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
Configuring Layer 2 Switching
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 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 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.
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:
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.
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.
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
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 BeginSUMMARY STEPSBefore you configure static MAC addresses, ensure that you are in the correct VDC (or enter the switchto vdc command).
2. mac address-table static mac-address vlan vlan-id {[drop | interface {type slot/port} | port-channel number]}
4. (Optional) show mac address-table static
5. (Optional) copy running-config startup-config
DETAILED STEPS
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 1/2Specifies 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 show mac address-table static
Example:switch# show mac address-table static(Optional) Displays the static MAC addresses.
Step 5 copy running-config startup-config
Example:switch# copy running-config startup-config(Optional) Copies the running configuration to the startup configuration.
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 BeginSUMMARY STEPSBefore you configure static MAC addresses, ensure that you are in the correct VDC (or enter the switchto vdc command).
2. interface [ethernet slot/port | ethernet slot/port.number | port-channel number | vlan vlan-id]
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 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 7/3Specifies 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 21ab.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 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.
Step 6 copy running-config startup-config
Example:switch# copy running-config startup-config(Optional) Copies the running configuration to the startup configuration.
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 BeginSUMMARY STEPSBefore you configure the aging time for the MAC table, ensure that you are in the correct VDC (or enter the switchto vdc command).
2. mac address-table aging-time seconds [vlan vlan_id]
4. (Optional) show mac address-table aging-time
5. (Optional) copy running-config startup-config
DETAILED STEPS
Command or Action Purpose Step 1 config t
Example:switch# config t switch(config)#Enters configuration mode.
Step 2 mac address-table aging-time seconds [vlan vlan_id]
Example:switch(config)# mac address-table aging-time 600Specifies 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.
Step 3 exit
Example:switch(config)# exit switch#Exits the configuration mode.
Step 4 show mac address-table aging-time
Example:switch# show mac address-table aging-time(Optional) Displays the aging time configuration for MAC address retention.
Step 5 copy running-config startup-config
Example:switch# copy running-config startup-config(Optional) Copies the running configuration to the startup configuration.
Checking Consistency of MAC Address Tables
SUMMARY STEPSBeginning with Release 4.1(2). you can check the match between the MAC address table on the supervisor and all the modules.
1. show forwarding consistency l2 {module_number}
DETAILED STEPS
Command or Action Purpose Step 1 show forwarding consistency l2 {module_number}
Example:switch# show forwarding consistency l2 7 switch#Displays the discrepant, missing, and extra MAC addresses between the supervisor and the specified module.
Clearing Dynamic Addresses from the MAC Table
Before You BeginSUMMARY STEPSBefore you clear the dynamic MAC table, ensure that you are in the correct VDC (or enter the switchto vdc command).
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 Step 1 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 dynamicClears the dynamic address entries from the MAC address table in Layer 2.
Step 2 show mac address-table
Example:switch# show mac address-table(Optional) Displays the MAC address table.
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.
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
Feature History for Configuring Layer 2 Switching -- CLI Version
This table lists the release history for this feature.
Table 2 Feature History for Configuring Layer 2 SwitchingFeature 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.