BigIron RX Series Configuration Guide 18553-1001986-01Configuring ARP parameters 7Displaying IP receive access listTo determine if IP receive access list has been configured on the device, enter the followingcommand.BigIron RX# show access-list bindingsL4 configuration:ip receive access-list 101Configuring ARP parametersAddress Resolution Protocol (ARP) is a standard IP protocol that enables the device to obtain theMAC address of another device’s interface when the device knows the IP address of the interface.ARP is enabled by default and cannot be disabled.How ARP worksThe device needs to know a destination’s MAC address when forwarding traffic, because the deviceencapsulates the IP packet in a Layer 2 packet (MAC layer packet) and sends the Layer 2 packet toa MAC interface on a device directly attached to the device. The device can be the packet’s finaldestination or the next-hop router toward the destination.The device encapsulates IP packets in Layer 2 packets regardless of whether the ultimatedestination is locally attached or is multiple router hops away. Since the device’s IP route table andIP forwarding cache contain IP address information but not MAC address information, the devicecannot forward IP packets based solely on the information in the route table or forwarding cache.The device needs to know the MAC address that corresponds with the IP address of either thepacket’s locally attached destination or the next-hop router that leads to the destination.For example, to forward a packet whose destination is multiple router hops away, the device mustsend the packet to the next-hop router toward its destination, or to a default route or defaultnetwork route if the IP route table does not contain a route to the packet’s destination. In eachcase, the device must encapsulate the packet and address it to the MAC address of a locallyattached device, the next-hop router toward the IP packet’s destination.To obtain the MAC address required for forwarding a datagram, the device does the following:• First, the device looks in the ARP cache (not the static ARP table) for an entry that lists the MACaddress for the IP address. The ARP cache maps IP addresses to MAC addresses. The cachealso lists the port attached to the device and, if the entry is dynamic, the age of the entry. Adynamic ARP entry enters the cache when the device receives an ARP reply or receives an ARPrequest (which contains the sender’s IP address and MAC address). A static entry enters theARP cache from the static ARP table (which is a separate table) when the interface for theentry comes up.To ensure the accuracy of the ARP cache, each dynamic entry has its own age timer. The timeris reset to zero each time the device receives an ARP reply or ARP request containing the IPaddress and MAC address of the entry. If a dynamic entry reaches its maximum allowable age,the entry times out and the software removes the entry from the table. Static entries do notage out and can be removed only by you.
