Dynamic Host Configuration Protocol (DHCP) is a protocol to assign an IP address to a client that requests for an IP address.
There are 4 stages in assigning a IP address using DHCP:
1. Client to server --> DHCPDiscover. In this process the client broadcasts a request asking if there are any DHCP servers available.
2. Server to client --> DHCPOffer. Here the server chooses an IP address from its pool and sends the assigned IP address with additional information (gateway, lease time etc) to client. If there are multiple DHCP server, the client will process and accept the first offer it receives. Before the DHCP server offers the address, it will firstly make sure that the IP address is currently not used (or being given by another DHCP server) by sending 2 ping commands to the IP address with an RTO=500 ms (can be modified by the command (config)#ip dhcp ping packets and (config)#ip dhcp ping timeout). If there is no reply the IP address is clear to use, but if there is a reply the address is being used by someone and DHCP server will choose another IP address from its pool and check the ip address again before offering it to the client.
3. Client to server --> DHCPRequest. In this process, the client accepts the IP address by sending packets with the IP address to the DHCP server.
4. Server to client --> Check. If the IP address is the same as given by the server, then the DHCP server will send DHCPAck with additional info. If it is not the same, the DHCP server will take the offered address back to the pool, meaning that the client has already another IP address.
To configure DHCP service on a router type the command:
(config)#service dhcp
This will enable cisco router to do dhcp by being a DHCP server. It does not necessarily acts as a main dhcp server, but the router can act as a secondary/standby dhcp server in case the main DHCP server goes down.
To disable dhcp service simply put the command:
(config)#no service dhcp
There are several DHCP Assignment Methods which are:
1. Dynamic --> This is the default assignment method where the client is assigned an ip address for a given period of time.
2. Manual --> This method uses static mapping/binding ip addresses to mac address
3. Automatic --> In this method DHCP server chooses IP address from pool to a mac address of the client and permanently binding them
The methods of binding itself is divided in 2 ways:
1. Manual binding, where the IP address is binded to the MAC address if the host is found in the DHCP database. The binding info database is kept in the NVRAM, which means it will be kept when the router is reloaded.
2. Automatic binding, where the IP address is binded to the MAC address, but the binding database is kept on a DHCP Database Agent (a remote device e.g. tftp server).
To create binding:
(dhcp-config)#host
(dhcp-config)#hardware-address xxxx.xxxx.xxxx
(dhcp-config)#client-name
To set dhcp server database location use the command
(config)#ip dhcp database
To create dhcp pool type the command:
(config)#ip dhcp pool
(dhcp-config)#network
There is a possibility to exclude an IP range from the subnet mask to prevent that IP address being given as an DHCP ip address by typing the command:
(config)#ip dhcp exluded-address // if single ip, no high ip address required
To set the lease time of the assigned IP address:
(dhcp-config)# lease
The IP helper address is another useful feature. Basically, a host broadcasts its DHCP request to search for DHCP servers. However, routers are L3 devices which do not forward broadcasts. The IP helper is ther to resolve this problem, because the hosts' broadcasts are translated to unicasts and therefore makes it possible for the router to send the packet to the desired DHCP server destination. The router here acts as dhcp relay agent.
To configure on interface of the router of the incoming host/receiving the broadcast:
(config-if)#ip helper-address
For more advanced networks use DHCP relay agent (insert its own router information when forwarding dhcp packages in the network) called option 82:
(config)#ip dhcp relay information option
There are 4 stages in assigning a IP address using DHCP:
1. Client to server --> DHCPDiscover. In this process the client broadcasts a request asking if there are any DHCP servers available.
2. Server to client --> DHCPOffer. Here the server chooses an IP address from its pool and sends the assigned IP address with additional information (gateway, lease time etc) to client. If there are multiple DHCP server, the client will process and accept the first offer it receives. Before the DHCP server offers the address, it will firstly make sure that the IP address is currently not used (or being given by another DHCP server) by sending 2 ping commands to the IP address with an RTO=500 ms (can be modified by the command (config)#ip dhcp ping packets and (config)#ip dhcp ping timeout). If there is no reply the IP address is clear to use, but if there is a reply the address is being used by someone and DHCP server will choose another IP address from its pool and check the ip address again before offering it to the client.
3. Client to server --> DHCPRequest. In this process, the client accepts the IP address by sending packets with the IP address to the DHCP server.
4. Server to client --> Check. If the IP address is the same as given by the server, then the DHCP server will send DHCPAck with additional info. If it is not the same, the DHCP server will take the offered address back to the pool, meaning that the client has already another IP address.
To configure DHCP service on a router type the command:
(config)#service dhcp
This will enable cisco router to do dhcp by being a DHCP server. It does not necessarily acts as a main dhcp server, but the router can act as a secondary/standby dhcp server in case the main DHCP server goes down.
To disable dhcp service simply put the command:
(config)#no service dhcp
There are several DHCP Assignment Methods which are:
1. Dynamic --> This is the default assignment method where the client is assigned an ip address for a given period of time.
2. Manual --> This method uses static mapping/binding ip addresses to mac address
3. Automatic --> In this method DHCP server chooses IP address from pool to a mac address of the client and permanently binding them
The methods of binding itself is divided in 2 ways:
1. Manual binding, where the IP address is binded to the MAC address if the host is found in the DHCP database. The binding info database is kept in the NVRAM, which means it will be kept when the router is reloaded.
2. Automatic binding, where the IP address is binded to the MAC address, but the binding database is kept on a DHCP Database Agent (a remote device e.g. tftp server).
To create binding:
(dhcp-config)#host
(dhcp-config)#hardware-address xxxx.xxxx.xxxx
(dhcp-config)#client-name
To set dhcp server database location use the command
(config)#ip dhcp database
To create dhcp pool type the command:
(config)#ip dhcp pool
(dhcp-config)#network
There is a possibility to exclude an IP range from the subnet mask to prevent that IP address being given as an DHCP ip address by typing the command:
(config)#ip dhcp exluded-address
To set the lease time of the assigned IP address:
(dhcp-config)# lease
The IP helper address is another useful feature. Basically, a host broadcasts its DHCP request to search for DHCP servers. However, routers are L3 devices which do not forward broadcasts. The IP helper is ther to resolve this problem, because the hosts' broadcasts are translated to unicasts and therefore makes it possible for the router to send the packet to the desired DHCP server destination. The router here acts as dhcp relay agent.
To configure on interface of the router of the incoming host/receiving the broadcast:
(config-if)#ip helper-address
For more advanced networks use DHCP relay agent (insert its own router information when forwarding dhcp packages in the network) called option 82:
(config)#ip dhcp relay information option
-- 9 December 2010 --
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