Lab Objective:
Learn how to troubleshoot duplicate MAC addresses.
Lab Purpose:
You will have read in your study guide I’m sure that every MAC address should be unique. Each vendor is given a unique code (OUI) which comprises half of the MAC address, and it allocates the other half. Duplicate MAC address issues can arise if you buy cheap equipment from disreputable vendors or due to a configuration issue.
Packets need a MAC address in order to encapsulate a frame properly. Switches build a table of MAC addresses to ports. If the same MAC address is allocated to multiple ports, then either frame will be dropped or misdirected. Constant table rewrites on a switch may lead to its crashing.
Lab Tool:
Packet Tracer
Lab Topology:
Please use the following topology to complete this lab exercise:
Lab Walkthrough:
Task 1:
Drag three desktops. Connect them all with straight-through cables.
Task 2:
Add the IP addresses on the PCs as indicated. Here is the process for PC0:
Task 3:
Copy-and-paste the MAC address from PC0 to PC1. Yours will differ from mine of course.
Task 4:
Ping from 192.168.1.1 to .3.
Task 5:
Now check the MAC table on the switch. You will see the 3587 MAC address is associated with F0/1.
Switch#show mac-address-table
Mac Address Table
——————————————-
Vlan Mac Address Type Ports
—- ———– ——– —–
1 0004.9aac.3587 DYNAMIC Fa0/1
1 00d0.ba48.a838 DYNAMIC Fa0/3
Task 6:
Now ping 192.168.1.3 from 192.168.1.2.
Task 7:
Check the MAC table on the switch again.
Switch#show mac-address-table
Mac Address Table
——————————————-
Vlan Mac Address Type Ports
—- ———– ——– —–
1 0004.9aac.3587 DYNAMIC Fa0/2
1 00d0.ba48.a838 DYNAMIC Fa0/3
Task 8:
You can repeat this lab, but with a sniffer attached to the switch (see next lab). You can capture ARP and ICMP to see the two different IP addresses claiming the same source MAC. You may need to clear the switch MAC table with the command clear mac-address-table.
Note:
If you see multiple MAC addresses associated with one port, it usually indicates a trunk interface connected to another switch (or possibly a hub).
