How to check IP and MAC addresses in a core switch

To find the MAC address for a given IP on a core switch, use ARP to resolve the IP to a MAC, then check the MAC address table to identify the switch port. Step 1: Resolve the IP to a MAC AddressOn the core switch, first determine the MAC address associated with the IP address using the ARP table: Cisco IOS: CodeCopyshow ip arp This command displays the MAC address corresponding to the IP, along with the VLAN and interface if directly connected. Juniper JunOS: CodeCopyshow arp | match This will return the MAC address for the specified IP.

Step 2: Locate the MAC in the Switch MAC Address TableOnce you have the MAC address, identify the port on the switch where the device is connected: Cisco IOS: CodeCopyshow mac address-table address This command shows the VLAN and the port (e.g., Gi1/0/1) where the MAC is learned.

If the MA

C appears on a trunk port, it indicates the device is connected to a downstream access switch, and you may need to repeat the command on that switch to find the exact access port. Juniper JunOS: CodeCopyshow ethernet-switching table | match This displays the VLAN and interface associated with the MAC. Step 3: Trace Through Multiple Switches (if needed)In larger networks, the MAC may appear on a trunk port of the core switch.

To locate the end device: Identify the trunk port on the core switch where the MAC is learned. Log into the connected access switch.

Repeat the MA

C address table lookup on the access switch to find the exact port of the end device.

Additional Tips

Ensure the device is active; MAC addresses age out after a default period (e.g., 300 seconds on Cisco).

If the MA

C is static, it will appear in the table permanently. For Layer 3 devices, the ARP table is essential to map IPs to MACs before checking the MAC table. By following these steps, you can reliably determine the MAC address corresponding to an IP and trace it to the specific switch port, even in multi-switch topologies.