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Rowell Dionicio

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MAC Address Table on Cisco Switches

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Network switching fundamentals are required for CCNP Enterprise Core studies. We’ll review the MAC address table on a Cisco switch to learn how a device to port mapping is created and why it is needed.

We don’t think much about connecting our hosts to a Cisco switch. Whether 1 Gbps or 10 Gbps Ethernet ports, MAC address learning is performed the same way. 

Each host connecting to a switch port will have its MAC address entered into the switch’s MAC address table. 

The MAC address table is a way to map each port to a MAC address. This makes it efficient to forward traffic directly to a host. Without the MAC address table, traffic would be forwarded out each port, like a hub (hopefully you haven’t used one of those in a long time.)

MAC address table on a switch for CCNP ENCOR
MAC Address Table

Host A has a fictitious MAC address of AA:AA:AA:AA:AA:AA and it wants to send traffic to Host B with a fictitious MAC address of BB:BB:BB:BB:BB:BB. When the network switch receives the traffic destined for Host B, it knows to forward that traffic destined to interface g1/0/27 because the MAC address table lists Host B’s MAC address for interface g1/0/27. Assume they are also on the same VLAN.

What happens if a destination MAC address is not in the MAC address table? The switch must flood the traffic out of all ports in what’s called unknown unicast flooding. The switch wants the host with the destination MAC address to respond. 

What happens if the host is no longer connected? The host’s MAC address remains in the table until it ages out. There is a default aging timer. 

What happens if the host changes to a different port? The MAC address table is updated accordingly. 

How to view the Cisco MAC address table

First, let’s see what’s connected to my Cisco switch.

sw1#show interface status | include connected
Gi1/0/1                      connected    129        a-full  a-100 10/100/1000BaseTX
Gi1/0/3                      connected    trunk      a-full a-1000 10/100/1000BaseTX
Gi1/0/9   3504               connected    trunk      a-full a-1000 10/100/1000BaseTX
Gi1/0/14                     connected    trunk      a-full a-1000 10/100/1000BaseTX

Next, we use a show command to view the MAC address table of all dynamically learned addresses:

show mac address-table dynamic

The dynamic MAC addresses on my switch:

sw1#show mac address-table dynamic
          Mac Address Table
-------------------------------------------

Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
 129    0017.88a9.b5dc    DYNAMIC     Gi1/0/1
 129    7483.c279.3a4c    DYNAMIC     Gi1/0/14
 129    c869.cd81.2307    DYNAMIC     Gi1/0/14
 103    000c.2979.60af    DYNAMIC     Gi1/0/14
 103    38f9.d329.a785    DYNAMIC     Gi1/0/14
 103    3c52.82af.08b6    DYNAMIC     Gi1/0/14
 103    5032.37d2.9089    DYNAMIC     Gi1/0/14
 103    6cae.f6b0.3fd2    DYNAMIC     Gi1/0/14
 103    701f.53b7.da81    DYNAMIC     Gi1/0/9
 103    7483.c279.3a4c    DYNAMIC     Gi1/0/14
 103    8e0a.c4f3.5e49    DYNAMIC     Gi1/0/14
 103    9c20.7bb9.6f35    DYNAMIC     Gi1/0/14
 103    b02a.4357.9868    DYNAMIC     Gi1/0/14
 120    18e8.29b0.84b8    DYNAMIC     Gi1/0/14
 140    000c.2979.60a5    DYNAMIC     Gi1/0/14
 140    000c.2979.60b9    DYNAMIC     Gi1/0/14
 140    0011.329f.c5a1    DYNAMIC     Gi1/0/14
   1    18e8.29b0.84b8    DYNAMIC     Gi1/0/14
   1    18e8.29b0.84b9    DYNAMIC     Gi1/0/14
   1    7483.c273.d835    DYNAMIC     Gi1/0/14
   3    5c5b.3550.0776    DYNAMIC     Gi1/0/3
   3    7483.c279.3a4c    DYNAMIC     Gi1/0/14
Total Mac Addresses for this criterion: 22

Viewing the MAC address seen on a specific interface

What if we want to verify what MAC address is seen off an individual port. This is useful for troubleshooting and verifying where a host might be located. The command syntax is:

show mac address-table <interface-name>

Here’s the output on my switch for interface g1/0/9:

sw1#show mac address-table interface g1/0/9
           Mac Address Table
 Vlan    Mac Address       Type        Ports
 ----    -----------       --------    -----
  103    701f.53b7.da81    DYNAMIC     Gi1/0/9

View MAC addresses for a specific VLAN

It’s possible to view all learned MAC addresses for a specific VLAN. I use this command to ensure I’ve trunked a VLAN across all necessary uplinks. The command syntax is:

show mac address-table vlan <vlan-id>

The output on my switch for vlan 103:

sw1#show mac address-table dynamic vlan 103
           Mac Address Table
 Vlan    Mac Address       Type        Ports
 ----    -----------       --------    -----
  103    000c.2979.60af    DYNAMIC     Gi1/0/14
  103    38f9.d329.a785    DYNAMIC     Gi1/0/14
  103    5032.37d2.9089    DYNAMIC     Gi1/0/14
  103    6cae.f6b0.3fd2    DYNAMIC     Gi1/0/14
  103    701f.53b7.da81    DYNAMIC     Gi1/0/9
  103    7483.c279.3a4c    DYNAMIC     Gi1/0/14
  103    8e0a.c4f3.5e49    DYNAMIC     Gi1/0/14
  103    b02a.4357.9868    DYNAMIC     Gi1/0/14
  103    f65a.0212.e051    DYNAMIC     Gi1/0/14
 Total Mac Addresses for this criterion: 9

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