Amer Networks E5Web GUI - Manual (Page 185)

The Expires Column
The third column in the table, Expires, is used to indicate how much longer the ARP entry will be
valid for.
For example, the first entry has an expiry value of 45 which means that this entry will be rendered
invalid and removed from the ARP Cache in 45 seconds. If traffic is going to be sent to the
192.168.0.10 address after the expiration, cOS Core will issue a new ARP request.
The default expiration time for dynamic ARP entries is 900 seconds (15 minutes). This can be
changed by modifying the advanced setting ARP Expire.
The advanced setting ARP Expire Unknown specifies how long cOS Core will remember
addresses that cannot be reached. This limit is needed to ensure that cOS Core does not
continuously request such addresses. The default value for this setting is 3 seconds.
Example 3.20. Displaying the ARP Cache
The contents of the ARP Cache can be displayed from within the CLI.
Command-Line Interface
Device:/> arp -show
ARP cache of iface lan
Dynamic 10.4.0.1 = 1000:0000:4009 Expire=196
Dynamic 10.4.0.165 = 0002:a529:1f65 Expire=506
Flushing the ARP Cache
If a host in a network is replaced with new hardware and retains the same IP address then it will
probably have a new MAC address. If cOS Core has an old ARP entry for the host in its ARP cache
then that entry will become invalid because of the changed MAC address and this will cause data
to be sent to the host over Ethernet which will never reach its destination.
After the ARP entry expiration time, cOS Core will learn the new MAC address of the host but
sometimes it may be necessary to manually force the update. The easiest way to achieve this is
by flushing the ARP cache. This deletes all dynamic ARP entries from the cache and forces cOS
Core to issue new ARP queries to discover the MAC/IP address mappings for connected hosts.
Flushing can be done with the CLI command arp -flush.
Example 3.21. Flushing the ARP Cache
This example shows how to flush the ARP Cache from within the CLI.
Command-Line Interface
Device:/> arp -flush
ARP cache of all interfaces flushed.
Chapter 3: Fundamentals
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