To perform an ARP lookup on a core switch, use the switch's ARP table to map IP addresses to MAC addresses, then correlate with the MAC address table to identify physical ports.

Understanding ARP and MAC Tables

• ARP Table: Populated on Layer 3 interfaces, it maps IP addresses to MAC addresses. It is built when the switch or router communicates with devices in its subnet or receives traffic from them .
• MAC Address Table: Populated on Layer 2 interfaces, it maps MAC addresses to physical switch ports. It is built as frames are received on the switch ports . A core switch that performs Layer 3 routing will maintain an ARP table for its VLAN interfaces, but may not automatically have all MAC addresses of connected hosts unless traffic has passed through it .

Commands for ARP Lookup

1. View ARP Table:

   show ip arp This displays IP-to-MAC mappings for devices the switch has communicated with .

2. View MAC Address Table:

   show mac address-table This shows which MAC addresses are associated with which physical ports .

3. Correlate IP to Port:
   • First, find the MAC address of the IP in the ARP table.
   • Then, locate that MAC in the MAC address table to determine the switch port .

Forcing ARP Discovery

• Ping devices in the subnet from the switch management interface to populate ARP entries. Only active hosts will appear in the ARP table .
• Proxy ARP can be enabled with:

  ip proxy-arp to allow the switch to respond to ARP requests on behalf of other devices .

• Tools like nmap followed by arp -an can also help discover active hosts and populate the ARP table .

Notes and Best Practices

• Only managed switches with a Layer 3 interface can maintain an ARP table. Unmanaged switches cannot be queried for ARP entries .
• ARP entries have a timeout (commonly 4 hours on Cisco devices), after which they are removed unless refreshed .
• For mapping IP addresses to switch ports in VLANs, ensure the switch has an IP address configured in the same VLAN as the devices . By combining the ARP table and MAC address table, network administrators can effectively identify which devices are connected to which ports on a core switch and troubleshoot connectivity issues.