Configuring Layer 2 Switching (2024)

Contents

• Configuring Layer 2 Switching
• Information About Layer 2 Switching
• Layer 2 Ethernet Switching Overview
• Switching Frames Between Segments
• Building the Address Table and Address Table Changes
• Consistent MAC Address Tables on the Supervisor and on the Modules
• Layer 3 Static MAC Addresses
• High Availability for Switching
• Virtualization Support for Layer 2 Switching
• Licensing Requirements for Layer 2 Switching
• Prerequisites for Configuring MAC Addresses
• Guidelines and Limitations for Configuring MAC Addresses
• Default Settings for Layer 2 Switching
• Configuring Layer 2 Switching by Steps
• Configuring a Static MAC Address
• Configuring a Static MAC Address on a Layer 3 Interface
• Configuring the Aging Time for the MAC Table
• Checking Consistency of MAC Address Tables
• Clearing Dynamic Addresses from the MAC Table
• Verifying the Layer 2 Switching Configuration
• Configuration Example for Layer 2 Switching
• Additional References for Layer 2 Switching -- CLI Version
• Feature History for Configuring Layer 2 Switching -- CLI Version

This chapter describes how to configure Layer 2 switching using Cisco NX-OS.

This chapter includes the following sections:

• Information About Layer 2 Switching
• Licensing Requirements for Layer 2 Switching
• Prerequisites for Configuring MAC Addresses
• Guidelines and Limitations for Configuring MAC Addresses
• Default Settings for Layer 2 Switching
• Configuring Layer 2 Switching by Steps
• Verifying the Layer 2 Switching Configuration
• Configuration Example for Layer 2 Switching
• Additional References for Layer 2 Switching -- CLI Version
• Feature History for Configuring Layer 2 Switching -- CLI Version

Information About Layer 2 Switching

Note	See the Cisco Nexus 7000 Series NX-OS Interfaces Configuration Guide, Release 5.x, 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.

Note	See the Cisco Nexus 7000 Series NX-OS High Availability and Redundancy Guide, Release 5.x, for complete information on high-availability features.

• Layer 2 Ethernet Switching Overview
• High Availability for Switching
• Virtualization Support for Layer 2 Switching

Layer 2 Ethernet Switching Overview

The device supports simultaneous, parallel connections between Layer2 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 10-,100-, 1000-Mbps, or 10-Gigabit 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. Typically, 10/100-Mbps Ethernet operates in half-duplex mode, which means that stations can either receive or transmit. 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. 1/10-Gigabit Ethernet operates in full duplex only.

• Switching Frames Between Segments
• Building the Address Table and Address Table Changes
• Consistent MAC Address Tables on the Supervisor and on the Modules
• Layer 3 Static MAC Addresses

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 or multicast addresses as static MAC addresses. The static MAC entries are retained across a reboot of the device.

Beginning with Cisco NX-OS Release 4.1(5), you must manually configure identical static MAC addresses on both devices connected by a virtual port channel (vPC) peer link. The MAC address table display is enhanced to display information on MAC addresses when you are using vPCs.

The address table can store up to 128,000 address entries. 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.

Consistent MAC Address Tables on the Supervisor and on the Modules

Optimally, all the MAC address tables on each module exactly match the MAC address table on the supervisor. Beginning with NX-OS 4.1(2), when you enter the show forwarding consistency l2 command, the device displays discrepant, missing, and extra MAC address entries.

Layer 3 Static MAC Addresses

Beginning with Release 4.2, you can configure a static MAC address for all Layer 3 interfaces. The default MAC address for the Layer 3 interfaces is the VDC MAC address.

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

• Layer 3 interfaces

• Layer 3 subinterfaces

• Layer 3 port channels

• VLAN network interface

Note	You cannot configure static MAC address on tunnel interfaces.

See the Cisco Nexus 7000 Series NX-OS Interfaces Configuration Guide, Release 5.x, for information on configuring Layer 3 interfaces.

High Availability for Switching

You can upgrade or downgrade the software seamlessly, with respect to classical Ethernet switching. Beginning with Release 4.2(1), if you have configured static MAC addresses on Layer 3 interfaces, you must unconfigure those ports in order to downgrade the software.

Note	See the Cisco Nexus 7000 Series NX-OS High Availability and Redundancy Guide, Release 5.x, for complete information on high availability features.

Virtualization Support for Layer 2 Switching

The device supports virtual device contexts (VDCs), and the configuration and operation of the MAC address table are local to the VDC.

Note	See the Cisco Nexus 7000 Series NX-OS Virtual Device Context Configuration Guide, Release 5.x, for complete information on VDCs and assigning resources.

Licensing Requirements for Layer 2 Switching

This table shows the licensing requirements for this feature:

Product	License Requirement
Cisco NX-OS	Layer 2 switching require no license. Any feature not included in a license package is bundled with the Cisco NX-OS system images and is provided at no extra charge to you. For a complete explanation of the Cisco NX-OS licensing scheme, see the Cisco NX-OS Licensing Guide.

However, using VDCs requires an Advanced Services license.

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 and enter the desired VDC.

Guidelines and Limitations for Configuring MAC Addresses

MAC addresses have the following configuration guidelines and limitations:

• The MAC address table stores up to 128,000 entries.

• You can configure up to 14 different aging times on the device.

Default Settings for Layer 2 Switching

This table lists the default setting for Layer 2 switching parameters.
Table 1Default 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
• Configuring a Static MAC Address on a Layer 3 Interface
• Configuring the Aging Time for the MAC Table
• Checking Consistency of MAC Address Tables
• Clearing Dynamic Addresses from the MAC Table

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.

Before You Begin

Before you configure static MAC addresses, ensure that you are in the correct VDC (or enter the switchto vdc command).

SUMMARY STEPS

1. config t

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

3. exit

4. (Optional) show mac address-table static

5. (Optional) copy running-config startup-config

DETAILED STEPS

	Command or Action	Purpose
Step1	config t Example: switch# config tswitch(config)#	Enters configuration mode.
Step2	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.
Step3	exit Example: switch(config)# exitswitch#	Exits the configuration mode.
Step4	show mac address-table static Example: switch# show mac address-table static	(Optional) Displays the static MAC addresses.
Step5	copy running-config startup-config Example: switch# copy running-config startup-config	(Optional) Copies the running configuration to the startup configuration.

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

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

Configuring a Static MAC Address on a Layer 3 Interface

Beginning with Release 4.2(1), you can configure static MAC addresses on Layer 3 interfaces. You cannot configure broadcast or multicast addresses as static MAC addresses.

Note	You cannot configure static MAC addresses on tunnel interfaces.

See the Cisco Nexus 7000 Series NX-OS Interfaces Configuration Guide, Release 5.x, for information on configuring Layer 3 interfaces.

Before You Begin

Before you configure static MAC addresses, ensure that you are in the correct VDC (or enter the switchto vdc command).

SUMMARY STEPS

1. config t

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

3. mac-address mac-address

4. exit

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

6. (Optional) copy running-config startup-config

DETAILED STEPS

	Command or Action	Purpose
Step1	config t Example: switch# config tswitch(config)#	Enters configuration mode.
Step2	interface [ethernet slot/port | ethernet slot/port.number | port-channel number | vlan vlan-id] Example: switch(config)# interface ethernet 7/3	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.
Step3	mac-address mac-address Example: switch(config-if)# mac-address 21ab.47dd.ff89switch(config-if)#	Specified a static MAC address to add to the Layer 3 interface.
Step4	exit Example: switch(config-if)# exitswitch(config)#	Exits the interface mode.
Step5	show interface [ethernet slot/port | ethernet slot/port.number | port-channel number | vlan vlan-id] Example: switch# show interface ethernet 7/3	(Optional) Displays information about the Layer 3 interface.
Step6	copy running-config startup-config Example: switch# copy running-config startup-config	(Optional) Copies the running configuration to the startup configuration.

This example shows how to configure the Layer 3 interface on slot 7, port 3 with a static MAC address:

switch# config tswitch(config)# interface ethernet 7/3switch(config-if)# mac-address 21ab.47dd.ff89switch(config-if)# 

Configuring the Aging Time for the MAC Table

You can configure the amount of time that a MAC address entry (the packet source MAC address and port on which that packet was learned) remains in the MAC table, which contains the Layer 2 information.

Note	You can also configure the MAC aging time in interface configuration mode or VLAN configuration mode.

Before You Begin

Before you configure the aging time for the MAC table, ensure that you are in the correct VDC (or enter the switchto vdc command).

SUMMARY STEPS

1. config t

2. mac address-table aging-time seconds [vlan vlan_id]

3. exit

4. (Optional) show mac address-table aging-time

5. (Optional) copy running-config startup-config

DETAILED STEPS

	Command or Action	Purpose
Step1	config t Example: switch# config tswitch(config)#	Enters configuration mode.
Step2	mac address-table aging-time seconds [vlan vlan_id] Example: switch(config)# mac address-table aging-time 600	Specifies the time before an entry ages out and is discarded from the Layer 2 MAC address table. The range is from 120 to 918000; the default is 1800 seconds. Entering the value 0 disables the MAC aging.
Step3	exit Example: switch(config)# exitswitch#	Exits the configuration mode.
Step4	show mac address-table aging-time Example: switch# show mac address-table aging-time	(Optional) Displays the aging time configuration for MAC address retention.
Step5	copy running-config startup-config Example: switch# copy running-config startup-config	(Optional) Copies the running configuration to the startup configuration.

This example shows how to set the ageout time for entries in the Layer 2 MAC address table to 600 seconds (10 minutes):

switch# config t switch(config)# mac address-table aging-time 600switch(config)#

Checking Consistency of MAC Address Tables

Beginning with Release 4.1(2). you can check the match between the MAC address table on the supervisor and all the modules.

SUMMARY STEPS

1. show forwarding consistency l2 {module_number}

DETAILED STEPS

	Command or Action	Purpose
Step1	show forwarding consistency l2 {module_number} Example: switch# show forwarding consistency l2 7switch#	Displays the discrepant, missing, and extra MAC addresses between the supervisor and the specified module.

This example shows how to display discrepant, missing, and extra entries in the MAC address tables between the supervisor and the specified module:

switch# show forwarding consistency l2 7switch#

Clearing Dynamic Addresses from the MAC Table

You can clear all dynamic Layer 2 entries in the MAC address table.

Before You Begin

Before you clear the dynamic MAC table, ensure that you are in the correct VDC (or enter the switchto vdc command).

SUMMARY STEPS

1. clear mac address-table dynamic {address mac_addr} {interface [ethernet slot/port | loopback number | port-channel channel-number]} {vlan vlan_id}

2. (Optional) show mac address-table

DETAILED STEPS

	Command or Action	Purpose
Step1	clear mac address-table dynamic {address mac_addr} {interface [ethernet slot/port | loopback number | port-channel channel-number]} {vlan vlan_id} Example: switch# clear mac address-table dynamic	Clears the dynamic address entries from the MAC address table in Layer 2.
Step2	show mac address-table Example: switch# show mac address-table	(Optional) Displays the MAC address table.

This example shows how to clear the dynamic entries in the Layer 2 MAC address table:

switch# clear mac address-table dynamicswitch# 

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.
show forwarding consistency l2 {module}	Displays discrepant, missing, and extra MAC addresses between the tables on the module and the supervisor.

For information on the output of these commands, see the Cisco Nexus 7000 Series NX-OS Layer 2 Switching Command Reference.

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 terminalswitch(config)# mac address-table static 0000.0000.1234 vlan 10 interface ethernet 2/15switch(config)# mac address-table aging-time 120

Additional References for Layer 2 Switching -- CLI Version

Related Documents

Related Topic	Document Title
Port security, static MAC addresses	Cisco Nexus 7000 Series NX-OS Security Configuration Guide, Release 5.x
Interfaces	Cisco Nexus 7000 Series NX-OS Interfaces Configuration Guide, Release 5.x
Command reference	Cisco Nexus 7000 Series NX-OS Layer 2 Switching Command Reference
High availability	Cisco Nexus 7000 Series NX-OS High Availability and Redundancy Guide, Release 5.x
VDCs	Cisco Nexus 7000 Series NX-OS Virtual Device Context Configuration Guide, Release 5.x
System management	Cisco Nexus 7000 Series NX-OS System Management Configuration Guide, Release 5.x
Licensing	Cisco NX-OS Licensing Guide
Release Notes	Cisco Nexus 7000 Series NX-OS Release Notes, Release 5.x

Standards

Standards	Title
No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.	—

Feature History for Configuring Layer 2 Switching -- CLI Version

This table lists the release history for this feature.
Table 2Feature History for Configuring Layer 2 Switching

Feature Name	Releases	Feature Information
Layer 3 interface static MAC addresses	4.2(1)	You can configure a Layer 3 interface with a static MAC address.
show mac address-table	4.1(2)	This display provides additional information when vPC is enabled and running.
Layer 2 consistency	4.1(2)	The show forwarding consistency l2 command displays inconsistent entries on the MAC address table between the modules.