3.1.a Identify, implement and troubleshoot IPv4 addressing and subnetting

3.1.a [ii] ARP

The Address Resolution Protocol (ARP) was developed to enable communications on an internetwork. Layer 3 devices need ARP to map IP network addresses to MAC hardware addresses so that IP packets can be sent across networks. Before a device sends a datagram to another device, it looks in its ARP cache to see if there is a MAC address and corresponding IP address for the destination device . If there is no entry, the source device sends a broadcast message to every device on the network. Each device compares the IP address to its own . Only the device with the matching IP address replies to the sending device with a packet containing the MAC address for the device (except in the case of “proxy ARP” where a router can reply to ARP request on behalf of another host). The source device adds the destination device MAC address to its ARP table for future reference, creates a data-link header and trailer that encapsulates the packet, and proceeds to transfer the data.

High CPU utilization in the ARP input process occurs if the router has to originate an excessive number of ARP requests. The router uses ARP for all hosts, not just those on the local subnet, and ARP requests are sent out as broadcasts , which causes more CPU utilization on every host in the network. ARP requests for the same IP address are rate-limited to one request every two seconds, so an excessive number of ARP requests would have to originate for different IP addresses. This can happen if an IP route has been configured pointing to a broadcast interface (as opposed to next-hop). A most common example is a default route such as:

ip route Fastethernet0/ 0

In this case, the router generates an ARP request for each IP address that is not reachable through more specific routes, which practically means that the router generates an ARP request for almost every address on the Internet.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 2252-2253).  . Kindle Edition.