DHCP Commands

Dynamic Host Configuration Protocol (DHCP) allows hosts to join a network without having a preconfigured IP address. The setup requires a DHCP server to be present on the network. In large networks, DHCP servers provide IP configuration information for several subnets using DHCP relay agents running on each subnet.

DHCP Relay

RCP100 implements the relay agent functionality for each router interface. The functionality can be enabled on a per interface basis.

For redundancy and load balancing purposes, the relay agent can talk to multiple server groups. A group consists of not more than four DHCP servers. The relay sends the requests to all servers in the group. The servers in a group can implement any failover scheme available, such as the one described in RFC 2131.

If multiple groups are defined, the relay agent will load-balance randomly between groups. If the client fails to acquire an IP address, the request will be sent to all groups after a wait time specified by service-delay command.

DHCP Server

RCP100 also implements DHCP server functionality.


Table Of Contents

clear ip dhcp
default-router
domain-name
dns-server
host
ip dhcp relay
ip dhcp relay enable
ip dhcp server
lease
max-hops
network
ntp-server
option
range
server-group
service-delay
service dhcp
show ip dhcp
Example: DHCP relay
Example: DHCP server

clear ip dhcp

Clear DHCP statistics and leases.

clear ip dhcp statistics

Clear DHCP statistics

.

clear ip dhcp lease [<ip address>]

Clear DHCP leases. If no IP address is specified, all leases are removed

.

Example:

rcp#clear ip dhcp statistics 
rcp#clear ip dhcp lease 10.0.0.78

default-router

Set the default router advertized by DHCP server (DHCP option 3) for a specific network.

default-router <ip address>

no default-router

The no form removes the default router configuration.

Example:

rcp(dhcp server)#network 10.0.0.0/24
rcp(dhcp 10.0.0.0/24)#default-router 10.0.0.1

domain-name

Set the domain name advertized by DHCP server (DHCP option 15).

domain-name <domain>

no domain-name

The no form removes the domain name configuration.

Example:

rcp(dhcp server)#domain-name mycompany.com

dns-server

Set Domain Name Servers (DNS) advertized by DHCP server (DHCP option 6).

dns-server <ip address> [<ip address>]

Up to two DNS servers can be specified

no dns-server

The no form removes the DNS server configuration.

Example:

rcp(dhcp server)#dns-server 8.8.8.8 8.8.4.4

host

Configure a static lease.

host <ip address> <mac address>

no host <ip address>

The no form removes the static lease.

Example:

rcp(dhcp server)#host 10.25.20.50 00:16:af:b5:39:a2

ip dhcp relay

Enter DHCP relay configuration mode.

ip dhcp relay

Use exit to return to global configuration mode.

no ip dhcp relay

The no form clears all the configuration entered in DHCP relay mode.

Example:

rcp(config)#ip dhcp relay 
rcp(dhcp relay)# ?
  exit                        Exit the current mode
  max-hops                    Maximum number of hops
  no                          Negate a command or set its defaults
  option                      Add DHCP Relay Agent Information (option 82)
  server                      DHCP primary server
  service-delay               Delay before sending the request to all servers
  show                        Show running system information
rcp(dhcp relay)#
rcp(dhcp relay)#exit
rcp(config)#

ip dhcp relay enable

The command is executed in interface configuration mode. It enables DHCP relay agent functionality for the interface.

ip dhcp relay enable
no ip dhcp relay enable

Example:

rcp(config)#interface ethernet eth0
rcp(config-if eth0)#ip dhcp relay enable 
rcp(config-if eth0)#

ip dhcp server

Enter DHCP server configuration mode.

ip dhcp server

Use exit to return to global configuration mode.

no ip dhcp server

The no form clears all the configuration entered in DHCP server mode.

Example:

rcp(config)#ip dhcp server
rcp(dhcp server)#
  dns-server                  DNS server
  domain-name                 Domanin name
  exit                        Exit the current mode
  network                     Network
  no                          Negate a command or set its defaults
  ntp-server                  Network Time Protocol server
  show                        Show running system information
rcp(dhcp server)#exit
rcp(config)#

lease

Enter the lease for DHCP clients.

lease <days> <hours> <minutes>

no lease

The no form restores the default lease of one day.

Example:

rcp(dhcp server)#network 10.0.0.0/24
rcp(dhcp 10.0.0.0/24)#lease 0 4 0
The example sets a lease of four hours.

max-hops

If the hop count field in DHCP packet is greater than max-hops, the packet is discarded.

max-hops <count>
no max-hops

The no form of the command restores the default of 4 hops.


network

Set a network for DHCP server and enter network mode.

network <CIDR address>

The network address is specified in CIDR format.

no network <CIDR address>

The no form removes the network configuration.

Example:

rcp(dhcp server)#network 10.0.0.0/24
rcp(dhcp 10.0.0.0/24)#
  default-router              Default router
  exit                        Exit the current mode
  lease                       Lease time
  no                          Negate a command or set its defaults
  range                       Address range
  show                        Show running system information
rcp(dhcp 10.0.0.0/24)#exit
rcp(dhcp server)#

ntp-server

Set Network Time Protocol (NTP) servers advertized by DHCP server (DHCP option 42).

ntp-server <ip address> [<ip address>]

Up to two NTP servers can be specified.

no ntp-server

The no form removes the NTP server configuration.

Example:

rcp(dhcp server)#ntp-server 10.0.0.10 10.0.0.11

option

Insert a DHCP Relay Agent Information element (option 82) in the packet going to DHCP server. The element contains a Agent Circuit ID Sub-option (sub-option 1) in hostname.interface format. (e.g. rcp.eth0). If an option 82 is already present in the incoming packet, the packet is forwarded to the server as is.

option
no option


range

Set a range of IP addresses for DHCP clients on a specific network.

range <ip address> <ip address>

no range

The no form removes the client range configuration.

Example:

rcp(dhcp server)#network 10.0.0.0/24
rcp(dhcp 10.0.0.0/24)#range 10.0.0.50 10.0.0.200

server-group

Define a server group.

[no] server-group <address> [<address>] [<address>] [<address>]

The no form of the command removes the server group.

Example:

rcp(config)#ip dhcp relay 
rcp(dhcp relay)#server-group 10.1.20.70 10.1.20.71 
rcp(dhcp relay)#server-group 10.1.20.72 10.1.20.73 
rcp(dhcp relay)#show configuration dhcp
ip dhcp relay
  server-group 10.1.20.70 10.1.20.71
  server-group 10.1.20.72 10.1.20.73
!
rcp(dhcp relay)#

Two server groups are defined. Each group has two servers. Load balancing is done between the two groups with a default service-delay of 10 seconds.


service-delay

Delay before sending the requests to all server groups.

service-delay <seconds>
no service-delay

The no form of the command restores the default of 10 seconds.

When load-balancing between multiple server groups, if the client fails to acquire an IP address, the request is forwarded to all server groups. service-delay configures the time to wait before forwarding the request to everybody.


service dhcp

Enable DHCP server.

service dhcp
no service dhcp

The no form of the command disables the server.

Example:

rcp(config)#service dhcp

show ip dhcp

Show DHCP packet counts and statistics.

show ip dhcp
show ip dhcp interfaces
show ip dhcp leases
show ip dhcp servers

Example:

rcp(dhcp relay)#show ip dhcp interfaces 
Interface eth0
Total packets:
	RX 16004, TX 16001
Packets from/to clients:
	DHCPDISCOVER 8003             DHCPOFFER 8001
	DHCPREQUEST 8001              DHCPACK 8000
	DHCPDECLINE 0                 DHCPNACK 0
	DHCPRELEASE 0
	DHCPINFORM 0

Interface eth1
Total packets:
	RX 8103, TX 0

Interface eth2
Total packets:
	RX 7898, TX 0

rcp(dhcp relay)#show ip dhcp servers 
Interface eth0
Total packets:
	RX 16004, TX 16001

Interface eth1
Total packets:
	RX 8103, TX 0
Packets from servers:
	DHCPOFFER 4052
	DHCPACK 4051
	DHCPNACK 0

Interface eth2
Total packets:
	RX 7898, TX 0
Packets from servers:
	DHCPOFFER 3949
	DHCPACK 3949
	DHCPNACK 0

rcp(dhcp relay)#

Example: DHCP Relay


Two DHCP load-balancing servers serving two subnetst
Two DHCP load balancing servers servicing two subnets


DHCP1 and DHCP2 are ISC DHCP servers running on regular Linux/*BSD computers. The IP ranges served are non-overlapping. Each server can service all the clients in the network, in case the other one dies. RCP router forwards requests randomly to both servers. If the client doesn't manage to get an IP address in 10 seconds, his requests are sent to both servers.

ISC DHCP server configuration, usually located in /etc/dhcp/dhcpd.conf, could look something like: 
# DHCP1 server
subnet 10.20.0.0 netmask 255.255.255.0 {
}
subnet 10.1.1.0 netmask 255.255.255.0 {
	range 10.1.1.50 10.1.1.149;
}
subnet 10.1.2.0 netmask 255.255.255.0 {
	range 10.1.2.50 10.1.2.149;
}

and

# DHCP2 server
subnet 10.20.0.0 netmask 255.255.255.0 {
}
subnet 10.1.1.0 netmask 255.255.255.0 {
	range 10.1.1.150 10.1.1.250;
}
subnet 10.1.2.0 netmask 255.255.255.0 {
	range 10.1.2.150 10.1.2.250;
}

RCP configuration:

hostname rcp
!
administrator rcp encrypted password QFSNKPHJ$kNBAqXTxlxZhwKWIpt1e61
!
ip dhcp relay
  server-group 10.20.0.50
  server-group 10.20.0.51
!
interface loopback lo
  ip address 127.0.0.1/8
  ip mtu 16436
  no shutdown
!
interface ethernet eth0
  ip address 10.1.1.1/24
  ip mtu 1500
  ip dhcp relay enable
  no shutdown
!
interface ethernet eth1
  ip address 10.1.2.1/24
  ip mtu 1500
  ip dhcp relay enable
  no shutdown
!
interface ethernet eth2
  ip address 10.20.0.1/24
  ip mtu 1500
  no shutdown
!

Example: DHCP Server

This is a sample configuration for a DHCP server handling two network segments.

rcp#show configuration dhcp
service dhcp
ip dhcp server
  domain-name mycompany.com
  dns-server 8.8.8.8 8.8.4.4
  ntp-server 10.0.0.22 10.1.0.22
  network 10.0.0.0/24
    range 10.0.0.205 10.0.0.250
    default-router 10.0.0.1
    lease 0 4 0
  !
  network 10.1.0.0/24
    range 10.1.0.50 10.1.0.100
    default-router 10.1.0.1
    lease 0 4 0
  !
!
rcp#

The leases can be displayed using show ip dhcp leases command:

rcp#show ip dhcp leases 
MAC Address         IP Address                      Lease (hh:mm:ss)
00:58:22:33:00:09   10.0.0.214                      03:59:54
00:58:22:33:00:08   10.0.0.213                      03:59:54
00:58:22:33:00:03   10.0.0.208                      03:59:54
00:58:22:33:00:02   10.0.0.207                      03:59:54
00:58:22:33:00:01   10.0.0.206                      03:59:54
00:58:22:33:00:00   10.0.0.205                      03:59:54
00:58:22:33:00:07   10.0.0.212                      03:59:54
00:58:22:33:00:06   10.0.0.211                      03:59:54
00:58:22:33:00:05   10.0.0.210                      03:59:54
00:58:22:33:00:04   10.0.0.209                      03:59:54
rcp#

Specific leases can be removed using clear ip dhcp lease command:

rcp#clear ip dhcp lease 10.0.0.210