186 BigIron RX Series Configuration Guide53-1001986-01Configuring ARP parameters7• If the ARP cache does not contain an entry for the destination IP address, the devicebroadcasts an ARP request out all its IP interfaces. The ARP request contains the IP address ofthe destination. If the device with the IP address is directly attached to the device, the devicesends an ARP response containing its MAC address. The response is a unicast packetaddressed directly to the device. The device places the information from the ARP response intothe ARP cache.ARP requests contain the IP address and MAC address of the sender, so all devices thatreceive the request learn the MAC address and IP address of the sender and can update theirown ARP caches accordingly.NOTEThe ARP request broadcast is a MAC broadcast, which means the broadcast goes only to devicesthat are directly attached to the device. A MAC broadcast is not routed to other networks. However,some routers, including the device, can be configured to reply to ARP requests from one network onbehalf of devices on another network. Refer to “Enabling proxy ARP” on page 189.NOTEIf the router receives an ARP request packet that it is unable to deliver to the final destinationbecause of the ARP timeout and no ARP response is received (the device knows of no route to thedestination address), the router sends an ICMP Host Unreachable message to the source.Rate limiting ARP packetsYou can limit the number of ARP packets the device accepts during each second. By default, thesoftware does not limit the number of ARP packets the device can receive. Since the device sendsARP packets to the CPU for processing, if a device in a busy network receives a high number of ARPpackets in a short period of time, some CPU processing might be deferred while the CPU processesthe ARP packets.To prevent the CPU from becoming flooded by ARP packets in a busy network, you can restrict thenumber 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 ARPpackets received during the one-second interval. When a new one-second interval starts, thecounter restarts at zero, so the device again accepts up to the maximum number of ARP packetsyou 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 asthe following at the global CONFIG level of the CLI.BigIron RX(config)# arp-port-rate-limit 100This command configures the device to accept up to 100 ARP packets each second. If the devicereceives more than 100 ARP packets during a one-second interval, the device drops the additionalARP packets during the remainder of that one-second interval.Syntax: [no] arp-port- rate-limit The parameter specifies the number of ARP packets and can be from 0 – 30,000. If youspecify 0, the device will not accept any ARP packets.
