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PowerConnect B-Series FCX Configuration Guide 811
53-1002266-01
Configuring IP parameters – Layer 3 Switches
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• If the ARP cache does not contain an entry for the destination IP address, the Layer 3 Switch
broadcasts an ARP request out all its IP interfaces. The ARP request contains the IP address of
the destination. If the device with the IP address is directly attached to the Layer 3 Switch, the
device sends an ARP response containing its MAC address. The response is a unicast packet
addressed directly to the Layer 3 Switch. The Layer 3 Switch places the information from the
ARP response into the ARP cache.
ARP requests contain the IP address and MAC address of the sender, so all devices that
receive the request learn the MAC address and IP address of the sender and can update their
own ARP caches accordingly.
NOTE
The ARP request broadcast is a MAC broadcast, which means the broadcast goes only to
devices that are directly attached to the Layer 3 Switch. A MAC broadcast is not routed to other
networks. However, some routers, including Layer 3 Switches, can be configured to reply to
ARP requests from one network on behalf of devices on another network. Refer to “Enabling
proxy ARP” on page 812.
NOTE
If the router receives an ARP request packet that it is unable to deliver to the final destination
because of the ARP timeout and no ARP response is received (the Layer 3 Switch knows of no route
to the destination address), the router sends an ICMP Host Unreachable message to the source.
Rate limiting ARP packets
You can limit the number of ARP packets the Dell PowerConnect device accepts during each
second. By default, the software does not limit the number of ARP packets the device can receive.
Since the device sends ARP packets to the CPU for processing, if a device in a busy network
receives a high number of ARP packets in a short period of time, some CPU processing might be
deferred while the CPU processes the ARP packets.
To prevent the CPU from becoming flooded by ARP packets in a busy network, you can restrict the
number of ARP packets the device will accept each second. When you configure an ARP rate limit,
the device accepts up to the maximum number of packets you specify, but drops additional ARP
packets received during the one-second interval. When a new one-second interval starts, the
counter restarts at zero, so the device again accepts up to the maximum number of ARP packets
you specified, but drops additional packets received within the interval.
To limit the number of ARP packets the device will accept each second, enter a command such as
the following at the global CONFIG level of the CLI.
PowerConnect(config)# rate-limit-arp 100
This command configures the device to accept up to 100 ARP packets each second. If the device
receives more than 100 ARP packets during a one-second interval, the device drops the additional
ARP packets during the remainder of that one-second interval.
Syntax: [no] rate-limit-arp <num>
The <num> parameter specifies the number of ARP packets and can be from 0 through 100. If you
specify 0, the device will not accept any ARP packets.