Dynamic host control protocol - Metasys - LIT-12013055 - Gateway/Router - Cisco Switch - 12.0

Cisco IE 2000 and IE 4010 Ethernet Switches for Metasys Networks Installation Instructions and Troubleshooting Guide

Product name
Cisco Switch
Document type
Troubleshooting Guide
Document number
Revision date

In older network configurations, all IP addresses were assigned statically. Computers never moved, so their subnet assignments were always correct. Also, the supply of 32-bit addresses seemed inexhaustible. In the present day, devices are more plentiful, and it is customary to allocate pools of addresses and let computers make the final assignments. The usual mechanism is dynamic host control protocol (DHCP). It is worth discussing DHCP in detail because it is useful in troubleshooting network issues.

The switches are configured as DHCP servers. The switch configurations on a site are unique so that each switch has a unique pool of addresses that it can assign to controllers. All IP addresses across the building network are unique. They may duplicate addresses in the customer’s IT network, but that is not a problem. The IE 2000 switches provide a layer 3 boundary between the controllers and the customer network. The switches have routing statements to forward every packet where it needs to go.

Configure all controllers so that the DHCP client software is enabled. When first powered up on the network, a controller sends a broadcast message soliciting a DHCP server. If, after retries, there is still no DHCP server, then the controller assigns itself an unused APIPA address beginning with 169.254. It continues indefinitely looking for a DHCP server, but in the meantime can communicate with other devices on the same network segment using its self-assigned address.

Alternatively, if the controller finds a DHCP server, the client and server exchange a series of messages, at the end of which, the controller is assigned an IP address from the server’s pool of addresses, assigned a default gateway, and given other information. However, the assignment is a lease, not a permanent allocation. The switch configuration files generated by the configuration tool have the DHCP lease set to ten minutes. Half way through the lease period, the controller contacts the DHCP server and tries to renew the lease. If necessary, it tries again when the remaining time is only one quarter of the original term. When only one eighth of the original term remains, it broadcasts a request for DHCP. When the original term expires, it gives up its address and assigns itself a 169.254 address.

Because the lease term is short and controllers are not very mobile, the DHCP server is configured to respond to a renewal request by giving the client a new term on the address it already has. If a controller is power-cycled and tries to get a new address, the switch recognizes the MAC address and assigns the controller its previous address. The switch remembers assignments even when it is power-cycled. However abandoned addresses are available for reuse. When the there are no longer any previously unused addresses left in the DHCP pool of addresses, a new device is assigned to the oldest previously used address in the DHCP pool.