The Definitive Guide To ARP, IARP, RARP, and Proxy ARP
When I first started studying for my CCNA years ago, one of the (many) things
that confused me was ARP. Or rather, what ARP did as opposed to Reverse ARP,
Inverse ARP, and Proxy ARP! One book would mention ARP without mentioning the
other variations, one would mention RARP but not Proxy ARP, and so on...
I got through my Intro and ICND exams, but I never forgot how confusing this was
to me when I started. (And we all start somewhere!) To help current CCNA candidates
with this confusing topic, let's take a look at each one of these technologies.
ARP - Address Resolution Protocol
You may well know what ARP does from your networking studies or work on a LAN,
but to effectively troubleshoot ARP issues on a WAN (and pass the 640-801, 640-
811, and 640-821 exams!), you need to take network devices into account that may
be separating the workstations in question.
The basic ARP operation is simple enough. We concentrate on IP addressing a
great deal in our studies and our jobs, but it's not enough to have a destination
IP address in order to send data; the transmitting device must have a destination
MAC address as well.
If the sender doesn't know the MAC address of the destination, it has to get
that address before data can be sent. To obtain the unknown Layer Two address
when the Layer Three address is known, the sender transmits an ARP Request. This
is a Layer Two broadcast, which has a destination address of ff-ff-ff-ff-ff-ff.
Since Ethernet is a broadcast media, every other device on the segment will see
it. However, the only device that will answer it is the device with the matching
Layer Three address. That device will send an ARP Reply, unicast back to the
device that sent the original ARP Request. The sender will then have a MAC
address to go with the IP address and can then transmit.
There are several network devices that may be between our two hosts, and for the
most part, there is no impact on ARP. Since this is Cisco, though, there's gotta
be an exception! Let's take a look at how these devices impact ARP.
Repeaters and Hubs are Layer One (Physical Layer) devices, and they have no
impact on ARP. A repeater's job is simply to regenerate a signal to make it
stronger, and a hub is simply a multiport repeater. Therefore, neither a
repeater nor a hub have impact on ARP.
Switches are Layer Two devices, so you might think they impact ARP's operation;
after all, ARP deals with getting an unknown MAC address to correspond with a
known IP address. While that's certainly true, switches don't impact ARP for one
simple reason: Switches forward broadcasts out every port except the one it was
originally received on. The ARP Reply will be unicast to the device requesting
it, as with the previous example.
Now here's the exception - a router. Routers accept broadcasts, but routers
will not forward them. For example, consider a PC with the address 126.96.36.199 /16.
That host assumes it's on the same physical segment as the device 188.8.131.52
/16, since their IP addresses are both on the same subnet (184.108.40.206 /16). The
problem here is that a router separates the two devices, and the router will not
forward the ARP broadcast.
The Cisco router will answer the ARP Request, however, with the MAC address of
the router interface the ARP Request was received on. In this case, the router
will respond to the ARP Request with its own E1 interface's MAC address.
When the device at 220.127.116.11 receives this ARP Response, it thinks the MAC
address of 18.104.22.168 is 11-11-11-11-11-11. Therefore, the destination IP for
traffic destined for the remote host will be 22.214.171.124, but the MAC destination
will actually be that of the router's E1 interface.
Proxy ARP runs by default on a Cisco 2500 router, but it can be turned off at
the interface level with the no ip proxy-arp command.
RARP and Inverse ARP
Reverse ARP is a lot simpler! RARP obtains a device's IP address when it already
knows its own MAC address. (If the device doesn't know it's own MAC address, you
have bigger problems than RARP!) A separate device, a RARP Server, tells the
device what its MAC address is in response to the RARP Request. As you can see,
RARP and DHCP have a lot in common.
Inverse ARP doesn't deal with MAC or IP addresses. Inverse ARP dynamically maps
local DLCIs to remote IP addresses when you configure Frame Relay. Many
organizations prefer to statically create these mappings; you can turn this
default behavior off with the interface-level command no frame inverse-arp.
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage, home of free
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