Configuring Layer 3 Interfaces

Information About Layer 3 Interfaces

Layer 3 interfaces forward packets to another device using static or dynamic routing protocols. You can use Layer 3 interfaces for IP routing and inter-VLAN routing of Layer 2 traffic.

Routed Interfaces

You can configure a port as a Layer 2 interface or a Layer 3 interface. A routed interface is a physical port that can route IP traffic to another device. A routed interface is a Layer 3 interface only and does not support Layer 2 protocols, such as the Spanning Tree Protocol (STP).

All Ethernet ports are Layer 2 (switchports) by default. You can change this default behavior using the no switchport command from interface configuration mode. To change multiple ports at one time, you can specify a range of interfaces and then apply the no switchport command.

You can assign an IP address to the port, enable routing, and assign routing protocol characteristics to this routed interface.

You can also create a Layer 3 port channel from routed interfaces.

Routed interfaces and subinterfaces support exponentially decayed rate counters. Cisco NX-OS tracks the following statistics with these averaging counters:

  • Input packets/sec

  • Output packets/sec

  • Input bytes/sec

  • Output bytes/sec

Subinterfaces

You can create virtual subinterfaces on a parent interface configured as a Layer 3 interface. A parent interface can be a physical port or a port channel.

Subinterfaces divide the parent interface into two or more virtual interfaces on which you can assign unique Layer 3 parameters such as IP addresses and dynamic routing protocols. The IP address for each subinterface should be in a different subnet from any other subinterface on the parent interface.

You create a subinterface with a name that consists of the parent interface name (for example, Ethernet 2/1) followed by a period and then by a number that is unique for that subinterface. For example, you could create a subinterface for Ethernet interface 2/1 named Ethernet 2/1.1 where .1 indicates the subinterface.

Cisco NX-OS enables subinterfaces when the parent interface is enabled. You can shut down a subinterface independent of shutting down the parent interface. If you shut down the parent interface, Cisco NX-OS shuts down all associated subinterfaces as well.

One use of subinterfaces is to provide unique Layer 3 interfaces to each VLAN that is supported by the parent interface. In this scenario, the parent interface connects to a Layer 2 trunking port on another device. You configure a subinterface and associate the subinterface to a VLAN ID using 802.1Q trunking.

The following figure shows a trunking port from a switch that connects to router B on interface E 2/1. This interface contains three subinterfaces that are associated with each of the three VLANs that are carried by the trunking port.

Figure 1. Subinterfaces for VLANs

VLAN Interfaces

A VLAN interface or a switch virtual interface (SVI) is a virtual routed interface that connects a VLAN on the device to the Layer 3 router engine on the same device. Only one VLAN interface can be associated with a VLAN, but you need to configure a VLAN interface for a VLAN only when you want to route between VLANs or to provide IP host connectivity to the device through a virtual routing and forwarding (VRF) instance that is not the management VRF. When you enable VLAN interface creation, Cisco NX-OS creates a VLAN interface for the default VLAN (VLAN 1) to permit remote switch administration.

You must enable the VLAN network interface feature before you can configure it. The system automatically takes a checkpoint prior to disabling the feature, and you can roll back to this checkpoint. For information about rollbacks and checkpoints, see the System Management Configuration Guide for your device.


Note

You cannot delete the VLAN interface for VLAN 1.

You can route across VLAN interfaces to provide Layer 3 inter-VLAN routing by configuring a VLAN interface for each VLAN that you want to route traffic to and assigning an IP address on the VLAN interface. For more information on IP addresses and IP routing, see the Unicast Routing Configuration Guide for your device.

The following figure shows two hosts connected to two VLANs on a device. You can configure VLAN interfaces for each VLAN that allows Host 1 to communicate with Host 2 using IP routing between the VLANs. VLAN 1 communicates at Layer 3 over VLAN interface 1and VLAN 10 communicates at Layer 3 over VLAN interface 10.

Figure 2. Connecting Two VLANs with VLAN Interfaces

Loopback Interfaces

A loopback interface is a virtual interface with a single endpoint that is always up. Any packet that is transmitted over a loopback interface is immediately received by this interface. Loopback interfaces emulate a physical interface.

You can use loopback interfaces for performance analysis, testing, and local communications. Loopback interfaces can act as a termination address for routing protocol sessions. This loopback configuration allows routing protocol sessions to stay up even if some of the outbound interfaces are down.

Guidelines and Limitations for Layer 3 Interfaces

Layer 3 interfaces have the following configuration guidelines and limitations:

  • If you change a Layer 3 interface to a Layer 2 interface, Cisco NX-OS shuts down the interface, reenables the interface, and removes all configuration specific to Layer 3.

  • If you change a Layer 2 interface to a Layer 3 interface, Cisco NX-OS shuts down the interface, reenables the interface, and deletes all configuration specific to Layer 2.

Default Settings for Layer 3 Interfaces

The default setting for the Layer 3 Admin state is Shut.

Configuring Layer 3 Interfaces

Configuring a Routed Interface

SUMMARY STEPS

  1. switch# configure terminal
  2. switch(config)# interface ethernet slot/port
  3. switch(conifg-if)# no switchport
  4. switch(config-if)# ipip-address/length
  5. (Optional) switch(config-if)# medium {broadcast | p2p}
  6. (Optional) switch(config-if)# show interfaces
  7. (Optional) switch(config-if)# copy running-config startup-config

DETAILED STEPS

  Command or Action Purpose

Step 1

switch# configure terminal

Enters global configuration mode.

Step 2

switch(config)# interface ethernet slot/port

Enters interface configuration mode.

Step 3

switch(conifg-if)# no switchport

Configures the interface as a Layer 3 interface and deletes any configuration specific to Layer 2 on this interface.

Note

 

To convert a Layer 3 interface back into a Layer 2 interface, use the switchport command.

Step 4

switch(config-if)# ipip-address/length

Configures an IP address for this interface.

Step 5

(Optional) switch(config-if)# medium {broadcast | p2p}

(Optional)

Configures the interface medium as either point to point or broadcast.

Note

 

The default setting is broadcast, and this setting does not appear in any of the show commands. However, if you do change the setting to p2p , you will see this setting when you enter the show running-config command.

Step 6

(Optional) switch(config-if)# show interfaces

(Optional)

Displays the Layer 3 interface statistics.

Step 7

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

(Optional)

Saves the change persistently through reboots and restarts by copying the running configuration to the startup configuration.

Example

This example shows how to configure an IPv4-routed Layer 3 interface:


switch# configure terminal
switch(config)# interface ethernet 2/1
switch(config-if)# no switchport
switch(config-if)# ip address 192.0.2.1/8
switch(config-if)# copy running-config startup-config

Configuring a Subinterface

Before you begin

  • Configure the parent interface as a routed interface.

  • Create the port-channel interface if you want to create a subinterface on that port channel.

SUMMARY STEPS

  1. (Optional) switch(config-if)# copy running-config startup-config
  2. switch(config)# interface ethernet slot/port.number
  3. switch(config-if)# ip address ip-address/length
  4. switch(config-if)# encapsulation dot1Q vlan-id
  5. (Optional) switch(config-if)# show interfaces
  6. (Optional) switch(config-if)# copy running-config startup-config

DETAILED STEPS

  Command or Action Purpose

Step 1

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

(Optional)

Saves the change persistently through reboots and restarts by copying the running configuration to the startup configuration.

Step 2

switch(config)# interface ethernet slot/port.number

Enters interface configuration mode. The range for the slot is from 1 to 255. The range for the port is from 1 to 128.

Step 3

switch(config-if)# ip address ip-address/length

Configures an IP address for this interface.

Step 4

switch(config-if)# encapsulation dot1Q vlan-id

Configures IEEE 802.1Q VLAN encapsulation on the subinterface. The range for the vlan-id is from 2 to 4093.

Step 5

(Optional) switch(config-if)# show interfaces

(Optional)

Displays the Layer 3 interface statistics.

Step 6

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

(Optional)

Saves the change persistently through reboots and restarts by copying the running configuration to the startup configuration.

Example

This example shows how to create a subinterface:


switch# configure terminal
switch(config)# interface ethernet 2/1
switch(config-if)# ip address 192.0.2.1/8
switch(config-if)# encapsulation dot1Q 33
switch(config-if)# copy running-config startup-config

Configuring the Bandwidth on an Interface

You can configure the bandwidth for a routed interface, port channel, or subinterface.

SUMMARY STEPS

  1. switch# configure terminal
  2. switch(config)# interface ethernet slot/port
  3. switch(conifg-if)# bandwidth [value | inherit [value]]
  4. (Optional) switch(config-if)# copy running-config startup-config

DETAILED STEPS

  Command or Action Purpose

Step 1

switch# configure terminal

Enters global configuration mode.

Step 2

switch(config)# interface ethernet slot/port

Enters interface configuration mode. The range for the slot is from 1 to 255. The range for the port is from 1 to 128.

Step 3

switch(conifg-if)# bandwidth [value | inherit [value]]

Configures the bandwidth parameter for a routed interface, port channel, or subinterface, as follows:

  • value—Size of the bandwidth in kilobytes. The range is from 1 to 10000000.

  • inherit —Indicates that all subinterfaces of this interface inherit either the bandwidth value (if a value is specified) or the bandwidth of the parent interface (if a value is not specified).

Step 4

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

(Optional)

Saves the change persistently through reboots and restarts by copying the running configuration to the startup configuration.

Example

This example shows how to configure Ethernet interface 2/1 with a bandwidth value of 80000:


switch# configure terminal
switch(config)# interface ethernet 2/1
switch(config-if)# bandwidth 80000
switch(config-if)# copy running-config startup-config

Configuring a VLAN Interface

SUMMARY STEPS

  1. switch# configure terminal
  2. switch(config)# feature interface-vlan
  3. switch(config)# interface vlan number
  4. switch(config-if)# ip address ip-address/length
  5. switch(config-if)# no shutdown
  6. (Optional) switch(config-if)# show interface vlan number
  7. (Optional) switch(config-if)# copy running-config startup-config

DETAILED STEPS

  Command or Action Purpose

Step 1

switch# configure terminal

Enters global configuration mode.

Step 2

switch(config)# feature interface-vlan

Enables VLAN interface mode.

Step 3

switch(config)# interface vlan number

Creates a VLAN interface. The number range is from 1 to 4094.

Step 4

switch(config-if)# ip address ip-address/length

Configures an IP address for this interface.

Step 5

switch(config-if)# no shutdown

Brings the interface up administratively.

Step 6

(Optional) switch(config-if)# show interface vlan number

(Optional)

Displays the VLAN interface statistics. The number range is from 1 to 4094.

Step 7

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

(Optional)

Saves the change persistently through reboots and restarts by copying the running configuration to the startup configuration.

Example

This example shows how to create a VLAN interface:


switch# configure terminal
switch(config)# feature interface-vlan
switch(config)# interface vlan 10
switch(config-if)# ip address 192.0.2.1/8
switch(config-if)# copy running-config startup-config

Configuring a Loopback Interface

Before you begin

Ensure that the IP address of the loopback interface is unique across all routers on the network.

SUMMARY STEPS

  1. switch# configure terminal
  2. switch(config)# interface loopback instance
  3. switch(config-if)# ip address ip-address/length
  4. (Optional) switch(config-if)# show interface loopback instance
  5. (Optional) switch(config-if)# copy running-config startup-config

DETAILED STEPS

  Command or Action Purpose

Step 1

switch# configure terminal

Enters global configuration mode.

Step 2

switch(config)# interface loopback instance

Creates a loopback interface. The instance range is from 0 to 1023.

Step 3

switch(config-if)# ip address ip-address/length

Configures an IP address for this interface.

Step 4

(Optional) switch(config-if)# show interface loopback instance

(Optional)

Displays the loopback interface statistics. The instance range is from 0 to 1023.

Step 5

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

(Optional)

Saves the change persistently through reboots and restarts by copying the running configuration to the startup configuration.

Example

This example shows how to create a loopback interface:


switch# configure terminal
switch(config)# interface loopback 0
switch(config-if)# ip address 192.0.2.100/8
switch(config-if)# copy running-config startup-config

Assigning an Interface to a VRF

Before you begin

Assign the IP address for a tunnel interface after you have configured the interface for a VRF.

SUMMARY STEPS

  1. switch# configure terminal
  2. switch(config)# interface interface-typenumber
  3. switch(conifg-if)#vrf member vrf-name
  4. switch(config-if)# ipip-address/length
  5. (Optional) switch(config-if)# show vrf [vrf-name] interface interface-type number
  6. (Optional) switch(config-if)# show interfaces
  7. (Optional) switch(config-if)# copy running-config startup-config

DETAILED STEPS

  Command or Action Purpose

Step 1

switch# configure terminal

Enters global configuration mode.

Step 2

switch(config)# interface interface-typenumber

Enters interface configuration mode.

Step 3

switch(conifg-if)#vrf member vrf-name

Adds this interface to a VRF.

Step 4

switch(config-if)# ipip-address/length

Configures an IP address for this interface. You must do this step after you assign this interface to a VRF.

Step 5

(Optional) switch(config-if)# show vrf [vrf-name] interface interface-type number

(Optional)

Displays VRF information.

Step 6

(Optional) switch(config-if)# show interfaces

(Optional)

Displays the Layer 3 interface statistics.

Step 7

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

(Optional)

Saves the change persistently through reboots and restarts by copying the running configuration to the startup configuration.

Example

This example shows how to add a Layer 3 interface to the VRF: 

switch# configure terminal 
switch(config)# interface loopback 0 
switch(config-if)# vrf member RemoteOfficeVRF 
switch(config-if)# ip address 209.0.2.1/16 
switch(config-if)# copy running-config startup-config

Verifying the Layer 3 Interfaces Configuration

Use one of the following commands to verify the configuration:

Command

Purpose

show interface ethernet slot/port

Displays the Layer 3 interface configuration, status, and counters (including the 5-minute exponentially decayed moving average of inbound and outbound packet and byte rates).

show interface ethernet slot/port brief

Displays the Layer 3 interface operational status.

show interface ethernet slot/port capabilities

Displays the Layer 3 interface capabilities, including port type, speed, and duplex.

show interface ethernet slot/port description

Displays the Layer 3 interface description.

show interface ethernet slot/port status

Displays the Layer 3 interface administrative status, port mode, speed, and duplex.

show interface ethernet slot/port.number

Displays the subinterface configuration, status, and counters (including the f-minute exponentially decayed moving average of inbound and outbound packet and byte rates).

show interface port-channel channel-id.number

Displays the port-channel subinterface configuration, status, and counters (including the 5-minute exponentially decayed moving average of inbound and outbound packet and byte rates).

show interface loopback number

Displays the loopback interface configuration, status, and counters.

show interface loopback number brief

Displays the loopback interface operational status.

show interface loopback number description

Displays the loopback interface description.

show interface loopback number status

Displays the loopback interface administrative status and protocol status.

show interface vlan number

Displays the VLAN interface configuration, status, and counters.

show interface vlan number brief

Displays the VLAN interface operational status.

show interface vlan number description

Displays the VLAN interface description.

show interface vlan number status

Displays the VLAN interface administrative status and protocol status.

Monitoring Layer 3 Interfaces

Use one of the following commands to display statistics about the feature:

Command

Purpose

show interface ethernet slot/port counters

Displays the Layer 3 interface statistics (unicast, multicast, and broadcast).

show interface ethernet slot/port counters brief

Displays the Layer 3 interface input and output counters.

show interface ethernet slot/port counters detailed [all]

Displays the Layer 3 interface statistics. You can optionally include all 32-bit and 64-bit packet and byte counters (including errors).

show interface ethernet slot/port counters error

Displays the Layer 3 interface input and output errors.

show interface ethernet slot/port counters snmp

Displays the Layer 3 interface counters reported by SNMP MIBs. You cannot clear these counters.

show interface ethernet slot/port.number counters

Displays the subinterface statistics (unicast, multicast, and broadcast).

show interface port-channel channel-id.number counters

Displays the port-channel subinterface statistics (unicast, multicast, and broadcast).

show interface loopback number counters

Displays the loopback interface input and output counters (unicast, multicast, and broadcast).

show interface loopback number counters detailed [all]

Displays the loopback interface statistics. You can optionally include all 32-bit and 64-bit packet and byte counters (including errors).

show interface loopback number counters errors

Displays the loopback interface input and output errors.

show interface vlan number counters

Displays the VLAN interface input and output counters (unicast, multicast, and broadcast).

show interface vlan number counters detailed [all]

Displays the VLAN interface statistics. You can optionally include all Layer 3 packet and byte counters (unicast and multicast).

show interface vlan counters snmp

Displays the VLAN interface counters reported by SNMP MIBs. You cannot clear these counters.

Configuration Examples for Layer 3 Interfaces

This example shows how to configure Ethernet subinterfaces: 

switch# configuration terminal
switch(config)# interface ethernet 2/1.10
switch(config-if)# description Layer 3 for VLAN 10
switch(config-if)# encapsulation dot1q 10
switch(config-if)# ip address 192.0.2.1/8
switch(config-if)# copy running-config startup-config
This example shows how to configure a VLAN interface: 

switch# configuration terminal
switch(config)# interface vlan 100
switch(config-if)# no switchport
switch(config-if)# ip address 192.0.2.1/8
switch(config-if)# copy running-config startup-config
This example shows how to configure a loopback interface: 

switch# configuration terminal
switch(config)# interface loopback 3
switch(config-if)# no switchport
switch(config-if)# ip address 192.0.2.2/32
switch(config-if)# copy running-config startup-config

Related Documents for Layer 3 Interfaces

Related Topics Document Title

Command syntax

Cisco Nexus 3548 Switch NX-OS Interfaces Command Reference

IP

“Configuring IP” chapter in the Cisco Nexus 3548 Switch NX-OS Unicast Routing Configuration Guide

VLAN

“Configuring VLANs” chapter in the Cisco Nexus 3548 Switch NX-OS Layer 2 Switching Configuration Guide

Standards for Layer 3 Interfaces

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