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664 CHAPTER E: DHCP SERVER
The MSS DHCP server is configurable on an individual VLAN basis only,
and operates only on the subnets for which you configure it.
Use of the MSS DHCP server to allocate client addresses is intended for
temporary, demonstration deployments and not for production networks.
3Com recommends that you do not use the MSS DHCP server to allocate
client addresses in a production network.
How the MSS DHCP
When MSS receives a DHCP Discover packet, the DHCP server allocates
an address from the configured range according to RFC 2131 and ARPs
the address to ensure that it is not already in use. If the address is in use,
the server allocates the next address in the range, and ARPs again. The
process continues until MSS finds an address that is not in use. MSS then
offers the address to the Distributed MAP or client that sent the DHCP
Discover. If there are no unused addresses left in the range, MSS ignores
the DHCP Discover and generates a log message.
If the client does not respond to the DHCP Offer from the MSS DHCP
server within 2 minutes, the offer becomes invalid and MSS returns the
address to the pool.
The siaddr value in the DHCP exchanges is the IP address of the VLAN.
The yiaddr value is an unused address within the range the server is
allowed to use.
In addition to an IP address, the Offer message from the MSS DHCP
server also contains the following options:
Option 54—Server Identifier, which has the same value as siaddr.
Option 51—Address Lease, which is 12 hours and cannot be
Option 1—Subnet Mask of the VLAN’s IP interface.
Option 15—Domain Name. If this option is not set with the set
interface dhcp-server command’s dns-domain option, the MSS
DHCP server uses the value set by the set ip dns domain command.