CompTIA Network+ Certification Exam Tutorial:

ARP, Broadcasts, & Proxy ARP

By Chris Bryant, CCIE #12933

In a previous CompTIA Network+ exam tutorial on ARP, we discussed how ARP arrives at a destination MAC address when only the destination IP address is known. If there is no network connectivity device between the source and destination of the data, or if there is a switch between the two, ARP functions normally since switches forward broadcasts.

This is also the case if the switch in the following exhibit was a hub or repeater, since these Layer One OSI Model devices have no effect on ARP.

If the device between the hosts was a router, we do have a problem, since routers don't forward broadcasts. Don't get forwarding confused with accepting, however, as the router will examine the packet. When a router receives an ARP Request, the overall effect is shown in the following network diagram.

The ARP Request successfully passes through the switch, and is seen by the router on its Ethernet0 (E0) interface. The router can tell by the destination IP address of the ARP Request that it's destined for Host B, but the router will not forward the ARP Request to Host B.

This doesn't mean that Host A can never get a destination MAC address for a transmission to Host B. If the router is configured to run Proxy ARP, the router will respond to the ARP Request with the MAC address of the router interface that received the packet!

Just as a proxy vote is cast on behalf of someone who can't physically cast a vote on election day, the router sends a proxy response of its own on behalf of Host B. This entire process is transparent to Host A - Host A thinks it just received the MAC address of Host B. Therefore, when Host A sends traffic destined for Host B, the destination IP address will be that of Host B, but the destination MAC address will be that of the router's E0 interface.

To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com