Addressing via DHCP
10.1 Principle of address assignment via DHCP
Communication
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Function Manual, 05/2021, A5E03735815-AJ
DHCP address assignment options
For the S7-1500 CPU you can configure that the following options are obtained via a DHCP
server:
•
Addresses of up to four DNS servers
Get addresses of the DNS servers via DHCP (Page 370)
•
Address of up to four NTP servers
Get addresses of the NTP servers via DHCP (Page 370)
•
Host and domain name
Obtain host and domain name via DHCP (Page 371)
If necessary, the DHCP server also supplies the address of a router (default gateway) as
option.
How long can the S7-1500 CPU use the DHCP address parameters?
In addition to the address parameters, the DHCP server also notifies the S7-1500 CPU (DHCP
client)of the lease time. The lease time defines how long the CPU can use the address
parameters.
When the lease time has fully expired, the CPU returns the assigned address parameters. The
CPU has an internal time monitoring for the lease time.
At certain times when the lease time expires, the CPU has the option of extending the lease
time:
•
Renewal: Half of the lease time has expired: The CPU contacts the original DHCP server
and asks for an extension of the lease time. The original DHCP server can either confirm
the existing lease time or assign a new lease time. With a new lease time, the time
monitoring in the CPU is reset.
•
Rebinding: 7/8 of the lease term has expired: The CPU contacts all available DHCP servers
via broadcast and asks for an extension of the lease time. A DHCP server can either
confirm the existing lease time or assign a new lease time. With a new lease time, the
time monitoring in the CPU is reset.
In the event of a negative response from DHCP server during rebinding or if no
DHCP server replies, the CPU returns the address parameters after 8/8 of the lease time.
If the CPU has returned the address parameters after the lease time has expired, the CPU
starts a new cycle for DHCP addressing with a new DHCP discover.
See also
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