Understanding ARP and Proxy ARP on Cisco Routers

Objective

Learn and understand how ARP and Proxy ARP are used by routers to encapsulate packets before sending them to a neighboring device.

Purpose

Understanding ARP is crucial for passing the CCNA exam. You may encounter ARP-related issues to troubleshoot both in the exam and in real-world scenarios.

Lab Topology

Use the following topology to complete this lab exercise:

Router Setup 1

Task 1: Configure Hostnames

Objective: Set hostnames on routers R1, R2, and R3 as per the topology.

Configuration Steps:

Router#config t
Enter configuration commands, one per line. End with CTRL/Z.
Router(config)#hostname R1
R1(config)#

Router#config t
Enter configuration commands, one per line. End with CTRL/Z.
Router(config)#hostname R2
R2(config)#

Router#config t
Enter configuration commands, one per line. End with CTRL/Z.
Router(config)#hostname R3
R3(config)#

Task 2: Configure IP Addresses and Static Routes

Objective: Configure the IP addresses on the Ethernet interfaces of R1, R2, and R3 as illustrated in the topology (.1 for R1 and .2 for R2, and then .1 and .2 between R2 and R3). Add static routes so that R1 can ping the host address on R3 and R3 can return the ping.

Configuration Steps:

R1(config)#int f0/0
R1(config-if)#ip address 10.0.0.1 255.0.0.0
R1(config-if)#no shutdown
R1(config-if)#ip route 0.0.0.0 0.0.0.0 f0/0
R1(config)#end
R1#copy running-config startup-config

R2(config)#int f0/0
R2(config-if)#ip address 10.0.0.2 255.0.0.0
R2(config-if)#no shutdown

R2(config)#int f0/1
R2(config-if)#ip address 192.168.1.1 255.255.255.0
R2(config-if)#no shutdown
R1(config)#end
R1#copy running-config startup-config

R3(config)#int f0/1
R3(config-if)#ip address 192.168.1.2 255.255.255.0
R3(config-if)#no shutdown
R3(config-if)#ip route 0.0.0.0 0.0.0.0 f0/1
R1(config)#end
R1#copy running-config startup-config

Task 3: Verify ARP Configuration

Objective: Use the correct show commands to check:

  • The ARP cache on R1. Note the times for the learned addresses. Identify which will not timeout and how you can tell.
  • The entry for R3 and why it is the same as the R2 Ethernet interface.
  • The meaning of the “–” in the ARP table.

    • Verification Commands:

    R1#show arp
Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet  10.0.0.1                -   00D0.5898.7B01  ARPA   FastEthernet0/0

R1#ping 10.0.0.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.0.0.2, timeout is 2 seconds:
.!!!!
Success rate is 80 percent (4/5), round-trip min/avg/max = 0/0/0 ms

R1#show arp
Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet  10.0.0.1                -   00D0.5898.7B01  ARPA   FastEthernet0/0
Internet  10.0.0.2                0   0001.4313.3501  ARPA   FastEthernet0/0

R1#ping 192.168.1.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.2, timeout is 2 seconds:
...!!
Success rate is 40 percent (2/5), round-trip min/avg/max = 0/0/1 ms

R1#show arp
Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet  10.0.0.1                -   00D0.5898.7B01  ARPA   FastEthernet0/0
Internet  10.0.0.2                11  0001.4313.3501  ARPA   FastEthernet0/0
Internet  192.168.1.2             0   0001.4313.3501  ARPA   FastEthernet0/0

R3#show int f0/1
FastEthernet0/1 is up, line protocol is up (connected)
  Hardware is Lance, address is 0002.1769.e102 (bia 0002.1769.e102)
  Internet address is 192.168.1.2/24

R1#show arp
Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet  10.0.0.1                -   00D0.5898.7B01  ARPA   FastEthernet0/0
Internet  10.0.0.2                13  0001.4313.3501  ARPA   FastEthernet0/0
Internet  192.168.1.2             1   0001.4313.3501  ARPA   FastEthernet0/0
  • In the ARP table, the “–” symbol under the "Age" column indicates that the entry is permanent and will not timeout.
  • the entry with the “–” in the ARP table could also indicate that the IP address is assigned to an interface on the same device. In this case, the router does not need to send ARP requests for its own IP addresses, so these entries are treated as permanent and do not age out.

    • Cisco Packet Tracer file:
    Load and open the .pkt Lab file in Cisco Packet Tracer from here: ARP_and_Proxy_ARP.pkt