How Automatic IP Configuration Works | 71 |
Automatic Process To detect its IP information using the automatic configuration process, the Switch goes through the following sequence of steps:
1The DHCP client that resides in the Switch makes up to four attempts to contact a DHCP server on the network requesting IP information from the server. The attempts are at 0, 4, 12, 28 second intervals.
■If a DHCP server is on the network and working correctly it responds to the clients request with an IP address (allocated from a pool of available addresses) and other parameters such as a subnet mask, default gateway, lease time, and any other options configured in the DHCP server.
The way a DCHP server responds is dependant on the DHCP server settings. Therefore the way your DHCP server responds may be different to the process outlined.
■If the DHCP process fails after 30 seconds on all four attempts, then the Switch activates its Auto-IP configuration feature.
2The Auto-IP feature starts with an IP address of 169.254.100.100. It uses the Address Resolution Protocol (ARP) to check to make sure this address is not already in use on the network. If not, it will allocate this default address to the Switch.
If this IP address is already in use, Auto-IP will check once every second for three seconds for an IP address on the 169.254.x.y subnet (where x = 1-254 and y = 0-255) (Auto-IP only uses addresses in the range 169.254.1.0 through to 169.254.254.255 as valid addresses.) Once Auto-IP has ensured that an IP address is not already in use on the network, it assigns it to the Switch with a subnet mask of 255.255.0.0 and a default gateway of 0.0.0.0.
3While the Auto-IP assigned address is in use:
■The Auto-IP client continues to check every 30 seconds (using ARP) to ensure that any other Auto-IP hosts have not mistakenly configured themselves using the same Auto-IP address.
■DHCP and BOOTP requests also continue in the background. The requests begin 3 minutes after either the Auto-IP address is assigned, or 125 attempts to establish a valid Auto-IP address, whichever occurs first. The requests proceed with DHCP requests for 1 minute; a 3 minute pause; DHCP requests for another minute; a 3 minute pause; BOOTP requests for one minute; a 3 minute pause; then the process repeats until a DHCP or BOOTP server answers the requests.
