Basic Operation Guide 2014/02

Possible scenarios for updating the configuration file
The following table shows various network configurations and how Option 66 is handled.
BehaviorScenario
Single Server serving
Multiple VLANs
• Each DHCP-enabled VLAN interface initiates DHCPDISCOVER message, receives
DHCPOFFER from the server, and send DHCPREQUEST to obtain the offered parameters.
• If multiple interfaces send DHCPREQUESTs, it’s possible that more than one DHCPACK
is returned with a valid Option 66.
• Evaluating and updating the configuration file occurs only on the primary VLAN.
• Option 66 is ignored by any interfaces not belonging to the primary VLAN.
Multiple Servers serving a
Single VLAN
• Each DHCP-enabled VLAN interface initiates one DHCPDISCOVER and receives one
or more DHCPOFFER messages.
• Each interface accepts the best offer.
• Option 66 is processed only for the interface belonging to the primary VLAN.
Multiple Servers serving
Multiple VLANs
• Each DHSP-enabled VLAN interface initiates DHCPDISCOVER and receives one or
more DHCPOFFER messages.
• Each interface accepts the best offer.
• Option 66 is processed only for the interface belonging to the primary VLAN.
Multi-homed Server serving
Multiple VLANs
• The switch perceives the multi-homed server as multiple separate servers.
• Each DHCP-enabled VLAN interface initiates DHCPDISCOVER and receives one
DHCPOFFER message.
• Each interface accepts the offer.
• Option 66 is processed only for the interface belonging to the primary VLAN.
Operating notes
Replacing the existing configuration file: After the DHCP client downloads the configuration file,
the switch compares the contents of that file with the existing configuration file. If the content is
different, the new configuration file replaces the existing file and the switch reboots.
Option 67 and the configuration file name: Option 67 includes the name of the configuration file.
If the DHCPACK contains this option, it overrides the default name for the configuration file
(switch.cfg)
Global DHCP parameters: Global parameters are processed only if received on the primary VLAN.
Best Offer: The "Best Offer" is the best DHCP or BootP offer sent by the DHCP server in response
to the DHCPREQUEST sent by the switch. The criteria for selecting the "Best Offer" are:
• DHCP is preferred over BootP
• If two BootP offers are received, the first one is selected
• For two DHCP offers:
The offer from an authoritative server is selected◦
◦ If there is no authoritative server, the offer with the longest lease is selected
Log messages
The file transfer is implemented by the existing TFTP module. The system logs the following message
if an incorrect IP address is received for Option 66:
"Invalid IP address <ip-address> received for DHCP Option 66"
90 Switch Memory and Configuration