ARP, Proxy ARP, Reverse ARP, BOOTP, and DHCP
The branch for this area may assume like a laundry account of a lot of altered protocols. However,
these bristles protocols do accept one axial theme, namely that they advice a host apprentice advice so
that it can auspiciously accelerate and accept IP packets. Specifically, ARP and proxy ARP define
methods for a host to apprentice addition host’s MAC address, admitting the amount functions of RARP,
BOOTP, and DHCP ascertain how a host can ascertain its own IP address, added additional related
information.
ARP and Proxy ARP
You would brainstorm that anyone accepting this far in their CCIE abstraction would already accept a solid
understanding of the Abode Resolution Agreement (ARP, RFC 826). However, proxy ARP (RFC
1027) is generally ignored, in allotment because of its abridgement of use today. To see how they both work, Figure
5-1 shows an archetype of each, with Fred and Barney both aggravating to ability the web server at IP
address 10.1.2.200.
Figure 5-1 Comparing ARP and Proxy ARP
Fred follows a accustomed ARP process, broadcasting an ARP request, with R1’s E1 IP abode as the
target. The ARP bulletin has a Ambition acreage of all 0s for the MAC abode that needs to be learned,
and a ambition IP abode of the IP abode whose MAC abode it is searching, namely 10.1.1.1 in
R1
ARP Reply
Source = R1-E1-MAC Web Server:
10.1.2.200/24
GW = 10.1.2.1
Interface: E1
MAC: R1-E1-MAC
10.1.1.1/24
Barney: 10.1.1.102
Mask: /8
GW = 10.1.1.1
Fred: 10.1.1.101
Mask: /24
GW = 10.1.1.1
Interface: E2
MAC: R1-E2-MAC
10.1.2.1/24
DHCP Server:
10.1.2.202/24
DNS Server:
10.1.2.203/24
ARP Request
Target = 10.1.1.1
ARP Reply
Source = R1-E1-MAC
Proxy logic
used by R1!
ARP Request
Target = 10.1.2.200
this case. The ARP acknowledgment lists the MAC abode associated with the IP address, in this case, the
MAC abode of R1’s E1 interface.
Proxy ARP uses the exact aforementioned ARP bulletin as ARP, but the ARP appeal is absolutely requesting
a MAC abode that is not on the bounded subnet. Because the ARP appeal is advertisement on the local
subnet, it will not be heard by the ambition host—so if a router can avenue packets to that ambition host,
the router issues a proxy ARP acknowledgment on account of that target.
For instance, Barney places the web server’s IP abode (10.1.2.200) in the Ambition field, because
Barney thinks that he is on the aforementioned subnet as the web server due to Barney’s affectation of 255.0.0.0.
The ARP appeal is a LAN broadcast, so R1, actuality a affable router, does not advanced the
ARP broadcast. However, alive that the ARP appeal will never get to the subnet where
10.1.2.200 resides, R1 saves the day by acknowledging to the ARP on account of the web server. R1 takes
the web server’s abode in the ARP process, appropriately the name proxy ARP. Also, agenda that R1’s ARP
reply contains R1’s E1 MAC address, so that Barney will advanced frames to R1 back Barney
wants to accelerate a packet to the web server.
Before the appearance of DHCP, abounding networks relied on proxy ARP, configuring hosts to use the
default masks in their corresponding networks. Regardless of whether the proxy adaptation is used, the
end aftereffect is that the host learns a router’s MAC abode to advanced packets to addition subnet.
NOTE The ARP bulletin itself does not accommodate an IP header, although it does have
destination and antecedent IP addresses in the aforementioned about position as an IP header. The ARP
request lists an IP destination of 255.255.255.255. The ARP Ethernet agreement blazon is 0x0806,
whereas IP packets accept an Ethernet agreement blazon of 0x0800.