Lab Objective
Observe normal ARP resolution between a router and a directly connected host, then enable Proxy ARP on a router interface and observe how it allows a misconfigured host with no default gateway to still reach a remote subnet.
Lab Purpose
ARP is the invisible mechanism behind nearly every Layer 3 to Layer 2 translation, and Proxy ARP is a legacy feature that occasionally causes confusing behavior in real networks — a host with no gateway configured can sometimes still reach remote networks, and understanding Proxy ARP is essential to explaining why.
Lab Topology
R1 ---- Gi0/0 (192.168.30.1/24) ---- Switch ---- PC1 (192.168.30.10/24)
R1 ---- Gi0/1 (192.168.40.1/24) ---- R2 ---- LAN 192.168.40.0/24
PC1's default gateway is intentionally left
UNCONFIGURED (0.0.0.0) for this labTask 1: Configure Basic Addressing
Configure R1's interfaces as shown, and configure PC1 with IP 192.168.30.10/24 but no default gateway.
Task 2: Observe Normal ARP Behavior
From R1, ping PC1's address, then examine R1's ARP table to confirm the resolved MAC address.
Task 3: Confirm PC1 Cannot Reach the Remote Subnet by Default
Attempt to ping 192.168.40.x from PC1 and confirm it fails due to the missing default gateway.
Task 4: Enable Proxy ARP on R1
Enable Proxy ARP on R1's Gi0/0 interface (note: it is enabled by default on most IOS versions, so this task also covers explicitly verifying and, if needed, re-enabling it).
Task 5: Verify Proxy ARP Resolves the Issue
Attempt the same ping from PC1 to a host on 192.168.40.0/24 and observe that it now succeeds, then explain why using the ARP table.
Solution and Verification
R1(config)# interface gigabitethernet0/0
R1(config-if)# ip address 192.168.30.1 255.255.255.0
R1(config-if)# no shutdown
R1(config-if)# exit
R1(config)# interface gigabitethernet0/1
R1(config-if)# ip address 192.168.40.1 255.255.255.0
R1(config-if)# no shutdownR1# ping 192.168.30.10
!!!!!
Success rate is 100 percent (5/5)
R1# show ip arp
Protocol Address Age Hardware Addr Type Interface
Internet 192.168.30.10 0 00aa.bb00.1010 ARPA GigabitEthernet0/0
Internet 192.168.30.1 - 00aa.bb00.0001 ARPA GigabitEthernet0/0PC1> ping 192.168.40.20
(without a gateway configured, and without
Proxy ARP, this would fail entirely -- but
because PC1 has no gateway set, it will
ARP directly for 192.168.40.20 as if it
were on its own local subnet)R1(config)# interface gigabitethernet0/0
R1(config-if)# ip proxy-arp
R1# show ip interface gigabitethernet0/0 | include Proxy
Local Proxy ARP is disabled
Proxy ARP is enabledPC1> ping 192.168.40.20
Reply from 192.168.40.20: bytes=32 time=2ms
PC1> arp -a
Interface: 192.168.30.10
Internet Address Physical Address
192.168.40.20 00aa.bb00.0001
-- Notice: PC1 believes it has directly
-- resolved 192.168.40.20's MAC address,
-- but the MAC address shown actually
-- belongs to R1's Gi0/0 interface --
-- R1 answered the ARP request on behalf
-- of the remote host, then routed the
-- traffic normally once it arrivedKey Takeaway
Proxy ARP allows a router to answer ARP requests for addresses it can route to, even though those addresses are not physically on the local segment — this can mask a misconfigured default gateway on a host, which is exactly why unexpectedly working connectivity despite a missing gateway is a classic sign to check for Proxy ARP during troubleshooting.