Basic 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
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. The figure illustrates the use of VLAN 99 as the management VLAN; however, it is important to consider that an interface other than VLAN 99 can be considered for the management interface.
Note: You will learn more about VLANs in the next chapter. Here the focus is on providing management access to the switch using an alternative VLAN. Some of the commands introduced here are explained more thoroughly in the next chapter.
For now, VLAN 99 is created and assigned an IP address. Then the appropriate port on switch S1 is assigned to VLAN 99. The figure also shows this configuration information.
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.
Note that a Layer 2 switch, such as the Cisco Catalyst 2960, only permits a single VLAN interface to be active at a time. This means that the Layer 3 interface, interface VLAN 99, is active, but the Layer 3 interface, interface VLAN 1, is not active.
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 172.17.99.1.
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.
Verify Configuration
The top screen shot in the figure is an abbreviated screen output showing that VLAN 99 has been configured with an IP address and subnet mask, and Fast Ethernet port F0/18 has been assigned the VLAN 99 management interface.
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 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. The figure shows a sample MAC address table from the output of the show mac-address-table command that includes static and dynamic MAC addresses.
Note: The MAC address table was previously referred to as content addressable memory (CAM) or as the CAM table.
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.

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