Administrator Guide

ARP Learning via Gratuitous ARP
Gratuitous ARP can mean an ARP request or reply.
In the context of ARP learning via gratuitous ARP on the system, 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. Gratuitous ARP can:
• 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. When a gratuitous ARP is received, the system installs an ARP entry on the CPU.
To enable ARP learning via gratuitous ARP, use the arp learn-enable command in CONFIGURATION
mode.
ARP Learning via ARP Request
In the Dell Networking OS versions prior to 8.3.1.0, the system 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.
Figure 50. ARP Learning via ARP Request
IPv4 Routing 459