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CLI
Management Guide
TigerAccess™ EE
222
SMC7824M/VSW
7.12.5 Proxy-ARP
The switch supports Proxy Address Resolution Protocol. Proxy ARP is the technique in
which one host, usually a router, answers ARP requests intended for another machine. By
“faking” its identity, the router accepts responsibility for routing packets to the “real” desti-
nation. Proxy ARP can help the switches on a subnet reach remote subnets without con-
figuring routing or a default gateway.
Fig. 7.15
Proxy-ARP
As shown in the diagram above, 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 switches if believes are directly connected, it will send 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.
Host A needs the MAC address of Host D to reach Host D. Therefore, Host A broadcasts
an ARP request on Subnet A, including the switch’s Br1 interface, but does not reah Host
D. This switch does not forward broadcasts by default. Since the switch knows that the
target address (Host D’s IP address) is on another subnet and can reach Host D, it will
reply with its own MAC address to Host A.
The Proxy ARP reply that switch sends to Host A. The proxy ARP reply packet is encap-
sulated in an Ethernet frame with its MAC address as the source address and Host A’s
MAC address as the destination address. The ARP replies are always unicast to the
original requester. On receiving this ARP reply, Host A updates its ARP table.
From now on Host A will forward all the packets that it wants to reach Host D to the MAC
address of switch. Since the switch 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 switch for all the hosts on Subnet B. Hence, all packets destined to Subnet
B are sent to the router. The switch forwards those packets to the hosts in Subnet B.
Host A
172.16.10.100/16
Host B
172.16.10.200/24
Host C
172.16.20.100/24
Host D
172.16.20.200/24
br1
172.16.10.99/24
br2
172.16.20.99/24
subnet A
subnet B
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