ARP and Proxy ARP
You would imagine that anyone getting this far in their CCIE study would already have a solid understanding of the Address Resolution Protocol (ARP, RFC 826). However, proxy ARP (RFC 1027) is often ignored, in part because of its lack of use today. To see how they both work, Figure 5-1 shows an example of each, with Fred and Barney both trying to reach the web server at IP address 10.1.2.200.
Figure 5-1 Comparing ARP and Proxy ARP
Figure 5-1 Comparing ARP and Proxy ARP
Fred follows a normal ARP process, broadcasting an ARP request, with R1's E1 IP address as the target. The ARP message has a Target field of all 0s for the MAC address that needs to be learned, and a target IP address of the IP address whose MAC address it is searching, namely 10.1.1.1 in this case. The ARP reply lists the MAC address associated with the IP address, in this case, the MAC address of R1's E1 interface.
NOTE The ARP message itself does not include an IP header, although it does have destination and source IP addresses in the same relative position as an IP header. The ARP request lists an IP destination of 255.255.255.255. The ARP Ethernet protocol type is 0x0806, whereas IP packets have an Ethernet protocol type of 0x0800.
Proxy ARP uses the exact same ARP message as ARP, but the ARP request is actually requesting a MAC address that is not on the local subnet. Because the ARP request is broadcast on the local subnet, it will not be heard by the target host—so if a router can route packets to that target host, the router issues a proxy ARP reply on behalf of that target.
For instance, Barney places the web server's IP address (10.1.2.200) in the Target field, because Barney thinks that he is on the same subnet as the web server due to Barney's mask of 255.0.0.0. The ARP request is a LAN broadcast, so R1, being a well-behaved router, does not forward the ARP broadcast. However, knowing that the ARP request will never get to the subnet where 10.1.2.200 resides, R1 saves the day by replying to the ARP on behalf of the web server. R1 takes the web server's place in the ARP process, hence the name proxy ARP. Also, note that R1's ARP reply contains R1's E1 MAC address, so that Barney will forward frames to R1 when Barney wants to send a packet to the web server.
Before the advent of DHCP, many networks relied on proxy ARP, configuring hosts to use the default masks in their respective networks. Regardless of whether the proxy version is used, the end result is that the host learns a router's MAC address to forward packets to another subnet.
Continue reading here: Dhcp
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