config-if)# no shutdown | Enables the interface |
Edmonton(config-if)# exit | Returns to global configuration mode |
Edmonton(config)# ip route 192.168.3.0 255.255.255.0 serial 0/0/0 | Creates a static route to the destination network |
Edmonton(config)# service dhcp | Verifies that the router can use DHCP services and that DHCP is enabled |
Edmonton(config)# ip dhcp pool 10NETWORK | Creates a DHCP pool called 10NETWORK |
Edmonton(dhcp-config)# network 10.0.0.0 255.0.0.0 | Defines the range of addresses to be leased |
Edmonton(dhcp-config)# default-router 10.0.0.1 | Defines the address of the default router for clients |
Edmonton(dhcp-config)# netbios-name-server 10.0.0.2 | Defines the address of the NetBIOS server for clients |
Edmonton(dhcp-config)# dns-server 10.0.0.3 | Defines the address of the DNS server for clients |
Edmonton(dhcp-config)# domain-name fakedomainname.com | Defines the domain name for clients |
Edmonton(dhcp-config)# lease 12 14 30 | Sets the lease time to be 12 days, 14 hours, 30 minutes |
Edmonton(dhcp-config)# exit | Returns to global configuration mode |
Edmonton(config)# ip dhcp excluded-address 10.0.0.1 10.0.0.5 | Specifies the range of addresses not to be leased out to clients |
Edmonton(config)# ip dhcp pool 192.168.3NETWORK | Creates a DHCP pool called the 192.168.3NETWORK |
Edmonton(dhcp-config)# network 192.168.3.0 255.255.255.0 | Defines the range of addresses to be leased |
Edmonton(dhcp-config)# default-router 192.168.3.1 | Defines the address of the default router for clients |
Edmonton(dhcp-config)# netbios-name-server 10.0.0.2 | Defines the address of the NetBIOS server for clients |
Edmonton(dhcp-config)# dns-server 10.0.0.3 | Defines the address of the DNS server for clients |
Edmonton(dhcp-config)# domain-name fakedomainname.com | Defines the domain name for clients |
Edmonton(dhcp-config)# lease 12 14 30 | Sets the lease time to be 12 days, 14 hours, 30 minutes |
Edmonton(dhcp-config)# exit | Returns to global configuration mode |
Edmonton(config)# exit | Returns to privileged EXEC mode |
Edmonton# copy running-config startup-config | Saves the configuration to NVRAM |
Gibbons Router
Router> enable | Moves to privileged EXEC mode |
Router# configure terminal | Moves to global configuration mode |
Router(config)# hostname Gibbons | Sets the host name |
Gibbons(config)# interface gigabitethernet 0/0 | Moves to interface configuration mode |
Gibbons(config-if)# description LAN Interface | Sets the local description of the interface |
Gibbons(config-if)# ip address 192.168.3.1 255.255.255.0 | Assigns an IP address and netmask |
Gibbons(config-if)# ip helper-address 192.168.1.2 | Forwards DHCP broadcast messages as unicast messages to this specific address instead of having them be dropped by the router |
Gibbons(config-if)# no shutdown | Enables the interface |
Gibbons(config-if)# interface serial 0/0/1 | Moves to interface configuration mode |
Gibbons(config-if)# description Link to Edmonton Router | Sets the local description of the interface |
Gibbons(config-if)# ip address 192.168.1.1 255.255.255.252 | Assigns an IP address and netmask |
Gibbons(config-if)# no shutdown | Enables the interface |
Gibbons(config-if)# exit | Returns to global configuration mode |
Gibbons(config)# ip route 0.0.0.0 0.0.0.0 serial 0/0/1 | Creates a default static route to the destination network |
Gibbons(config)# exit | Returns to privileged EXEC mode |
Gibbons# copy running-config startup-config | Saves the configuration to NVRAM |
