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Logical Interfaces

Learn about logical interfaces and how to configure various logical interface properties.

A logical interface (often called a unit) is a sub-interface defined under a physical interface. You can have multiple logical interfaces on one physical port, each with different encapsulation, VLANs, or IP subnets. To enable a physical interface device, configure at least one logical interface and specify its protocol family.

Logical interface properties depend on the Physical Interface Card (PIC) and encapsulation type, and whether the interface supports multicast traffic, data-link connection identifiers (DLCI), virtual channel identifiers (VCI) and virtual path identifiers (VPI), and traffic shaping.

Specify the Logical Interface Number

Each logical interface must have a logical unit number. The logical unit number corresponds to the logical unit part of the interface name. For more information, see Interface Naming.

Point-to-point Protocol (PPP), Cisco High-level Data Link Control (HDLC), and Ethernet circuit cross-connect (CCC) encapsulations support only a single logical interface, whose logical unit number must be 0. Frame Relay and ATM encapsulations support multiple logical interfaces, so you can configure one or more logical unit numbers.

The range of number available for the logical unit number varies for different interface types. See unit (interfaces) for current range values.

Add a Logical Unit Description

You can include a text description of each logical unit in the configuration file. The description can be a single line of text. If the text contains spaces, enclose it in quotation marks.

You can configure the extended DHCP relay to include the interface description in the option 82 Agent Circuit ID suboption. See DHCP Relay Agent Information Option (Option 82).

For information about describing physical interfaces, see Configure the Interface Description.

Interface Bandwidth

The default is Junos OS uses the physical interface speed for the MIB-II object, ifSpeed. You can configure the logical unit to populate the ifSpeed variable by configuring a bandwidth value for the logical interface.

rate is the peak rate, in bits per second (bps) or cells per second (cps). The value can be any positive integer.

  • You can specify a value in bps either as a complete decimal number or as a decimal number followed by the abbreviation k (1000), m (1,000,000), or g (1,000,000,000).

  • You can also specify a value in cps by entering a decimal number followed by the abbreviation c. Values expressed in cps are converted to bps using the formula 1 cps = 384 bps.

The bandwidthstatement sets an informational-only parameter; you cannot adjust the actual bandwidth of an interface with this statement. The bandwidth statement is valid for all logical interfaces except multilink interfaces.

Tip:

We recommend that you be careful when setting this value. Any interface bandwidth value that you configure using the bandwidth statement affects how the interface cost calculation for a dynamic routing protocol, such as OSPF. By default, the interface cost for a dynamic routing protocol is the following formula:

In the formula, bandwidth is the physical interface speed. However, if you specify a value for bandwidth using the bandwidth statement, that value is used to calculate the interface cost rather than the actual physical interface bandwidth.

Configure the Logical Interface

  • To configure logical interface properties:

    • [edit interfaces interface-name]

    • [edit logical-systems logical-system-name interfaces interface-name]

  • To configure one or more logical unit numbers, include the unit statement and specify the logical unit number:

    under the following hierarchy levels:

    • [edit interfaces interface-name]

    • [edit logical-systems logical-system-name interfaces interface-name]

  • To add a text description, include the description statement under the following hierarchy levels:

    • [edit interfaces interface-name unit logical-unit-number]

    • [edit logical-systems logical-system-name interfaces interface-name unit logical-unit-number]

    Display the output of added descriptive text using the show interfaces command. It is also exposed in the ifAlias Management Information Base (MIB) object. It has no impact on the interface’s configuration.

  • To configure the bandwidth value for a logical interface, include the bandwidth statement under the following hierarchy levels:

    • [edit interfaces interface-name unit logical-unit-number]

    • [edit logical-systems logical-system-name interfaces interface-name unit logical-unit-number]

Encapsulation on Logical Interfaces

An encapsulation is used with certain packet types. You can configure an encapsulation on a logical interface.

The following restrictions apply to logical interface encapsulation:

  • With the atm-nlpid, atm-cisco-nlpid, and atm-vc-mux encapsulations, you can configure the inet family only.

  • With the circuit cross-connect (CCC) circuit encapsulations, you cannot configure a family on the logical interface.

  • A logical interface cannot have frame-relay-ccc encapsulation unless the physical device also has frame-relay-ccc encapsulation.

  • A logical interface cannot have frame-relay-tcc encapsulation unless the physical device also has frame-relay-tcc encapsulation. In addition, you must assign this logical interface a data-link connection identifier (DLCI) from 512 through 1022 and configure it as point to point.

  • A logical interface cannot have frame-relay-ether-type or frame-relay-ether-type-tcc encapsulation unless the physical interface has flexible-frame-relay encapsulation and is also on an IQ or IQE PIC.

  • For frame-relay-ether-type-tcc encapsulation, you must assign this logical interface a DLCI from 512 through 1022.

  • For interfaces that carry IP version 6 (IPv6) traffic, you cannot configure ether-over-atm-llc encapsulation.

  • When you use ether-over-atm-llc encapsulation, you cannot configure multipoint interfaces.

  • A logical interface cannot have vlan-ccc or vlan-vpls encapsulation unless the physical device also has vlan-ccc or vlan-vpls encapsulation, respectively. In addition, you must assign this logical interface a VLAN ID from 512 through 1023; if the VLAN ID is 511 or lower, it is subject to the normal destination filter lookups in addition to source address filtering. For more information, see Configure VLAN and Extended VLAN Encapsulation.

  • You can create an ATM cell-relay circuit by configuring an entire ATM physical device or an individual virtual circuit (VC). When you configure an entire device, only cell-relay encapsulation is the only encapsulation type allowed on the logical interfaces. For more information, see Configuring an ATM1 Cell-Relay Circuit Overview.

Configure the Encapsulation on a Logical Interface

Generally, you configure an interface’s encapsulation at the [edit interfaces interface-name] hierarchy level. However, for some encapsulation types, such as Frame Relay, ATM, or Ethernet VLAN encapsulations, you can also configure the encapsulation type that is used inside the Frame Relay, ATM, or VLAN circuit itself.

To configure encapsulation on a logical interface:

  1. In configuration mode, go to the [edit interfaces interface-name unit logical-unit-number] or [edit logical-systems logical-system-name interfaces interface-name unit logical-unit-number] hierarchy level.
  2. Configure the encapsulation type as described in encapsulation (Logical Interface).

Display the Encapsulation on a Logical Interface

Purpose

To display the configured encapsulation and its associated set options on a physical interface when the following are set at the [edit interfaces interface-name] or [edit logical-systems logical-system-name interfaces interface-name] hierarchy level:

  • interface-name—at-1/1/0

  • Encapsulation—atm-ccc-cell-relay

  • Unit—120

Action

Run the show command at the [edit interfaces interface-name] hierarchy level.

Meaning

The configured encapsulation and its associated set options are displayed as expected.

See Also

Configure Interface Encapsulation on Routers

This topic describes how to configure interface encapsulation on Routers. Use the flexible-ethernet-services configuration statement to configure different encapsulation for different logical interfaces under a physical interface. With flexible Ethernet services encapsulation, you can configure each logical interface encapsulation without range restrictions for VLAN IDs.

Supported encapsulations for physical interfaces include:

  • flexible-ethernet-services

  • ethernet-ccc

  • ethernet-tcc

Supported encapsulations for logical interfaces include:

  • ethernet

  • vlan-ccc

  • vlan-tcc

To configure flexible Ethernet services encapsulation, include the encapsulation flexible-ethernet-services statement at the [edit interfaces et-fpc/pic/port] hierarchy level. For example:

Accounting for the Logical Interface

Juniper Networks routers and switches can collect various kinds of data about traffic passing through the router and switch. You can set up one or more accounting profiles that specify some common characteristics of this data, including the following:

  • The fields used in the accounting records

  • The number of files that the router or switch retains before discarding, and the number of bytes per file

  • The polling period that the system uses to record the data

You configure the profiles and define a unique name for each profile using statements at the [edit accounting-options] hierarchy level. There are two types of accounting profiles: interface profiles and filter profiles. You configure interface profiles by including the interface-profile statement at the [edit accounting-options] hierarchy level. You configure filter profiles by including the filter-profile statement at the [edit accounting-options] hierarchy level. For more information, see the Junos OS Network Management Administration Guide for Routing Devices.

You apply filter profiles by including the accounting-profile statement at the [edit firewall filter filter-name] and [edit firewall family family filter filter-name] hierarchy levels. For more information, see the Routing Policies, Firewall Filters, and Traffic Policers User Guide.

Configure Accounting for the Logical Interface

Before you begin

You must configure a profile to collect error and statistic information for input and output packets on a particular logical interface. An accounting profile specifies which statistics are collected and written to a log file. For more information about how to configure an accounting-data log file, see the Configuring Accounting-Data Log Files.

An interface profile specifies the information collected and written to a log file. You can configure a profile to collect error and statistic information for input and output packets on a particular logical interface.

  1. To configure which statistics are collected for an interface, include the fields statement at the [edit accounting-options interface-profile profile-name] hierarchy level.
  2. Each accounting profile logs its statistics to a file in the /var/log directory. To configure which file to use, include the file statement at the [edit accounting-options interface-profile profile-name] hierarchy level.

    You must specify a file statement for the interface profile that has already been configured at the [edit accounting-options] hierarchy level. For more information, see Configuring Accounting-Data Log Files.

  3. Each interface with an accounting profile enabled has statistics collected once per interval time specified for the accounting profile. Statistics collection time is scheduled evenly over the configured interval. To configure the interval, include the interval statement at the [edit accounting-options interface-profile profile-name] hierarchy level.

    The minimum interval allowed is 1 minute. Configuring a low interval in an accounting profile for a large number of interfaces might cause serious performance degradation.

  4. To configure the interfaces on which the accounting needs to be performed, apply the interface profile to a logical interface by including the accounting-profile statement at the [edit interfaces interface-name unit logical-unit-number] hierarchy level.

See Also

Display the Accounting Profile for the Logical Interface

Purpose

Displaying the configured accounting profile of a particular logical interface at the [edit accounting-options interface-profile profile-name] hierarchy level requires that you specify certain parameters:

  • interface-name—ge-1/0/1

  • Logical unit number—1

  • Interface profile —if_profile

  • File name—if_stats

  • Interval—15 minutes

Action

  • Run the show command at the [edit interfaces ge-1/0/1 unit 1] hierarchy level.

  • Run the show command at the [edit accounting-options] hierarchy level.

Meaning

The configured accounting and its associated set options are displayed as expected.

Configure a Point-to-Point Connection

By default, all interfaces are assumed to be point-to-point connections. You must ensure that the maximum transmission unit (MTU) sizes on both sides of the connection are the same.

For all interfaces except aggregated Ethernet, Fast Ethernet, and Gigabit Ethernet, you can explicitly configure an interface to be a point-to-point connection by including the point-to-point statement:

You can include this statement at the following hierarchy levels:

  • [edit interfaces interface-name unit logical-unit-number]

  • [edit logical-systems logical-system-name interfaces interface-name unit logical-unit-number]

Configure a Multipoint Connection

By default, all interfaces are assumed to be point-to-point connections. To configure an interface to be a multipoint connection, include the multipoint statement:

You can include this statement at the following hierarchy levels:

  • [edit interfaces interface-name unit logical-unit-number]

  • [edit logical-systems logical-system-name interfaces interface-name unit logical-unit-number]

Configure Dynamic Profiles for PPP

A dynamic profile acts as a template that enables you to create, update, or remove a configuration that includes attributes for client access (such as, interface or protocol) or service (such as, IGMP). Using dynamic profiles, you can consolidate all of the common attributes of a client (and eventually a group of clients) and apply the attributes simultaneously.

After dynamic profiles are created, the profiles reside in a profile library on the router. You can then use the dynamic-profile statement to attach profiles to interfaces. To assign a dynamic profile to a PPP interface, you can include the dynamic-profile statement at the [edit interfaces interface-name unit logical-unit-number ppp-options] hierarchy level:

To monitor the configuration, issue the show interfaces interface-name command.

For information about dynamic profiles, see Dynamic Profiles Overview in the Junos Subscriber Access Configuration Guide.

For information about creating dynamic profiles, see Configuring a Basic Dynamic Profile in the Junos Subscriber Access Configuration Guide.

For information about assigning a dynamic profile to a PPP interface, see Attaching Dynamic Profiles to Static PPP Subscriber Interfaces in the Junos Subscriber Access Configuration Guide.

For information about using dynamic profiles to authenticate PPP subscribers, see Configuring Dynamic Authentication for PPP Subscribers.

Enable or Disable SNMP Notifications on Logical Interfaces

By default, Simple Network Management Protocol (SNMP) notifications are sent when the state of an interface or a connection changes.

To explicitly enable these notifications on the logical interface, include the traps statement:

To explicitly disable these notifications on the logical interface, include the no-traps statement:

You can include these statements at the following hierarchy levels:

  • [edit interfaces interface-name unit logical-unit-number]

  • [edit logical-systems logical-system-name interfaces interface-name unit logical-unit-number]

Disable a Logical Interface

You can unconfigure a logical interface, effectively disabling that interface, without removing the logical interface configuration statements from the configuration. To unconfigure a logical interface, include the disable statement:

You can include this statement at the following hierarchy levels:

  • [edit interfaces interface-name unit logical-unit-number]

  • [edit logical-systems logical-system-name interfaces interface-name unit logical-unit-number]

When an interface is disabled, a route (pointing to the reserved target “REJECT”) with the IP address of the interface and a 32–bit subnet mask is installed in the routing table. See Routing Protocols.

Example: Disable a Logical Interface

Sample interface configuration:

Disabling the interface:

Verifying the interface configuration:

Platform-Specific disable Behavior

Use Feature Explorer to know if your platform supports this feature.

Platform

Difference

ACX Series routers

 Disabling IRB IFLs is not supported on ACX Series routers. You can deactivate IRB interfaces using [deactivate interfaces irb unit unit] as a workaround to block host-bound traffic to the RE.