Service Manual
ARP Learning via Gratuitous ARP
Gratuitous ARP can mean an ARP request or reply.
In the context of ARP learning via gratuitous ARP on Dell Networking OS, the gratuitous ARP is a request. A gratuitous ARP request
is an ARP request that is not needed according to the ARP specication, but one that hosts may send to:
• detect IP address conicts
• 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 elds.
In Dell Networking OS 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 Dell Networking OS sends an ARP request for IP A.B.C.D
2. At time t=1 Dell Networking OS receives an ARP request for IP A.B.C.D
3. At time t=2 Dell Networking OS installs an ARP entry for A.B.C.D only on RP2.
Beginning with Dell Networking OS version 8.3.1.0, when a gratuitous ARP is received, Dell Networking OS 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 Dell Networking OS versions prior to 8.3.1.0, Dell Networking OS learns via ARP requests only if the target IP specied 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 41. ARP Learning via ARP Request
IPv4 Routing
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