Goal:
● Understand internal MAC addresses on Cisco switches
Requirements:
● labsw1 and labsw2 configured in previous labs
● PC with console cable
This lab should be conducted using on the console port on each switch. Attach no devices to the FastEthernet interfaces.
In Lab 5 – Ethernet Address Management, we covered how a single switch builds and uses MAC Address Tables. The switch reads the source MAC address of every frame that enters the switch, and creates an entry in the MAC Address Table that identifies the address and the port that it used to enter the switch. Review Lab 5 if necessary.
We also covered how a switch handles broadcasts and unknown MAC addresses: the frame is forwarded on all active ports. This part of Lab 5 was not very exciting, considering that we only had two PCs.
Switches in a multi-switch network manage Ethernet addresses the same way, but the process now involves more Ethernet addresses. Labs 11 and 12 showed that trunk ports allow a computer on labsw1 to communicate with a device on labsw2, as long as the devices are in the same VLAN. This lab expands on that process to show what actually happens in a multi-switch environment. We will use two lab configurations for this lab.
When switches are connected by a trunk port, each switch learns MAC addresses associated with the “other” or neighboring switches. These MAC address will be displayed when the “show mac-address-table” command is used. These addresses are critical for switch function in a multi-switch environment, and future labs will covers the use of the addresses in detail. This lab provides an explanation of the internal MAC addresses.
Display the MAC address table on labsw1 and labsw2. (There may be a difference between Cisco switches, as well as the Packet Tracer display. By now, you should be able to understand the differences between the labs and your configuration.)
1. labsw1#sho mac-address-table dynamic
Mac Address Table
-------------------------------------------
Vlan Mac Address Type Ports
---- ----------- -------- -----
1 0001.c9c8.a118 DYNAMIC Fa0/24
2. labsw2# show mac-address-table dynamic
Mac Address Table
-------------------------------------------
Vlan Mac Address Type Ports
---- ----------- -------- -----
1 0009.43cb.3018 DYNAMIC Fa0/24
200 0009.43cb.3018 DYNAMIC Fa0/24
201 0009.43cb.3018 DYNAMIC Fa0/24
202 0009.43cb.3018 DYNAMIC Fa0/24
203 0009.43cb.3018 DYNAMIC Fa0/24
These MAC addresses were learned without connecting a computer to either switch. Where did these Ethernet addresses come from? We know that the switches are connected on interface Fa0/24 for both routers, and the MAC addresses (0009.43cb.3018, 0001.c9c8.a118) were learned on those ports. These MAC addresses can be verified by using the “show interface” command for each interface:
3. labsw1# show interface fa0/24
FastEthernet0/24 is up, line protocol is up (connected)
Hardware is Fast Ethernet, address is 0009.43cb.3018 (bia 0009.43cb.3018)
.
(Lines omitted)
4. labsw2#show interface fa0/24
FastEthernet0/24 is up, line protocol is up (connected)
Hardware is Fast Ethernet, address is 000a.8ae6.5198 (bia 000a.8ae6.5198)
.
(Lines omitted)
labsw1 knows MAC address 000a.8ae6.5198 because it receives information from labsw2 on interface Fa0/24.
labsw2 knows about MAC address 0009.43cb.3018 because it receives information from labsw1 on interface Fa0/24.
What data does each switch receive from the other on the respective interfaces Fa0/24? (Review Lab 12.) We know at this point that cdp updates are passed between the switches, and future labs will identify STP (Spanning Tree Protocol) data passed on the trunk connections, critical data for real networks as well for the exam.
Two critical points about switch internal MAC addresses:
- Switch internal MAC addresses have no impact on attached devices. Ethernet switches use transparent switching, meaning that internal switch MAC addresses are not used in Ethernet frames created by end devices. End devices learn the MAC addresses of other end devices, not the switch.
- Switch internal MAC addresses are critical to switch function.
That means that your PC can ignore internal switch MAC address, but you, hedgeling, cannot.
Each switch has a MAC address associated with the overall switch. It is identified as the CPU address, and is displayed by the “show mac-address-table” command (without the “dynamic” qualifier. Not available on Packet Tracer), as well as by the “show version” command. It is also the MAC address of interface VLAN 1.
In this example, labsw1 has an internal, or CPU, MAC address of 0009.43cb.3000. (Remember, MAC addresses are displayed in various formats. Pay attention to the address, not the format.)
5. labsw1# show version
Cisco Internetwork Operating System Software
(Lines omitted)
32K bytes of flash-simulated non-volatile configuration memory.
Base ethernet MAC Address: 00:09:43:CB:30:00
Motherboard assembly number: 73-5781-09
Power supply part number: 34-0965-01
.
(Lines omitted)
6. labsw1#show mac-address-table
Mac Address Table
-------------------------------------------
Vlan Mac Address Type Ports
---- ----------- -------- -----
All 0009.43cb.3000 STATIC CPU
All 0100.0ccc.cccc STATIC CPU
All 0100.0ccc.cccd STATIC CPU
All 0100.0cdd.dddd STATIC CPU
1 000a.8ae6.5198 DYNAMIC Fa0/24
7. labsw2# show version
Cisco Internetwork Operating System Software
(Lines omitted)
32K bytes of flash-simulated non-volatile configuration memory.
Base ethernet MAC Address: 00:0A:8A:E6:51:80
Motherboard assembly number: 73-5781-09
Power supply part number: 34-0965-01
.
(Lines omitted)
8. labsw2#show mac-address-table
Mac Address Table
-------------------------------------------
Vlan Mac Address Type Ports
---- ----------- -------- -----
All 000a.8ae6.5180 STATIC CPU
All 0100.0ccc.cccc STATIC CPU
All 0100.0ccc.cccd STATIC CPU
All 0100.0cdd.dddd STATIC CPU
1 0009.43cb.3018 DYNAMIC Fa0/24
200 0009.43cb.3018 DYNAMIC Fa0/24
201 0009.43cb.3018 DYNAMIC Fa0/24
202 0009.43cb.3018 DYNAMIC Fa0/24
203 0009.43cb.3018 DYNAMIC Fa0/24
(Note: Understanding the above is critical. Learn it)
In addition to the CPU address, each interface on the switch has a unique MAC address that is based on the CPU address. To view the interface MAC address on each switch, use the “show interface” command. The command will display information on all interfaces. Page through the display to review the interface MAC addresses (it’s a long display, so we won’t reproduce it here. However, you should review the output and make sure you understand the display.
labsw1 (The interface MAC addresses increment by a binary 1, but are expressed in hexadecimal.)
CPU MAC address: 00:09:43:CB:30:00
Interface VLAN 1: 0009.43cb.3000
Interface Fa0/1: 0009.43cb.3001
Interface Fa0/2: 0009.43cb.3002
Interface Fa0/3: 0009.43cb.3003
.
Interface Fa0/10: 0009.43cb.300a
Interface Fa0/11: 0009.43cb.300b
.
.
Interface Fa0/24: 0009.43cb.3018
labsw2
CPU MAC Address: 00:0A:8A:E6:51:80
Interface VLAN 1: 000a.8ae6.5180
Interface Fa0/1: 000a.8ae6.5181
Interface Fa0/2: 000a.8ae6.5182
Interface Fa0/3: 000a.8ae6.5183
.
(Interfaces omitted)
Interface Fa0/24: 000a.8ae6.5198
Understanding internal switch MAC addresses is critical. Learn it, live it, love it. It will become critical in future labs, on the exam, and in any network that has Cisco switches.
