ARP-Ping includes ARP-Ping IP and ARP-Ping MAC, which are used to maintain Layer 2 features.
ARP-Ping IP is on the basis of the ARP protocol. After obtaining the IP address and outbound interface (optional) input by the user through the configuration management plane, the system constructs an ARP Request packet and broadcasts the packet on the outbound interface. If no reply is received within the timeout period, the system prompts that this IP address is available; if a reply is received, the system fetches the MAC address of the peer carried in the Reply packet and displays it to the user.
ARP-Ping IP uses ARP packets to check whether an IP address is used by another device in the LAN.
To assign an IP address for a device, you need to make sure that this IP address is not used by other devices on the network by sending the ARP packets. Then, you can take appropriate actions.
You can also run the ping command to check whether this IP address is used by other devices on the network. If the destination host or the switch with the firewall function is disabled from replying to ping packets, the destination host or the switch does not reply to the ping packets and regards that the IP address is not in use. ARP is a Layer 2 protocol; therefore, the ARP packets can pass through the firewall that is configured not to reply to ping packets.
Principle of ARP-Ping IP
In ARP-Ping IP, ARP Request packets are sent. The following is the implementation process of ARP-Ping IP:
After setting the specified IP address through command lines, you can send the ARP Request packets and start the timeout timer of ARP Reply packets.
After receiving the ARP Request packets, each switch or host in the LAN returns ARP Reply packets.
After receiving the ARP Reply packets, the source device compares the source IP addresses contained in the Reply packets with the IP addresses input in the command lines. If they are consistent, the MAC address corresponding to the input IP address is displayed and the timeout timer of the ARP Reply packets is disabled.
If the timeout timer of the ARP Reply packets times out, it means that this IP address is not in use.
As shown in Figure 1, ARP-Ping IP can be used to check whether 10.1.1.2 has been used. After Switch A receives the ARP Reply packet from host A at 10.1.1.2, Switch A displays the MAC address of host A. The information displayed shows that 10.1.1.2 has been used by the hosts on the network.
Figure 1 Implementation process of ARP-Ping IP
The arp-ping ip command is applicable to only Ethernet, Gigabit Ethernet, Eth-Trunk, VLANIF interfaces, and the member interfaces of VLANIF interfaces.
ARP-Ping MAC is similar to the common ping operation. The difference is that ARP-Ping MAC is applicable to only directly connected Ethernet LANs or L2VPN Ethernet networks. After a host sends an ICMP Echo Request packet, it receives an ICMP Echo Reply packet, analyzes the packet, and compares the source MAC address in the data field of the packet with the locally saved MAC address. If they are consistent, the host displays the IP address of the Echo Reply packet, prompts that this MAC address is in use, and disables the timeout timer. Then, the ARP-Ping MAC operation finishes. If the time for waiting for a reply to the ICMP Echo Request packet times out, the host prompts that this MAC address is not used.
Principle of ARP-Ping MAC
In ARP-Ping MAC, broadcast ICMP Echo Request packets are sent. The following is the implementation process of ARP-Ping MAC:
After a MAC address is set through the command line, an ICMP Echo Request packet is broadcast and the timeout timer starts.
After receiving the ICMP Echo Request packet, each switch or host in the LAN returns an ICMP Echo Reply packet.
After receiving the ICMP Echo Reply packet, the source device compares the source MAC address contained in the Echo Reply packet with the MAC address input in the command line. If they are consistent, the system displays the IP address of the Echo Reply packet and prompts that this MAC address is in use and the timeout timer is disabled. Then, the ARP-Ping MAC operation finishes.
If the timeout timer of the ICMP Echo Reply packets times out, it means that this MAC address is not in use.
If the system disables returning replies to the requests from a specified network segment, the sender in the segment cannot receive the ICMP Echo Reply packet.
As shown in Figure 2, ARP-Ping IP MAC can check whether 10.1.1.2 has been used. After having received ICMP response packets sent by all hosts on the network, Switch A displays the IP address of the host whose MAC address is 0013-46E7-2EF5.
Figure 2 Implementation process of ARP-Ping MAC
