show mac-address-table

5.4 Troubleshooting Tools

show mac-address-table

📘CompTIA Network+ (N10-009)

---

---

Breaking it down: Key Terms

1. MAC Address (Media Access Control Address):
   • A unique identifier for every device on a network (like a network “fingerprint”).
   • Format: 00:1A:2B:3C:4D:5E
2. Port:
   • A physical connection on the switch where a device (like a computer, printer, or another switch) plugs in.
3. VLAN (Virtual LAN):
   • Switches often separate traffic into VLANs. Each MAC address entry belongs to a VLAN.
4. Dynamic vs. Static MAC Addresses:
   • Dynamic: Learned automatically by the switch when a device sends traffic.
   • Static: Manually configured by an admin (rarely used).

---

Why is it important?

The show mac-address-table command is very useful for:

1. Troubleshooting connectivity issues
   • If a device cannot communicate, you can check if the switch “knows” about it.
2. Identifying devices on the network
   • See which MAC addresses are connected to which ports.
3. Security monitoring
   • Detect unknown devices that may be connected to your network.
4. Network management
   • Helps in planning and mapping the network layout.

---

Typical Output of the Command

When you run:

Switch> show mac-address-table

You might see something like this:

VLAN	MAC Address	Type	Port
10	00:1A:2B:3C:4D:5E	Dynamic	Fa0/1
10	00:1B:2C:3D:4E:6F	Dynamic	Fa0/2
20	00:1C:2D:3E:4F:7A	Static	Fa0/3

• VLAN: The VLAN the device belongs to.
• MAC Address: Device identifier.
• Type: Dynamic (learned automatically) or Static (configured manually).
• Port: Physical port on the switch where the device is connected.

---

Common Variations

1. Show MAC addresses for a specific VLAN:

show mac-address-table vlan 10

• Only shows devices in VLAN 10.

2. Show MAC addresses for a specific port:

show mac-address-table interface Fa0/1

• Shows only the MAC addresses learned on that port.

3. Filtering by dynamic or static:

• Some switches allow filters like:

show mac-address-table dynamic
show mac-address-table static

• Useful to focus only on automatically learned devices or manually configured ones.

---

Exam Tip

For the Network+ exam, remember:

1. show mac-address-table = “Which devices are connected to my switch ports?”
2. Dynamic MACs are learned automatically.
3. Static MACs are manually configured by administrators.
4. You can filter by VLAN or port.
5. Used mainly for troubleshooting and mapping devices in a network.

---

Example Scenario in an IT Environment

• You are the network admin for a company.
• Users report that a printer on port Fa0/5 is not working.
• You run show mac-address-table interface Fa0/5 and see no MAC addresses.
• This tells you the switch is not seeing the printer, so you check the cable, power, or VLAN configuration.
• Another example: You see a MAC address you don’t recognize on a sensitive VLAN. This could be a device that shouldn’t be on your network, so you investigate.

---

✅ Key Points to Remember for the Exam

• Command: show mac-address-table
• Purpose: Displays MAC addresses learned by a switch and their associated ports.
• Filters: VLAN, interface, dynamic/static
• Usage: Troubleshooting connectivity, identifying devices, security checks, and network management