The Host A (172.16.10.100) on Subnet A needs to send packets to Host D
(172.16.20.200) on Subnet B. As shown in the diagram, Host A has a /16 subnet
mask. What this means is that Host A believes that it is directly connected to
all of network 172.16.0.0. When Host A needs to communicate with any devices it
believes are directly connected, it sends an ARP request to the destination.
Therefore, when Host A needs to send a packet to Host D, Host A believes that
Host D is directly connected, so it sends an ARP request to Host D.
of Host D.
Therefore, Host A broadcasts an ARP request on Subnet A, as shown:
Sender's MAC Address | Sender's IP Address | Target MAC Address | Target IP Address |
|---|---|---|---|
00-00-0c-94-36-aa | 172.16.10.100 | 00-00-00-00-00-00 | 172.16.20.200 |
In this ARP request, Host A (172.16.10.100) requests that Host D
(172.16.20.200) send its MAC address. The ARP request packet is then
encapsulated in an Ethernet frame with the MAC address of Host A as the source
address and a broadcast (FFFF.FFFF.FFFF) as the destination address. Since the
ARP request is a broadcast, it reaches all the nodes in the Subnet A, which
includes the e0 interface of the router, but does not reach Host D. The
broadcast does not reach Host D because routers, by default, do not forward
broadcasts.
Since the router knows that the target address (172.16.20.200) is on
another subnet and can reach Host D, it replies with its own MAC address to
Host A.
Sender's MAC Address | Sender's IP Address | Target MAC Address | Target IP Address |
|---|---|---|---|
00-00-0c-94-36-ab | 172.16.20.200 | 00-00-0c-94-36-aa | 172.16.10.100 |
This is the Proxy ARP reply that the router sends to Host A. The proxy
ARP reply packet is encapsulated in an Ethernet frame with MAC address of the
router as the source address and the MAC address of Host A as the destination
address. The ARP replies are always unicast to the original requester.
Upon receipt of this ARP reply, Host A updates its ARP table, as shown:
IP Address | MAC Address |
|---|---|
172.16.20.200 | 00-00-0c-94-36-ab |
From now on, Host A forwards all the packets that it wants to reach
172.16.20.200 (Host D) to the MAC address 00-00-0c-94-36-ab (router). Since the
router knows how to reach Host D, the router forwards the packet to Host D. The
ARP cache on the hosts in Subnet A is populated with the MAC address of the
router for all the hosts on Subnet B. Hence, all packets destined to Subnet B
are sent to the router. The router forwards those packets to the hosts in
Subnet B.
The ARP cache of Host A is shown in this table:
IP Address | MAC Address |
|---|---|
172.16.20.200 | 00-00-0c-94-36-ab |
172.16.20.100 | 00-00-0c-94-36-ab |
172.16.10.99 | 00-00-0c-94-36-ab |
172.16.10.200 | 00-00-0c-94-36-bb |
Note: Multiple IP addresses are mapped to a single MAC address, the MAC
address of this router, which indicates that proxy ARP is in use.
The interface of the Cisco must be configured to accept and respond to
proxy ARP. This is enabled by default. The no ip
proxy-arp command must be configured on the interface of the
router connected to the ISP router. Proxy ARP can be disabled on each interface
individually with the interface configuration command no ip
proxy-arp, as shown:
Router# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)# interface ethernet 0
Router(config-if)# no ip proxy-arp
Router(config-if)# ^Z
Router#
In order to enable proxy ARP on an interface, issue the ip
proxy-arp interface configuration command.
Note: When Host B (172.16.10.200/24) on Subnet A tries to send packets to
destination Host D (172.16.20.200) on Subnet B, it looks into its IP routing
table and routes the packet accordingly. Host B (172.16.10.200/24) does not ARP
for Host D IP address 172.16.20.200 because it belongs to a different subnet
than what is configured on Host B ethernet interface 172.16.20.200/24.
Advantages of Proxy ARP
The main advantage of proxy ARP is that it can be added to a single
router on a network and does not disturb the routing tables of the other
routers on the network.
with a default gateway or do not have any routing intelligence.
Disadvantages of Proxy ARP
it to be a flat network in which they can reach any destination simply by
sending an ARP request. But using ARP for everything has disadvantages. These
are some of the disadvantages:
It increases the amount of ARP traffic on your segment.
Hosts need larger ARP tables in order to handle IP-to-MAC address
mappings.Security can be undermined. A machine can claim to be another in
order to intercept packets, an act called "spoofing."It does not work for networks that do not use ARP for address
resolution.It does not generalize to all network topologies. For example, more
than one router that connects two physical networks.
.
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