270 | IPv4 Routing
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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 (Figure 14-8).
Beginning with FTOS version 8.3.1.0, when you enable ARP learning via gratuitous ARP, the system
installs a new ARP entry, or updates an existing entry for all received ARP requests (Figure 14-9).
Figure 14-8. Learning via Gratuitous ARP not Enabled
Figure 14-9. Learning via Gratuitous ARP Enabled
Whether you enable or disable ARP learning via gratuitous ARP, the system does not look up the Target IP.
It only updates the ARP entry for the Layer 3 interface with the source IP of the request.
Configurable ARP Retries
Beginning with FTOS version 8.3.1.0, the number of ARP retries is configurable.
Host 1
IP: 1.1.1.2
MAC: AA
Host 2
IP: 1.1.1.3
MAC: BB
ARP Request
Target IP: 1.1.1.3
Target IP is not the VLAN interface
IP. Update existing Host 1 entry.
Drop packet.
VLAN ID: 1.1.1.1
X
Host 1
IP: 1.1.1.2
MAC: AA
Host 2
IP: 1.1.1.3
MAC: BB
ARP Request
Target IP: 1.1.1.3
Target IP is not the VLAN interface
IP. Install new entry for Host 1, or
update existing Host 1 entry.
Drop packet.
VLAN ID: 1.1.1.1
ARP Learning via Gratuitous ARP enabled
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