A COM module stores the MAC addresses of the communication
partners in a MAC/IP address assignment table (ARP cache).
The MAC address remains stored in the ARP cache if messages
from the communication partner are received within 1x…2x ARP
Aging Time.
The MAC address is erased from the ARP cache if no messages
from the communication partner are received within 1x…2x ARP
Aging Time.
The typical value for the ARP Aging Time in a local network ranges
from 5…300 s.
The contents of the ARP cache cannot be read out.
Range of values: 1…3600 s
Default value: 60 s
Note:
If routers or gateways are used, the ARP Aging Time must be
adjusted (increased) due to the additional time required for two-
way transmission.
If the ARP Aging Time is too low, the MAC address of the
communication partner is erased from the ARP cache and
communication is delayed or interrupted. For an efficient
performance, the ARP Aging Time value must be greater than the
receive timeout set for the protocols in use.
MAC Learning and ARP Aging Time are used to set how quick the
Ethernet switch should learn the MAC address.
The following settings are possible:
▪ Conservative (recommended):
If the ARP cache already contains MAC addresses of
communication partners, these are locked and cannot be
replaced by other MAC addresses for at least 1 ARP Aging
Time and a maximum of 2 ARP Aging Time periods.
▪ Tolerant:
When a message is received, the IP address contained in the
message is compared to the data in the ARP cache and the
MAC address stored in the ARP cache is immediately
overwritten with the MAC address from the message.
The Tolerant setting must be used if the availability of
communication is more important than the authorized access to
the controller.
Default value: Conservative
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