H3C Technologies H3C S3600 Series Switches - Manual (Page 642)

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The DHCP server assigns an IP address to the client in the following order from an interface address
pool or a global address pool:
3) If there is an address pool where an IP address is statically bound to the MAC address or ID of the
client, the DHCP server will select this address pool and assign the statically bound IP address to
the client.
4) Otherwise, the DHCP server observes the following principles to select a dynamic address pool.
z If the client and the server reside in the same network segment, the smallest address pool that
contains the IP address of the receiving interface will be selected.
z If the client and the server do not reside in the same network segment (that is, a DHCP relay agent
is in-between), the smallest address pool that contains the IP address specified in the giaddr field
of the client’s request will be selected.
z If no assignable IP address is available in the selected address pool, the DHCP server will not
assign any IP address to the client because it cannot assign an IP address from the parent address
pool to the client.
DHCP IP Address Preferences
A DHCP server assigns IP addresses in interface address pools or global address pools to DHCP
clients in the following sequence:
1) IP addresses that are statically bound to the MAC addresses of DHCP clients or client IDs.
2) The IP address that was ever assigned to the client
3) The IP address designated by the Option 50 field in a DHCP-DISCOVER message
4) The first assignable IP address found in a proper DHCP address pool
5) If no IP address is available, the DHCP server queries lease-expired and conflicted IP addresses. If
the DHCP server finds such IP addresses, it assigns them; otherwise the DHCP server does not
assign an IP address.
IRF Support
In an IRF (intelligent resilient framework) system, DHCP servers operate in a centralized way to fit the
IRF environment.
z DHCP servers run (as tasks) on all the units (including the master unit and the slave 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.
z When a slave unit receives a DHCP-REQUEST packet, it redirects the packet to the DHCP server
on the master unit, which returns a DHCP-ACK or 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.
z DHCP is an UDP-based protocol operating at the application layer. When a DHCP 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.