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.
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 assign a static MAC address to a Layer 3 interface. The default MAC address for a Layer 3 interface is the MAC address of the virtual device context (VDC) that is associated with it. You can change the default MAC address of the Layer 3 interface by using the mac-address command from the interface configuration mode. A static MAC address can be configured on SVI, Layer 3 interfaces, port channels, Layer 3 subinterfaces, and tunnel interfaces. You can also configure static MAC addresses on a range of ports and port channels. However, all ports must be in Layer 3. Even if one port in the range of ports is in Layer 2, the command is rejected and an error message appears. For information on configuring MAC addresses, see the Layer 2 Switching Configuration Guide for your device.
You can also create a Layer 3 port channel from routed interfaces.
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.
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.
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.
Changing VRF Membership for an Interface
When you enter the vrf member command under an interface, you receive an alert regarding the deletion of interface configurations and to notify the clients/listeners (such as CLI-Server) to delete configurations with respect to the interface.
Entering the system vrf-member-change retain-l3-config command enables the retention of the Layer 3 configuration when the VRF member changes on the interface. It does this by sending notification to the clients/listeners to store (buffer) the existing configurations, delete the configurations from the old vrf context, and reapply the stored configurations under the new VRF context.
When the system vrf-member-change retain-l3-config command is enabled, the Layer 3 configuration is not deleted and remains stored (buffered). When this command is not enabled (default mode), the Layer 3 configuration is not retained when the VRF member changes.
You can disable the retention of the Layer 3 configuration with the no system vrf-member-change retain-l3-config command. In this mode, the Layer 3 configuration is not retained when the VRF member changes.
Notes About Changing VRF Membership for an Interface
Momentary traffic loss may occur when changing the VRF name.
Only the configurations under the interface level are processed when the system vrf-member-change retain-l3-config command is enabled. You must manually process any configurations at the router level to accommodate routing protocols after a VRF change.
The system vrf-member-change retain-l3-config command supports interface level configurations with:
Layer 3 configurations maintained by the CLI Server, such as ip address and ipv6 address (secondary) and all OSPF/ISIS/EIGRP CLIs available under the interface configuration.
DHCP Relay Agent CLIs, such as ip dhcp relay address [use-vrf] and ipv6 dhcp relay address [use-vrf] .
As a best practice, the client and server interface VRF should be changed one at a time. Otherwise, the DHCP packets cannot be exchanged on the relay agent.
When the client and server are in different VRFs, use the ip dhcp relay address [use-vrf] command to exchange the DHCP packets in the relay agent over the different VRFs.
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.
The IP unnumbered feature enables the processing of IP packets on a point to point (p2p) interface without explicitly configuring a unique IP address on it. This approach borrows an IP address from another interface and conserves address space on point to point links.
Any interface which conforms to the point to point mode can be used as an IP unnumbered interface. For 7.0(3)I3(1) and later, the IP unnumbered feature is supported only on Ethernet interfaces and sub-interfaces. The borrowed interface can only be a loopback interface and is known as the numbered interface.
A loopback interface is ideal as a numbered interface in that it is always functionally up. However, because loopback interfaces are local to a switch/router, the reachability of unnumbered interfaces first needs to be established through static routes or by using an interior gateway protocol, such as OSPF or ISIS.
Staring from 7.0(3)I5(1), IP unnumbered feature is supported on port channel interfaces and sub-interfaces. The borrowed interface can only be a loopback interface and is known as the numbered interface.
Cisco NX-OS supports tunnel interfaces as IP tunnels. IP tunnels can encapsulate a same- layer or higher layer protocol and transport the result over IP through a tunnel that is created between two routers.