Reference Guide

NOTE: Transit traffic may not be forwarded during the period when deleted ARP entries are resolved again and re-
installed in CAM. Use this option with extreme caution.
ARP Learning via Gratuitous ARP
Gratuitous ARP can mean an ARP request or reply.
In the context of ARP learning via gratuitous ARP on FTOS, the gratuitous ARP is a request. A gratuitous ARP request is
an ARP request that is not needed according to the ARP specification, but one that hosts may send to:
• detect IP address conflicts
• inform switches of their presence on a port so that packets can be forwarded
• update the ARP table of other nodes on the network in case of an address change
In the request, the host uses its own IP address in the Sender Protocol Address and Target Protocol Address fields.
In FTOS versions prior to 8.3.1.0, if a gratuitous ARP is received some time after an ARP request is sent, only RP2 installs
the ARP information. For example:
1. At time t=0 FTOS sends an ARP request for IP A.B.C.D
2. At time t=1 FTOS receives an ARP request for IP A.B.C.D
3. At time t=2 FTOS installs an ARP entry for A.B.C.D only on RP2.
Beginning with FTOS version 8.3.1.0, when a gratuitous ARP is received, FTOS installs an ARP entry on all three CPUs.
Enabling ARP Learning via Gratuitous ARP
To enable ARP learning via gratuitous ARP, use the following command.
• Enable ARP learning via gratuitous ARP.
CONFIGURATION mode
arp learn-enable
ARP Learning via ARP Request
In FTOS versions prior to 8.3.1.0, FTOS learns via ARP requests only if the target IP specified in the packet matches the IP
address of the receiving router interface. This is the case when a host is attempting to resolve the gateway address.
If the target IP does not match the incoming interface, the packet is dropped. If there is an existing entry for the
requesting host, it is updated.
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