Table Of Contents

interface fastethernet

interface group-async

interface port-channel

interface pos

interface vg-anylan

international bit

invert data

invert rxclock

invert-transmit-clock

invert txclock

keepalive

lex burned-in-address

lex input-address-list

lex input-type-list

lex priority-group

lex retry-count

lex timeout

linecode

line-termination

link-test

local-lnm

logging event

loopback (interface)

loopback (E3/T3 interface)

loopback (T1 interface)

loopback (T3 controller)

loopback applique

loopback dte

loopback line

interface fastethernet

To select a particular Fast Ethernet interface for configuration, use the interface fastethernet global configuration command.

Cisco 4500 and 4700 series

interface fastethernet number

Cisco 7200 series

interface fastethernet slot/port

Cisco 7500 series

interface fastethernet slot/port-adapter/port

Syntax Description

number	Port, connector, or interface card number. On a Cisco 4500 or 4700 series routers, specifies the NIM or NPM number. The numbers are assigned at the factory at the time of installation or when added to a system.
slot	Number of the slot being configured. Refer to the appropriate hardware manual for slot and port information.
port	Number of the port being configured. Refer to the appropriate hardware manual for slot and port information.
port-adapter	Number of the port-adapter being configured. Refer to the appropriate hardware manual for information about port adapter compatibility.

Defaults

No default behavior or values.

Command Modes

Global configuration

Command History

Release	Modification
11.2	This command was introduced.
11.3	Default encapsulation type changed to ARPA.

Usage Guidelines

There is not a no form of this command.

Examples

The following example configures Fast Ethernet interface 0 for standard ARPA encapsulation (the default setting) on a Cisco 4500 or 4700 series routers:

Router(config)# interface fastethernet 0

Related Commands

Command	Description
show interfaces fastethernet	Displays information about the Fast Ethernet interfaces.

interface group-async

To create a group interface that will serve as master, to which asynchronous interfaces can be associated as members, use the interface group-async global configuration command. To restore the default, use the no form of this command.

interface group-async unit-number

no interface group-async unit-number

Syntax Description

unit-number

Number of the asynchronous group interface being created.

Defaults

No interfaces are designated as group masters.

Command Modes

Global configuration

Usage Guidelines

Using the interface group-async command, you create a single asynchronous interface to which other interfaces are associated as members using the group-range command. This one-to-many configuration allows you to configure all associated member interfaces by entering one command on the group master interface, rather than entering this command on each individual interface. You can create multiple group masters on a device; however, each member interface can only be associated with one group.

Examples

The following example defines asynchronous group master interface 0:

Router(config)# interface group-async 0

Related Commands

Command	Description
group-range	Creates a list of member asynchronous interfaces (associated with a group interface).
member	Alters the configuration of an asynchronous interface that is a member of a group.

interface port-channel

To specify a Fast EtherChannel and enter interface configuration mode, use the interface port-channel global configuration command.

interface port-channel channel-number

Syntax Description

channel-number

Channel number assigned to this port-channel interface. Range is 1 to 4.

Defaults

No Fast EtherChannel is specified.

Command Modes

Global configuration

Command History

Release	Modification
11.1 CA	This command was introduced.

Usage Guidelines

The Fast EtherChannel feature allows multiple Fast Ethernet point-to-point links to be bundled into one logical link to provide bidirectional bandwidth of up to 800 Mbps. Fast EthernetChannel can be configured between networking devices that support FastEtherChannel capabilities.

You can configure the port-channel interface as you would do to any Fast Ethernet interface.

After you create a port-channel interface, you assign Fast Ethernet interfaces (up to four) to it. For information on how to assign a Fast Ethernet interface to a port-channel interface, refer to the channel-group interface configuration command.

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Caution The port-channel interface is the routed interface. Do not enable Layer 3 addresses on the physical Fast Ethernet interfaces. Do not assign bridge groups on the physical Fast Ethernet interfaces because it creates loops. Also, you must disable spanning tree.

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Caution With Release 11.1(20)CC, the Fast EtherChannel supports CEF/dCEF. We recommend that you clear all explicit ip route-cache distributed commands from the Fast Ethernet interfaces before enabling dCEF on the port-channel interface. Doing this gives the port-channel interface proper control of its physical Fast Ethernet links. When you enable CEF/dCEF globally, all interfaces that support CEF/dCEF are enabled. When CEF/dCEF is enabled on the port-channel interface, it is automatically enabled on each of the Fast Ethernet interfaces in the channel group. However, if you have previously disabled CEF/dCEF on the Fast Ethernet interface, CEF/dCEF is not automatically enabled. In this case, you must enable CEF/dCEF on the Fast Ethernet interface.

---

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Note As you work with the interface port-channel command, consider the following points:

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•If you configure ISL, you must assign the IP address to the subinterface (for example, interface port-channel 1.1—an IP address per VLAN) and you must specify the encapsulation with VLAN number under that subinterface (for example, encapsulation isl 100) for ISL to work.

•Currently, if you want to use the Cisco Discovery Protocol (CDP), you must configure it only on the port-channel interface and not on the physical Fast Ethernet interface.

•If you do not assign a static MAC address on the port-channel interface, the Cisco IOS software automatically assigns a MAC address. If you assign a static MAC address and then later remove it, the Cisco IOS software automatically assigns a MAC address.

There is not a no form of this command.

Examples

The following example creates a port-channel interface with a channel group number of 1 and adds 3 Fast Ethernet interfaces to port-channel 1:

Router(config)# interface port-channel 1

Router(config-if)# ip address 1.1.1.10 255.255.255.0

Router(config)# interface fastethernet 1/0/0

Router(config-if)# channel-group 1

Router(config)# interface fastethernet 4/0/0

Router(config-if)# channel-group 1

Router(config)# interface fastethernet 5/0/0

Router(config-if)# channel-group 1

Related Commands

Command	Description
channel-group	Defines the timeslots that belong to each T1 or E1 circuit.
show interfaces port-channel	Displays the information about the Fast EtherChannel on Cisco 7500 series routers and Cisco 7000 series routers with the RSP7000 and RSP7000CI.

interface pos

To specify the Packet OC-3 interface on the Packet-over-SONET (POS) interface processor and enter interface configuration mode, use the interface pos global configuration command.

Cisco 7000 and Cisco 7500 series routers with VIPs

interface pos slot/port-adapter/port

Cisco 7200 series routers

interface pos slot/port

Syntax Description

slot	Specifies the backplane slot number.
port	On Cisco 7000 series and Cisco 7500 series routers, specifies the ports on a VIP card. The value must be 0.
port-adapter	Port number on the interface. The value must be 0.

Defaults

No POS interface is specified.

Command Modes

Global configuration

Command History

Release	Modification
11.2	This command was introduced.

Usage Guidelines

There is not a no form of this command.

Examples

The following example specifies the single Packet OC-3 interface on the POS OC-3 port adapter in slot 2:

Router(config)# interface pos 2/0

interface vg-anylan

To specify the interface on a 100VG-AnyLAN port adapter and enter interface configuration mode on Cisco 7200 series routers and Cisco 7500 series routers, use the interface vg-anylan global configuration command.

Cisco 7500 series routers with VIPs

interface vg-anylan slot/port-adapter/port

Cisco 7200 series routers

interface vg-anylan slot/port

Syntax Description

slot	Number of the slot being configured. Refer to the appropriate hardware manual for slot and port information.
port	Number of the port being configured. Refer to the appropriate hardware manual for slot and port information.
port-adapter	Number of the port-adapter being configured. Refer to the appropriate hardware manual for information about port adapter compatibility.

Defaults

No interfaces are specified.

Command Modes

Global configuration

Command History

Release	Modification
11.3	This command was introduced.

Usage Guidelines

The 100VG-AnyLAN port adapter provides a single interface port that is compatible with and specified by IEEE 802.12. The 100VG-AnyLAN port adapter provides 100 Mbps over Category 3 or Category 5 unshielded twisted-pair (UTP) cable with RJ-45 terminators, and supports IEEE 802.3 Ethernet packets.

You configure the 100VG-AnyLAN port adapter as you would any Ethernet or Fast Ethernet interface. The 100VG-AnyLAN port adapter can be monitored with the IEEE 802.12 Interface MIB.

There is not a no form of this command.

Examples

The following example specifies the 100VG-AnyLAN port adapter in the first port adapter in slot 1:

Router(config)# interface vg-anylan 1/0/0

Related Commands

Command	Description
framing	Selects the frame type for the T1 or E1 data line.
show interfaces vg-anylan	Displays the information about the 100VG-AnyLAN port adapter on Cisco 7200 series routers and Cisco 7500 series routers.

international bit

To set the E3 international bit in the G.751 frame used by the PA-E3 port adapter, use the international bit interface configuration command. To return to the default international bit, use the no form of this command.

international bit {0 | 1} {0 | 1}

no international bit

Syntax Description

0 | 1	Specifies the value of the first international bit in the G.751 frame. The default is 0.
0 | 1	Specifies the value of the second international bit in the G.751 frame. The default is 0.

Defaults

The default value for each bit is 0.

Command Modes

Interface configuration

Command History

Release	Modification
11.1 CA	This command was introduced.

Usage Guidelines

The international bit command sets bits 6 and 8, respectively, of set II in the E3 frame.

To verify the international bit configured on the interface, use the show controller serial EXEC command.

Examples

The following example sets the international bit to 1 1 on the PA-E3 port adapter in slot 1, port-adapter slot 0, interface 0:

Router(config)# interface serial 1/0/0

Router(config-if)#  international bit 1 1

Related Commands

Command	Description
national bit	Sets the E3 national bit in the G.751 frame used by the PA-E3 port adapter.
show controllers serial	Displays information that is specific to the interface hardware.

invert data

To invert the data stream, use the invert data interface configuration command. This command applies only to the Cisco 7000 series routers with the RSP7000 and RSP7000CI, Cisco 7200 series routers, and Cisco 7500 series routers. To disable inverting the data stream, se the no form of this command.

invert data

no invert data

Syntax Description

This command has no arguments or keywords.

Defaults

Data is not inverted.

Command Modes

Interface configuration

Command History

Release	Modification
11.1 CA and 11.2 P	This command was introduced.

Usage Guidelines

T1 Line without B8ZS Encoding

If the interface on the PA-8T and PA-4T+ synchronous serial port adapters and the PA-T3 and PA-2T3 synchronous serial port adapters is used to drive a dedicated T1 line that does not have B8ZS encoding (a method to avoid 15 zeros), the data stream must be inverted (both TXD and RXD) either in the connecting CSU/DSU or the interface.

Inverting is a method of avoiding excessive zeroes that is superseded by the use of B8ZS encryption. This option could be needed for use with legacy equipment that supports this option. By inverting the HDLC data stream, the HDLC zero insertion algorithm becomes a ones insertion algorithm that satisfies the T1 requirements. Be careful not to invert data both on the interface and on the CSU/DSU as two data inversions will cancel each other out.

AMI Line Coding

If the interface on the CT3IP uses AMI line coding, you must also invert the data on the T1 channel. For more information, see the t1 linecode controller configuration command.

Examples

The following example inverts data on serial interface 3/1/0:

Router(config)# interface serial 3/1/0

Router(config-if)#  invert data

Related Commands

Command	Description
t1 linecode	Specifies the type of linecoding used by the T1 channels on the CT3IP in Cisco 7500 series routers.

invert rxclock

To configure UIO serial port 0 or 1 on the Cisco MC3810 when the cable connected is DCE type, use the invert rxclock interface configuration command. The command inverts the phase of the RX clock on the UIO serial interface, which does not use the T1/E1 interface. To disable the phase inversion, use the no form of this command.

invert rxclock

no invert rxclock

Syntax Description

This command has no arguments or keywords.

Defaults

Receive clock signal is not inverted.

Command Modes

Interface configuration

Command History

Release	Modification
11.3 MA	This command was introduced.

Examples

The following example inverts the clock signal on serial interface 1:

Router(config)# interface serial 1

Router(config-if)#  invert rxclock

invert-transmit-clock

The invert txclock command replaces this command. Refer to the description of invert txclock for information on the transmit clock signal.

invert txclock

To invert the transmit (TX) clock signal, use the invert txclock interface configuration command. To return the TX clock signal to its initial state, use the no form of this command.

invert txclock

no invert txclock

Syntax Description

This command has no arguments or keywords.

Defaults

Transmit clock signal is not inverted.

Command Modes

Interface configuration

Command History

Release	Modification
10.0	This command was introduced.
11.3	The invert-transmit-clock command was replaced by the invert txclock command.

Usage Guidelines

Delays between the SCTE clock and data transmission indicate that the transmit clock signal might not be appropriate for the interface rate and length of cable being used. Different ends of the wire can have variances that differ slightly. The invert txclock command compensates for these variances. This command replaces the invert-transmit-clock command.

Systems that use long cables or cables that are not transmitting the TxC signal (transmit echoed clock line, also known as TXCE or SCTE clock) can experience high error rates when operating at the higher transmission speeds. For example, if a PA-8T synchronous serial port adapter is reporting a high number of error packets, a phase shift might be the problem. Inverting the clock might correct this shift.

When a PA-8T or PA-4T+ port adapter interface is DTE, the invert txclock command inverts the TxC signal it received from the remote DCE. When the PA-8T or PA-4T+ port adapter interface is DCE, this command changes the signal back to its original phase.

Examples

The following example inverts the clock signal on serial interface 3/0:

Router(config)# interface serial 3/0

Router(config-if)# invert txclock

keepalive

To set the keepalive timer for a specific interface, use the keepalive interface configuration command. To turn off keepalives entirely, use the no form of this command.

keepalive [seconds]

no keepalive [seconds]

Syntax Description

seconds

(Optional) Integer value greater than 0. The default is 10.

Defaults

10 seconds

Command Modes

Interface configuration

Command History

Release	Modification
10.0	This command was introduced.

Usage Guidelines

You can configure the keepalive interval, which is the frequency at which the Cisco IOS software sends messages to itself (Ethernet and Token Ring) or to the other end (serial), to ensure a network interface is alive. The interval in previous software versions was 10 seconds; it is now adjustable in 1-second increments down to 1 second. An interface is declared down after three update intervals have passed without receiving a keepalive packet.

Setting the keepalive timer to a low value is very useful for rapidly detecting Ethernet interface failures (transceiver cable disconnecting, cable unterminated, and so on).

A typical serial line failure involves losing Carrier Detect (CD) signal. Because this sort of failure is typically noticed within a few milliseconds, adjusting the keepalive timer for quicker routing recovery is generally not useful.

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Note When adjusting the keepalive timer for a very low bandwidth serial interface, large datagrams can delay the smaller keepalive packets long enough to cause the line protocol to go down. You may need to experiment to determine the best value.

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Examples

The following example sets the keepalive interval to 3 seconds:

Router(config)# interface ethernet 0

Router(config-if)#  keepalive 3

lex burned-in-address

To set the burned-in MAC address for a LAN Extender interface, use the lex burned-in-address interface configuration command. To clear the burned-in MAC address, use the no form of this command.

lex burned-in-address ieee-address

no lex burned-in-address

Syntax Description

ieee-address

48-bit IEEE MAC address written as a dotted triplet of four-digit hexadecimal numbers.

Defaults

No burned-in MAC address is set.

Command Modes

Interface configuration

Command History

Release	Modification
10.3	This command was introduced.

Usage Guidelines

Use this command only on a LAN Extender interface that is not currently active (not bound to a serial interface).

Examples

The following example sets the burned-in MAC address on LAN Extender interface 0:

Router(config)# interface serial 4

Router(config-if)#  encapsulation ppp

Router(config)# interface lex 0

Router(config-if)#  lex burned-in-address 0000.0c00.0001

Router(config-if)#  ip address 131.108.172.21 255.255.255.0

lex input-address-list

To assign an access list that filters on MAC addresses, use the lex input-address-list interface configuration command. To remove an access list from the interface, use the no form of this command.

lex input-address-list access-list-number

no lex input-address-list

Syntax Description

access-list-number

Number of the access list assigned with the access-list global configuration command. It can be a number from 700 to 799.

Defaults

No access lists are preassigned to a LAN Extender interface.

Command Modes

Interface configuration

Command History

Release	Modification
10.0	This command was introduced.

Usage Guidelines

Use the lex input-address-list command to filter the packets that are allowed to pass from the LAN Extender to the core router. The access list filters packets based on the source MAC address.

The LAN Extender interface does not process MAC-address masks. Therefore, you should omit the mask from the access-list commands.

For LAN Extender interfaces, an implicit permit everything entry is automatically defined at the end of an access list. Note that this default differs from other access lists, which have an implicit deny everything entry at the end of each access list.

Examples

The following example applies access list 710 to LAN Extender interface 0. This access list denies all packets from MAC address 0800.0214.2776 and permits all other packets.

Router(config-if)#  access-list 710 deny 0800.0214.2776

Router(config)# interface lex 0

Router(config-if)#  lex input-address-list 710

Related Commands

Command	Description
access-list	Configures the access list mechanism for filtering frames by protocol type or vendor code.

lex input-type-list

To assign an access list that filters Ethernet packets by type code, use the lex input-type-list interface configuration command. To remove an access list from the interface, use the no form of this command.

lex input-type-list access-list-number

no lex input-type-list

Syntax Description

access-list-number

Number of the access list you assigned with the access-list command. It can be a number in the range 200 to 299.

Defaults

No access lists are preassigned to a LAN Extender interface.

Command Modes

Interface configuration

Command History

Release	Modification
10.3	This command was introduced.

Usage Guidelines

Filtering is done on the LAN Extender chassis.

The LAN Extender interface does not process masks. Therefore, you should omit the mask from the access-list commands.

For LAN Extender interfaces, an implicit permit everything entry is automatically defined at the end of an access list. Note that this default differs from other access lists, which have an implicit deny everything entry at the end of each access list.

Examples

The following example applies access list 220 to LAN Extender interface 0. This access list denies all AppleTalk packets (packets with a type field of 0x809B) and permits all other packets.

Router(config-if)#  access-list 220 deny 0x809B 0x0000

Router(config)# interface lex 0

Router(config-if)#  lex input-type-list 220

Related Commands

Command	Description
access-list	Configures the access list mechanism for filtering frames by protocol type or vendor code.

lex priority-group

To activate priority output queueing on the LAN Extender, use the lex priority-group interface configuration command. To disable priority output queueing, use the no form of this command.

lex priority-group group

no lex priority-group

Syntax Description

group

Number of the priority group. It can be a number in the range 1 to 10.

Defaults

Disabled

Command Modes

Interface configuration

Command History

Release	Modification
10.3	This command was introduced.

Usage Guidelines

To define queueing priorities, use the priority-list protocol global configuration command. Note that you can use only the following forms of this command:

priority-list list protocol protocol {high | medium | normal | low}

priority-list list protocol bridge {high | medium | normal | low} list list-number

If you specify a protocol that does not have an assigned Ethernet type code, such as x25, stun, or pad, it is ignored and will not participate in priority output queueing.

Examples

The following example activates priority output queueing on LAN Extender interface 0:

Router(config-if)# priority-list 5 protocol bridge medium list 701

Router(config-if)# lex interface 0

Router(config-if)# lex priority-group 5

Related Commands

Command	Description
priority-list protocol	Establishes queueing priorities based on the protocol type.

lex retry-count

To define the number of times to resend commands to the LAN Extender chassis, use the lex retry-count interface configuration command. To return to the default value, use the no form of this command.

lex retry-count number

no lex retry-count number

Syntax Description

number

Number of times to retry sending commands to the LAN Extender. It can be a number in the range 0 to 100. The default is 10.

Defaults

10 retries

Command Modes

Interface configuration

Command History

Release	Modification
10.3	This command was introduced.

Usage Guidelines

After the router has sent a command the specified number of times without receiving an acknowledgment from the LAN Extender, it stops sending the command altogether.

Examples

The following example resends commands 20 times to the LAN Extender:

Router(config-if)# lex interface 0

Router(config-if)# lex retry-count 20

Related Commands

Command	Description
lex timeout	Defines the amount of time to wait for a response from the LAN Extender.

lex timeout

To define the amount of time to wait for a response from the LAN Extender, use the lex timeout interface configuration command. To return to the default time, use the no form of this command.

lex timeout milliseconds

no lex timeout [milliseconds]

Syntax Description

milliseconds

Time, in milliseconds, to wait for a response from the LAN Extender before resending the command. It can be a number in the range 500 to 60000. The default is 2000 ms.

Defaults

2000 ms (2 seconds)

Command Modes

Interface configuration

Command History

Release	Modification
10.3	This command was introduced.

Usage Guidelines

The lex timeout command defines the amount of time that the router waits to receive an acknowledgment after having sent a command to the LAN Extender.

Examples

The following example causes unacknowledged packets to be resent at 4-second intervals:

Router(config-if)# lex interface 0

Router(config-if)# lex timeout 4000

Related Commands

Command	Description
lex retry-count	Defines the number of times to resend commands to the LAN Extender chassis.

linecode

To select the line-code type for T1 or E1 line, use the linecode controller configuration command.

linecode {ami | b8zs | hdb3}

Syntax Description

ami	Specifies alternate mark inversion (AMI) as the line-code type. Valid for T1 or E1 controllers. This is the default for T1 lines.
b8zs	Specifies B8ZS as the line-code type. Valid for T1 controller only.
hdb3	Specifies high-density bipolar 3 (hdb3) as the line-code type. Valid for E1 controller only. This is the default for E1 lines.

Defaults

AMI is the default for T1 lines.

High-density bipolar 3 is the default for E1 lines.

Command Modes

Controller configuration

Command History

Release	Modification
10.3	This command was introduced.

Usage Guidelines

Use this command in configurations where the router or access server must communicate with T1 fractional data lines. The T1 service provider determines which line-code type, either ami or b8zs, is required for your T1 circuit. Likewise, the E1 service provider determines which line-code type, either ami or hdb3, is required for your E1 circuit.

There is not a no form of this command.

Examples

The following example specifies B8ZS as the line-code type:

Router(config-controller)# linecode b8zs

line-termination

To specify the line termination for the E1 port on a trunk card, use the line-termination controller command. To return to the default line termination, use the no form of this command.

line-termination {75-ohm | 120-ohm}

no line-termination

Syntax Description

75-ohm	Specify 75-ohm unbalanced termination.
120-ohm	Specify 120-ohm balanced termination. This is the default.

Defaults

120-ohms

Command Modes

Controller configuration

Command History

Release	Modification
11.3(2)AA	This command was introduced.

Usage Guidelines

To determine the line termination setting for the port, use the show controller e1 command.

Examples

In the following example, the line termination is set to 75-ohms for the E1 port located in shelf 6, slot 0 port 0:

Router# configure terminal

Router(config)# controller e1 6/0/0

Router(config-controller)# line-termination 75-ohm

Router(config-controller)# exit

Router(config)# exit

Router#

Related Commands

Command	Description
show controllers e1	Displays information about the E1 links supported by the NPM (Cisco 4000) or MIP (Cisco 7500 series).

link-test

To reenable the link-test function on a port on an Ethernet hub of a Cisco 2505 or Cisco 2507 router, use the link-test hub configuration command. To disable this feature if a pre-10BaseT twisted-pair device not implementing link test is connected to the hub port, use the no form of this command.

link-test

no link-test

Syntax Description

This command has no arguments or keywords.

Defaults

Enabled

Command Modes

Hub configuration

Command History

Release	Modification
10.3	This command was introduced.

Usage Guidelines

This command applies to a port on an Ethernet hub only. Disable this feature if a 10BaseT twisted-pair device at the other end of the hub does not implement the link test function.

Examples

The following example disables the link test function on hub 0, ports 1 through 3:

Router(config)# hub ethernet 0 1 3

Router(config-hub)#  no link-test

Related Commands

Command	Description
hub	Enables and configures a port on an Ethernet hub of a Cisco 2505 or Cisco 2507 router.

local-lnm

To enable Lanoptics Hub Networking Management of a PCbus Token Ring interface, use the local-lnm interface configuration command. To disable Lanoptics Hub Networking Management, use the no form of this command.

local-lnm

no local-lnm

Syntax Description

This command has no arguments or keywords.

Defaults

Management is not enabled.

Command Modes

Interface configuration

Command History

Release	Modification
10.3	This command was introduced.

Usage Guidelines

The Token Ring interface on the AccessPro PC card can be managed by a remote LAN manager over the PCbus interface. At present, the Lanoptics Hub Networking Management software running on an IBM compatible PC is supported.

Examples

The following example enables Lanoptics Hub Networking Management:

Router(config-if)# local-lnm

logging event

To enable notification of interface, subinterface, and Frame Relay data link connection identifier (DLCI) data link status changes, use the logging event command in interface configuration mode. To disable notification, use the no form of this command.

logging event {dlci-status-change | link-status | subif-link-status}

no logging event {dlci-status-change | link-status | subif-link-status}

Syntax Description

dlci-status-change	Enables notification of Frame Relay DLCI status changes. Note This option is supported only when the encapsulation on the interface is Frame Relay.
link-status	Enables notification of interface data link status changes.
subif-link-status	Enables notification of subinterface data link status changes.

Defaults

For system images, notification of interface, subinterface, and Frame Relay DLCI data link status changes is enabled by default.

For boot images, notification of Frame Relay subinterface and DLCI data link status changes is disabled by default. Notification of interface data link status changes is enabled by default.

Command Modes

Interface configuration

Command History

Release	Modification
12.0	This command was introduced.

Examples

The following example shows how to enable notification of subinterface link status changes:

Router(config-if)# logging event subif-link-status

The following are examples of Frame Relay DLCI and subinterface status change notification messages filtered by the logging event command:

00:16:22: %FR-5-DLCICHANGE: Inteface Serial3/0/0:1 - DLCI 105 state changed to INACTIVE

00:16:22: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial3/0/0:1.5, changed state 
to down

loopback (interface)

To diagnose equipment malfunctions between the interface and device, use the loopback interface configuration command. To disable the test, use the no form of this command.

loopback

no loopback

Syntax Description

This command has no arguments or keywords.

Defaults

Disabled

Command Modes

Interface configuration

Command History

Release	Modification
10.0	This command was introduced.

Usage Guidelines

On HSSI serial interface cards, the loopback function configures a two-way internal and external loop on the HSA applique of the specific interface.

On MCI and SCI serial interface cards, the loopback functions when a CSU/DSU or equivalent device is attached to the router or access server. The loopback command loops the packets through the CSU/DSU to configure a CSU loop, when the device supports this feature.

On the MCI and MEC Ethernet cards, the interface receives back every packet it sends when the loopback command is enabled. Loopback operation has the additional effect of disconnecting network server functionality from the network.

On the CSC-FCI FDDI card, the interface receives back every packet it sends when the loopback command is enabled. Loopback operation has the additional effect of disconnecting network server functionality from the network.

On all Token Ring interface cards (except the 4-megabit CSC-R card), the interface receives back every packet it sends when the loopback command is enabled. Loopback operation has the additional effect of disconnecting network server functionality from the network.

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Note Loopback does not work on an X.21 DTE because the X.21 interface definition does not include a loopback definition.

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To show interfaces currently in loopback operation, use the show interfaces loopback EXEC command.

Examples

The following example configures the loopback test on Ethernet interface 4:

Router(config)# interface ethernet 4

Router(config-if)#  loopback

Related Commands

Command	Description
down-when-looped	Configures an interface to inform the system it is down when loopback is detected.
show interfaces loopback	Displays information about the loopback interface.

loopback (E3/T3 interface)

To loop the serial interface on a PA-E3 or a PA-T3 port adapter, use the loopback interface configuration command. To remove the loopback, use the no form of this command.

PA-E3 port adapter

loopback {dte | local | network {line | payload}}

no loopback

PA-T3 port adapter

loopback {dte | local | network {line | payload} | remote}

no loopback

Syntax Description

dte	Sets the loopback after the LIU toward the terminal.
local	Sets the loopback after going through the framer toward the terminal.
network {line | payload}	Sets the loopback toward the network before going through the framer (line) or after going through the framer (payload).
remote	Sends a far-end alarm control (FEAC) to set the remote framer in loopback.

Defaults

Disabled

Command Modes

Interface configuration

Command History

Release	Modification
11.1 CA	This command was introduced.

Usage Guidelines

Use this command for troubleshooting purposes.

To verify that a loopback is configured on the interface, use the show interfaces serial or show interfaces loopback EXEC command.

Examples

The following example configures the serial interface located in slot 3/0/0 for a local loopback:

Router(config)# interface serial 3/0/0

Router(config-if)#  loopback local

Related Commands

Command	Description
show controllers serial	Displays information that is specific to the interface hardware.

loopback (T1 interface)

To loop individual T1 channels on the CT3IP in Cisco 7000 series routers with the RSP7000 and RSP7000CI and in Cisco 7500 series routers, use the loopback interface configuration command. To remove the loopback, use the no form of this command.

loopback [local | network {line | payload} | remote {line {fdl {ansi | bellcore} | inband} | payload [fdl] [ansi]}]

no loopback

Syntax Description

local	(Optional) Loops the router output data back toward the router at the T1 framer and sends an AIS signal out toward the network.
network {line | payload}	(Optional) Loops the data back toward the network before the T1 framer and automatically sets a local loopback at the HDLC controllers (line) or loops the payload data back toward the network at the T1 framer and automatically sets a local loopback at the HDLC controllers (payload).
remote line fdl {ansi | bellcore}	(Optional) Sends a repeating, 16-bit ESF data link code word (00001110 11111111 for FDL ANSI and 00010010 11111111 for FDL Bellcore) to the remote end requesting that it enter into a network line loopback. Specify the ansi keyword to enable the remote line Facility Data Link (FDL) ANSI bit loopback on the T1 channel, per the ANSI T1.403 Specification. Specify the bellcore keyword to enable the remote SmartJack loopback on the T1 channel, per the TR-TSY-000312 Specification.
remote line inband	(Optional) Sends a repeating, 5-bit inband pattern (00001) to the remote end requesting that it enter into a network line loopback.
remote payload [fdl] [ansi]	(Optional) Sends a repeating, 16-bit ESF data link code word (00010100 11111111) to the remote end requesting that it enter into a network payload loopback. Enables the remote payload Facility Data Link (FDL) ANSI bit loopback on the T1 channel. You can optionally specify fdl and ansi, but it is not necessary.

Defaults

Disabled

Command Modes

Interface configuration

Command History

Release	Modification
11.1 CA	This command was introduced.

Usage Guidelines

Use this command for troubleshooting purposes.

To better diagnose T1 provisioning problems, you can place the remote CSU or remote SmartJack into loopback. The loopback remote line fdl interface configuration command allows you to place either the CSU or the SmartJack into loopback:

•ansi—Places the CSU into loopback, per the ANSI T1.403 Specification

•bellcore—Places the SmartJack into loopback, per the TR-TSY-000312 Specification

When both are configured, transmission of LOF indication (yellow alarm) takes priority over transmission of some FDL messages.

If the remote loopback appears not to be working, use the show cont t3 command to determine if the given T1 is currently attempting to transmit a LOF indication (yellow alarm):

Router# show controllers t3 0/0/0:2

 T3 0/0/0 is up.

   CT3 H/W Version: 5, CT3 ROM Version: 1.2, CT3 F/W Version: 2.5.9

   Mx H/W version: 2, Mx ucode ver: 1.34

   T1 2 is down, speed: 1536 kbs, non-inverted data