Configuring Layer 2 Switching

Information About Layer 2 Switching


Note

See the , for information on creating interfaces.

You can configure Layer 2 switching ports as access or trunk ports. Trunks carry the traffic of multiple VLANs over a single link and allow you to extend VLANs across an entire network. All Layer 2 switching ports maintain MAC address tables.

Layer 2 Ethernet Switching Overview

The device supports simultaneous, parallel connections between Layer 2 Ethernet segments. Switched connections between Ethernet segments last only for the duration of the packet. New connections can be made between different segments for the next packet.

The device solves congestion problems caused by high-bandwidth devices and a large number of users by assigning each device (for example, a server) to its own collision domain. Because each LAN port connects to a separate Ethernet collision domain, servers in a switched environment achieve full access to the bandwidth.

Because collisions cause significant congestion in Ethernet networks, an effective solution is full-duplex communication. In full-duplex mode, which is configurable on these interfaces, two stations can transmit and receive at the same time. When packets can flow in both directions simultaneously, the effective Ethernet bandwidth doubles.

Switching Frames Between Segments

Each LAN port on a device can connect to a single workstation, server, or to another device through which workstations or servers connect to the network.

To reduce signal degradation, the device considers each LAN port to be an individual segment. When stations connected to different LAN ports need to communicate, the device forwards frames from one LAN port to the other at wire speed to ensure that each session receives full bandwidth.

To switch frames between LAN ports efficiently, the device maintains an address table. When a frame enters the device, it associates the media access control (MAC) address of the sending network device with the LAN port on which it was received.

Building the Address Table and Address Table Changes

The device dynamically builds the address table by using the MAC source address of the frames received. When the device receives a frame for a MAC destination address not listed in its address table, it floods the frame to all LAN ports of the same VLAN except the port that received the frame. When the destination station replies, the device adds its relevant MAC source address and port ID to the address table. The device then forwards subsequent frames to a single LAN port without flooding all LAN ports.

You can configure MAC addresses, which are called static MAC addresses, to statically point to specified interfaces on the device. These static MAC addresses override any dynamically learned MAC addresses on those interfaces. You cannot configure broadcast addresses as static MAC addresses. The static MAC entries are retained across a reboot of the device.

The address table can store a number of MAC address entries depending on the hardware I/O module. The device uses an aging mechanism, defined by a configurable aging timer, so if an address remains inactive for a specified number of seconds, it is removed from the address table.

Layer 3 Static MAC Addresses

You can configure a static MAC address for the following Layer 3 interfaces:

  • Layer 3 interfaces

  • Layer 3 port channels

  • VLAN network interface


Note

You cannot configure static MAC address on tunnel interfaces.

See the Cisco Nexus Series NX-OS Interfaces Configuration Guide, for information on configuring Layer 3 interfaces.

Prerequisites for Configuring MAC Addresses

MAC addresses have the following prerequisites:

  • You must be logged onto the device.

  • If necessary, install the Advanced Services license.

Default Settings for Layer 2 Switching

This table lists the default setting for Layer 2 switching parameters.

Table 1. Default Layer 2 Switching Parameters

Parameters

Default

Aging time

1800 seconds

Configuring Layer 2 Switching by Steps


Note

If you are familiar with the Cisco IOS CLI, be aware that the Cisco NX-OS commands for this feature might differ from the Cisco IOS commands that you would use.

Configuring a Static MAC Address

You can configure MAC addresses, which are called static MAC addresses, to statically point to specified interfaces on the device. These static MAC addresses override any dynamically learned MAC addresses on those interfaces. You cannot configure broadcast or multicast addresses as static MAC addresses.

Procedure

  Command or Action Purpose
Step 1

config t

Example:

switch# config t
switch(config)#

Enters configuration mode.

Step 2

mac address-table static mac-address vlan vlan-id {[drop | interface {type slot/port} | port-channel number]}

Example:

switch(config)# mac address-table static 1.1.1 vlan 2 interface ethernet 1/2

Specifies a static MAC address to add to the Layer 2 MAC address table.

Step 3

exit

Example:

switch(config)# exit
switch#

Exits the configuration mode.

Step 4

(Optional) show mac address-table static

Example:

switch# show mac address-table static
 (Optional)

Displays the static MAC addresses.

Step 5

(Optional) copy running-config startup-config

Example:

switch# copy running-config startup-config
 (Optional)

Copies the running configuration to the startup configuration.

Example

This example shows how to put a static entry in the Layer 2 MAC address table: 

switch# config t
switch(config)# mac address-table static 1.1.1 vlan 2 interface ethernet 1/2
switch(config)#

Configuring a Static MAC Address on a Layer 3 Interface

You can configure static MAC addresses on Layer 3 interfaces. You cannot configure broadcast or multicast addresses as static MAC addresses.

See the Configuring Layer 3 Interfaces section, for information on configuring Layer 3 interfaces.

Procedure

  Command or Action Purpose
Step 1

config t

Example:

switch# config t
switch(config)#

Enters configuration mode.

Step 2

interface [ethernet slot/port | ethernet slot/port.number | port-channel number | vlan vlan-id]

Example:

switch(config)# interface ethernet 1/3

Specifies the Layer 3 interface and enters the interface configuration mode.

Note 

You must create the Layer 3 interface before you can assign the static MAC address.

Step 3

mac-address mac-address

Example:

switch(config-if)# mac-address 22ab.47dd.ff89
switch(config-if)#

Specified a static MAC address to add to the Layer 3 interface.

Step 4

exit

Example:

switch(config-if)# exit
switch(config)#

Exits the interface mode.

Step 5

(Optional) show interface [ethernet slot/port | ethernet slot/port.number | port-channel number | vlan vlan-id]

Example:

switch# show interface ethernet 1/3
 (Optional)

Displays information about the Layer 3 interface.

Step 6

(Optional) copy running-config startup-config

Example:

switch# copy running-config startup-config
 (Optional)

Copies the running configuration to the startup configuration.

Example

This example shows how to configure the Layer 3 interface on slot 1, port 3 with a static MAC address: 

switch# config t
switch(config)# interface ethernet 1/3
switch(config-if)# mac-address 22ab.47dd.ff89
switch(config-if)#

Verifying the Layer 2 Switching Configuration

To display Layer 2 switching configuration information, perform one of the following tasks:

Command

Purpose

show mac address-table

Displays information about the MAC address table.

show mac address-table aging-time

Displays information about the aging time set for the MAC address entries.

show mac address-table static

Displays information about the static entries on the MAC address table.

show interface [interface] mac-address

Displays the MAC addresses and the burn-in MAC address for the interfaces.

Configuration Example for Layer 2 Switching

The following example shows how to add a static MAC address and how to modify the default global aging time for MAC addresses: 

switch# configure terminal
switch(config)# mac address-table static 0000.0000.1234 vlan 10 interface ethernet 1/15
switch(config)# mac address-table aging-time 120

Additional References for Layer 2 Switching -- CLI Version

Related Documents

Related Topic

Document Title

Static MAC addresses

Cisco Nexus® 3550-T Security Configuration section

Interfaces

Cisco Nexus® 3550-T Interfaces Configuration section

System management

Cisco Cisco Nexus® 3550-T System Management Configuration section