Ethernet Switch Configuration

Ethernet Switch Configuration

Management  Interface Considerations
An access layer switch is much like a PC in that you need to configure an IP address, a subnet mask, and a default gateway. To manage a switch remotely using TCP/IP, you need to assign the switch an IP address. In the figure, you want to manage S1 from PC1, a computer used for managing the network. To do this, you need to assign switch S1 an IP address. This IP address is assigned to a virtual interface called a virtual LAN (VLAN), and then it is necessary to ensure the VLAN is assigned to a specific port or ports on the switch. The default configuration on the switch is to have the management of the switch controlled through VLAN 1. However, a best practice for basic switch configuration is to change the management VLAN to a VLAN other than VLAN 1. The implications and reasoning behind this action are explained in the next chapter.

Configure Management Interface
To configure an IP address and subnet mask on the management VLAN of the switch, you must be in VLAN interface configuration mode. Use the command interface vlan 99 and enter the ip address configuration command. You must use the no shutdown interface configuration command to make this Layer 3 interface operational. When you see "interface VLAN x", that refers to the Layer 3 interface associated with VLAN x. Only the management VLAN has an interface VLAN associated with it.

Configure Default Gateway
You need to configure the switch so that it can forward IP packets to distant networks. The default gateway is the mechanism for doing this. The switch forwards IP packets with destination IP addresses outside the local network to the default gateway. In the figure, router R1 is the next-hop router. Its IP address is

To configure a default gateway for the switch, use the ip default-gateway command. Enter the IP address of the next-hop router interface that is directly connected to the switch where a default gateway is being configured. Make sure you save the configuration running on a switch or router. Use the copy running-config startup-config command to back up your configuration.

Show the IP Interfaces
Use the show ip interface brief to verify port operation and status. You will practice using the switchport access vlan 99 command in a hands on lab and a Packet Tracer activity.

The mdix auto Command
You used to be required to use certain cable types (cross-over, straight-through) when connecting between specific devices, switch-to-switch or switch-to-router. Instead, you can now use the mdix auto interface configuration command in the CLI to enable the automatic medium-dependent interface crossover (auto-MDIX) feature.

When the auto-MDIX feature is enabled, the switch detects the required cable type for copper Ethernet connections and configures the interfaces accordingly. Therefore, you can use either a crossover or a straight-through cable for connections to a copper 10/100/1000 port on the switch, regardless of the type of device on the other end of the connection.

The auto-MDIX feature is enabled by default on switches running Cisco IOS Release 12.2(18)SE or later. For releases between Cisco IOS Release 12.1(14)EA1 and 12.2(18)SE, the auto-MDIX feature is disabled by default.

Configure Duplex and Speed
You can use the duplex interface configuration command to specify the duplex mode of operation for switch ports. You can manually set the duplex mode and speed of switch ports to avoid inter-vendor issues with autonegotiation. Although there can be issues when you configure switch port duplex settings to auto, in this example, S1 and S2 switches have the same duplex settings and speeds.

Configure a Web Interface
Modern Cisco switches have a number of web-based configuration tools that require that the switch is configured as an HTTP server. These applications include the Cisco web browser user interface, Cisco Router and Security Device Manager (SDM), and IP Phone and Cisco IOS Telephony Service applications.

To control who can access the HTTP services on the switch, you can optionally configure authentication. Authentication methods can be complex. You may have so many people using the HTTP services that you require a separate server specifically to handle user authentication. AAA and TACACS authentication modes are examples that use this type of remote authentication method. AAA and TACACS are authentication protocols that can be used in networks to validate user credentials. You may need to have a less complex authentication method. The enable method requires users to use the server's enable password. The local authentication method requires the user to use the login username, password, and privilege level access combination specified in the local system configuration (by the username global configuration command).

Managing the MAC Address Table
Switches use MAC address tables to determine how to forward traffic between ports. These MAC tables include dynamic and static addresses.

Dynamic addresses are source MAC addresses that the switch learns and then ages when they are not in use. You can change the aging time setting for MAC addresses. The default time is 300 seconds. Setting too short an aging time can cause addresses to be prematurely removed from the table. Then, when the switch receives a packet for an unknown destination, it floods the packet to all ports in the same LAN (or VLAN) as the receiving port. This unnecessary flooding can impact performance. Setting too long an aging time can cause the address table to be filled with unused addresses, which prevents new addresses from being learned. This can also cause flooding.

The switch provides dynamic addressing by learning the source MAC address of each frame that it receives on each port, and then adding the source MAC address and its associated port number to the MAC address table. As computers are added or removed from the network, the switch updates the MAC address table, adding new entries and aging out those that are currently not in use.

A network administrator can specifically assign static MAC addresses to certain ports. Static addresses are not aged out, and the switch always knows which port to send out traffic destined for that specific MAC address. As a result, there is no need to relearn or refresh which port the MAC address is connected to. One reason to implement static MAC addresses is to provide the network administrator complete control over access to the network. Only those devices that are known to the network administrator can connect to the network.

To create a static mapping in the MAC address table, use the mac-address-table static vlan {1-4096, ALL} interfaceinterface-id command.
To remove a static mapping in the MAC address table, use the no mac-address-table static vlan {1-4096, ALL} interfaceinterface-id command.

The maximum size of the MAC address table varies with different switches. For example, the Catalyst 2960 series switch can store up to 8,192 MAC addresses. There are other protocols that may limit the absolute number of MAC address available to a switch.

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