3Com 5500-SI Configuration Manual page 126

126 C 10: DHCP S
HAPTER
C
ERVER ONFIGURATION
IP address lease update
After a DHCP server dynamically assigns an IP address to a DHCP client, the IP address
keeps valid only within a specified lease time and will be reclaimed by the DHCP
server when the lease expires. If the DHCP client wants to use the IP address for a
longer time, it must update the IP lease.
By default, a DHCP client updates its IP address lease automatically by sending a
DHCP-REQUEST packet to the DHCP server when half of the lease time elapses. The
DHCP server, in turn, responds with a DHCP-ACK packet to notify the DHCP client of
a new IP lease if the server can assign the same IP address to the client. Otherwise, the
DHCP server responds with a DHCP-NAK packet to notify the DHCP client that the IP
address will be reclaimed when the lease time expires.
XRN support
In an XRN (expandable resilient networking) system, DHCP servers operate in a
centralized way to fit the XRN environment.
DHCP servers run (as tasks) on all the units (including the master unit and the slave
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units) in a Fabric system. But only the one running on the master unit
receives/sends packets and carries out all functions of a DHCP server. Those
running on the slave units only operate as the backup tasks of the one running on
the master unit.
When a slave unit receives a DHCP-REQUEST packet, it redirects the packet to the
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DHCP server on the master unit, which returns a DHCP-ACK/DHCP-NAK packet to
the DHCP client and at the same time backs up the related information to the
slave units. In this way, when the current master unit fails, one of the slaves can
change to the master and operates as the DHCP server immediately.
DHCP is an UDP-based protocol operating at the application layer. When a DHCP
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server in a fabric system runs on a Layer 2 network device, DHCP packets are
directly forwarded by hardware instead of being delivered to the DHCP server, or
being redirected to the master unit by UDP HELPER. This idles the DHCP server.
DHCP packets can be redirected to the DHCP server on the master unit by UDP
HELPER only when the Layer 2 device is upgraded to a Layer 3 device.
When you merge two or more XRN systems into one XRN system, a new master unit
is elected, and the new XRN system adopts new configurations accordingly. This may
result in the existing system configurations (including the address pools configured
for the DHCP servers) being lost. As the new XRN system cannot inherit the original
DHCP server configurations, you need to perform DHCP server configurations for it.
When an XRN system is split into multiple new XRN systems, some of the new XRN
systems may be degraded to Layer 2 devices. For a new XRN system degraded to
Layer 2 device, although the original DHCP server still exists in the new system, it run
idle for being unable to receive any packets. When the XRN system restores to a Layer
3 device due to being merged into a new XRN system, it adopts the configurations on
the new XRN system. And you need to perform DHCP server configurations if the
new XRN system does not have DHCP server-related configurations.
In an XRN system, the UDP HELPER function must be enabled on the DHCP servers
that are in fabric state.
After DHCP server is enabled on a device, the device processes the DHCP packet
received from a DHCP client in one of the following three modes depending on your
configuration.