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VLANs over IP Unnumbered Subinterfaces

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VLANs over IP Unnumbered Subinterfaces

Contents

Prerequisites for VLANs over IP Unnumbered Subinterfaces

Restrictions for VLANs over IP Unnumbered Subinterfaces

Information About VLANs over IP Unnumbered Subinterfaces

Support for VLANs over IP Unnumbered Subinterfaces

DHCP Option 82

Benefits of VLANs over IP Unnumbered Subinterfaces

How to Configure IP Unnumbered Interface Support on Ethernet VLAN Subinterfaces

Configuring IP Unnumbered Interface Support on an Ethernet VLAN Subinterface

Troubleshooting Tips

Configuring IP Unnumbered Interface Support on a Range of Ethernet VLAN Subinterfaces

Restrictions for Configuring IP Unnumbered Interface Support on a Range of Ethernet VLAN Subinterfaces

Troubleshooting Tips

Configuration Examples for IP Unnumbered Interface Support of Ethernet VLAN Subinterfaces

VLAN Configuration on a Single IP Unnumbered Subinterface: Example

VLAN Configuration on a Range of IP Unnumbered Subinterfaces: Example

Additional References

Related Documents

Standards

MIBs

RFCs

Technical Assistance

Command Reference

ip unnumbered


VLANs over IP Unnumbered Subinterfaces

First Published: November 3, 2003
Last Updated: March 16, 2006

The VLANs over IP Unnumbered Subinterfaces feature allows IP unnumbered interface support to be configured on Ethernet VLAN subinterfaces. This feature also provides support for Dynamic Host Configuration Protocol (DHCP) on VLAN subinterfaces. Configuring Ethernet VLANs on IP unnumbered subinterfaces can save IPv4 address space, simplify configuration and address management, and simplify migration for DSL providers from ATM networks to IP.

Release
Modification

12.3(4)T

This feature was introduced.

12.2(18)SXE

This feature was integrated into Cisco IOS Release 12.2(18)SXE.

12.2(28)SB

This feature was integrated into Cisco IOS Release 12.2(28)SB


Feature History for the VLANs over IP Unnumbered Subinterfaces Feature

Finding Support Information for Platforms and Cisco IOS Software Images

Use Cisco Feature Navigator to find information about platform support and Cisco IOS software image support. Access Cisco Feature Navigator at http://www.cisco.com/go/fn. You must have an account on Cisco.com. If you do not have an account or have forgotten your username or password, click Cancel at the login dialog box and follow the instructions that appear.

Contents

•Prerequisites for VLANs over IP Unnumbered Subinterfaces

•Restrictions for VLANs over IP Unnumbered Subinterfaces

•Information About VLANs over IP Unnumbered Subinterfaces

•How to Configure IP Unnumbered Interface Support on Ethernet VLAN Subinterfaces

•Configuration Examples for IP Unnumbered Interface Support of Ethernet VLAN Subinterfaces

•Additional References

•Command Reference

Prerequisites for VLANs over IP Unnumbered Subinterfaces

•DHCP must be configured and operational.

Restrictions for VLANs over IP Unnumbered Subinterfaces

•Only Ethernet VLAN subinterfaces, in addition to serial interfaces, can be configured as IP unnumbered interfaces.

•Interface ranges (interface range command) are not supported in Cisco IOS Release 12.2(18)SXE.

Information About VLANs over IP Unnumbered Subinterfaces

Before you configure VLANs over IP unnumbered subinterfaces, you should understand the following concepts:

•Support for VLANs over IP Unnumbered Subinterfaces

•Benefits of VLANs over IP Unnumbered Subinterfaces

Support for VLANs over IP Unnumbered Subinterfaces

IP unnumbered interface configuration allows you to enable IP processing on an interface without assigning it an explicit IP address. The IP unnumbered interface can "borrow" the IP address of another interface that is already configured on the router, thereby conserving network and address space. The VLANs over IP Unnumbered Subinterfaces feature allows Ethernet VLANs to be configured on IP unnumbered subinterfaces.

Figure 1 shows a sample network topology in which the VLANs over IP Unnumbered Subinterfaces feature would be implemented. In this topology, IP routes are dynamically established by the aggregation router when the DHCP server assigns IP addresses to the hosts.

Figure 1

Sample Network Topology Using the VLANs over IP Unnumbered Subinterfaces Feature

The VLANs over IP Unnumbered Subinterfaces feature supports the following functions:

•Peer IP address allocation through DHCP

•Ability to configure IP unnumbered interface support for a range of VLAN subinterfaces

•Service Selection Gateway support for VLANs over IP unnumbered subinterfaces

•DHCP relay agent information feature (Option 82)

DHCP Option 82

DHCP provides a framework for passing configuration information to hosts on a TCP/IP network. Configuration parameters and other control information are carried in tagged data items that are stored in the options field of the DHCP message. The data items are also called options. Option 82 is organized as a single DHCP option that contains information known by the relay agent.

This feature communicates information to the DHCP server using a suboption of the DHCP relay agent information option called agent remote ID. The information sent in the agent remote ID includes an IP address identifying the relay agent and information about the interface and the connection over which the DHCP request came in. The DHCP server can use this information to make IP address assignments and security policy decisions.

Figure 2 shows the agent remote ID suboption format that is used with the VLANs over IP Unnumbered Subinterfaces feature.

Figure 2 Format of the Agent Remote ID Suboption

Table 1 describes the agent remote ID suboption fields displayed in Figure 2.

Table 1 Agent Remote ID Suboption Field Descriptions

Field
Description

Type

Format type. The value 2 specifies the format for use with this feature. (1 byte)

Length

Length of the Agent Remote ID suboption, not including the type and length fields. (1 byte)

Reserved

Reserved. (2 bytes)

NAS IP Address

IP address of the interface specified by the ip unnumbered command. (4 bytes)

Interface

Physical interface. This field has the following format:
slot (4 bits) | module (1 bit) | port (3 bits).

For example, if the interface name is interface ethernet 2/1/1, the slot is 2, the module is 1, and the port is 1. (1 byte)

Reserved

Reserved. (1 byte)

VLAN ID

VLAN identifier for the Ethernet subinterface. (2 bytes)


Benefits of VLANs over IP Unnumbered Subinterfaces

The VLANs over IP Unnumbered Subinterfaces feature provides the following benefits:

•Easy migration for DSL providers to Gigabit Ethernet uplinks and an IP core

•Savings in IPv4 address space because all ports can share the same subnet

•Increased security because each user is on a separate VLAN, routing information comes from DHCP, and there is no Address Resolution Protocol (ARP) or MAC address spoofing

•Simpler address management with DHCP

•Easier configuration and savings in NVRAM because Ethernet VLAN subinterfaces can now be configured as interface ranges

How to Configure IP Unnumbered Interface Support on Ethernet VLAN Subinterfaces

This section contains the following procedures:

•Configuring IP Unnumbered Interface Support on an Ethernet VLAN Subinterface

•Configuring IP Unnumbered Interface Support on a Range of Ethernet VLAN Subinterfaces

Configuring IP Unnumbered Interface Support on an Ethernet VLAN Subinterface

Perform this task to configure IP unnumbered interface support on a single Ethernet VLAN subinterface.

SUMMARY STEPS

1. enable

2. configure terminal

3. interface type number [name-tag]

4. encapsulation dot1q vlan-id [native]

5. ip unnumbered type number

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

•Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

interface type number [name-tag]

Example:

Router(config)# interface fastethernet 1/0.1

Configures an interface type and enters interface or subinterface configuration mode.

Step 4 

encapsulation dot1q vlan-id [native]

Example:

Router(config-subif)# encapsulation dot1q 10

Enables IEEE 802.1Q encapsulation of traffic on a specified subinterface in a VLAN.

Step 5  

ip unnumbered type number
Example:

Router(config-subif)# ip unnumbered ethernet 3/0

Enables IP processing on an interface without assigning an explicit IP address to the interface.

•The type and number arguments specify another interface on which the router has an assigned IP address. The interface specified cannot be another unnumbered interface.

Troubleshooting Tips

Use the show running-config command to verify that IP unnumbered support has been configured correctly.

Configuring IP Unnumbered Interface Support on a Range of Ethernet VLAN Subinterfaces

Perform this task to configure IP unnumbered interface support on a range of Ethernet VLAN subinterfaces.

Restrictions for Configuring IP Unnumbered Interface Support on a Range of Ethernet VLAN Subinterfaces

•The interface range command is not supported in Cisco IOS Release 12.2(18)SXE.

SUMMARY STEPS

1. enable

2. configure terminal

3. interface range {{ethernet | fastethernet | gigabitethernet | vlan vlan} slot/interface.subinterface - {ethernet | fastethernet | gigabitethernet | vlan vlan} slot/interface.subinterface | macro macro-name}

4. encapsulation dot1q vlan-id [native]

5. ip unnumbered type number

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

•Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

interface range {{ethernet | fastethernet | gigabitethernet | vlan vlan} slot/interface.subinterface - {ethernet | fastethernet | gigabitethernet | vlan vlan} slot/interface.subinterface | macro macro-name}

Example:

Router(config)# interface range fastethernet 1/0.1 - fastethernet 1/0.100

Executes commands on multiple subinterfaces at the same time.

•A hyphen must be entered with a space on either side to separate the range information.

Step 4 

encapsulation dot1q vlan-id [native]

Example:

Router(config-if-range)# encapsulation dot1q 10

Applies a unique VLAN ID to each subinterface within the range.

•The VLAN ID specified by the vlan-id argument is applied to the first subinterface in the range. Each subsequent interface is assigned a VLAN ID, which is the specified vlan-id plus the subinterface number minus the first subinterface number (VLAN ID + subinterface number - first subinterface number).

Step 5  

ip unnumbered type number
Example:

Router(config-if-range)# ip unnumbered ethernet 3/0

Enables IP processing on an interface without assigning an explicit IP address to the interface.

•The type and number arguments specify another interface on which the router has an assigned IP address. The specified interface cannot be another unnumbered interface.

Troubleshooting Tips

Use the show running-config command to verify that IP unnumbered support has been configured correctly.

Configuration Examples for IP Unnumbered Interface Support of Ethernet VLAN Subinterfaces

•VLAN Configuration on a Single IP Unnumbered Subinterface: Example

•VLAN Configuration on a Range of IP Unnumbered Subinterfaces: Example

VLAN Configuration on a Single IP Unnumbered Subinterface: Example

In the following example, Ethernet VLAN subinterface 3/0.2 is configured as an IP unnumbered subinterface: 

interface ethernet 3/0.2

 encapsulation dot1q 200

 ip unnumbered ethernet 3/1

VLAN Configuration on a Range of IP Unnumbered Subinterfaces: Example

In the following example, Fast Ethernet subinterfaces in the range from 5/1.1 to 5/1.4 are configured as IP unnumbered subinterfaces: 

interface range fastethernet5/1.1 - fastethernet5/1.4 

 ip unnumbered ethernet 3/1

Additional References

The following sections provide references related to the VLANs over IP Unnumbered Subinterfaces feature.

Related Documents

Related Topic
Document Title

DHCP and other IP addressing configuration tasks

"IP Addressing and Services" section of the Cisco IOS IP Addressing Services Configuration Guide, Release 12.4

DHCP and other IP addressing commands

Cisco IOS IP Addressing Services Command Reference, Release 12.4 T

VLAN configuration tasks

"Virtual LANs" chapter of the Cisco IOS LAN Switching Configuration Guide, Release 12.4

VLAN configuration commands

Cisco IOS LAN Switching Command Reference, Release 12.4 T


Standards

Standards
Title

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

—


MIBs

MIBs
MIBs Link

No new or modified MIBs are supported by this feature. Support for existing MIBs has not been modified by this feature.

To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:

http://www.cisco.com/go/mibs


RFCs

RFCs
Title

RFC 1812

Requirements for IP Version 4 Routers, June 1995


Technical Assistance

Description
Link

The Cisco Technical Support & Documentation website contains thousands of pages of searchable technical content, including links to products, technologies, solutions, technical tips, and tools. Registered Cisco.com users can log in from this page to access even more content.

http://www.cisco.com/techsupport


Command Reference

This section documents the following command only:

•ip unnumbered

ip unnumbered

To enable IP processing on a serial interface without assigning an explicit IP address to the interface, use the ip unnumbered command in interface configuration mode or subinterface configuration mode. To disable the IP processing on the interface, use the no form of this command.

ip unnumbered interface-type interface-number

no ip unnumbered interface-type interface-number

Syntax Description

interface-type

Type of interface on which the router has an assigned IP address. The interface cannot be another unnumbered interface.

interface-number

Number of the interface on which the router has an assigned IP address. The interface cannot be another unnumbered interface.


Defaults

Disabled

Command Modes

Interface configuration
Subinterface configuration

Command History

Release
Modification

10.0

This command was introduced.

12.3(4)T

This command was modified to configure IP unnumbered support on Ethernet VLAN subinterfaces and subinterface ranges.

12.2(18)SXE

This command was incorporated into Release 12.2(18)SXE.

12.2(28)SB

This command was integrated into Cisco IOS Release 12.2(28)SB.


Usage Guidelines

When the unnumbered interface generates a packet (for example, for a routing update), it uses the address of the specified interface as the source address of the IP packet. It also uses the address of the specified interface in determining which routing processes are sending updates over the unnumbered interface. Restrictions are as follows:

•Serial interfaces using High-Level Data Link Control (HDLC), PPP, Link Access Procedure Balanced (LAPB), Frame Relay encapsulations, and Serial Line Internet Protocol (SLIP) and tunnel interfaces can be unnumbered. It is not possible to use this interface configuration command with X.25 or Switched Multimegabit Data Service (SMDS) interfaces.

•You cannot use the ping EXEC command to determine whether the interface is up, because the interface has no address. Simple Network Management Protocol (SNMP) can be used to remotely monitor interface status.

•It is not possible to netboot a Cisco IOS image over a serial interface that is assigned an IP address with the ip unnumbered command.

•You cannot support IP security options on an unnumbered interface.

The interface you specify by the interface-type and interface-number arguments must be enabled (listed as "up" in the show interfaces command display).

If you are configuring Intermediate System-to-Intermediate System (IS-IS) across a serial line, you should configure the serial interfaces as unnumbered. This configuration allows you to comply with RFC 1195, which states that IP addresses are not required on each interface.

Note Using an unnumbered serial line between different major networks (or majornets) requires special care. If at each end of the link there are different majornets assigned to the interfaces you specified as unnumbered, then any routing protocol running across the serial line must not advertise subnet information.

Examples

In the following example, the first serial interface is given the address of Ethernet 0: 

interface ethernet 0

 ip address 10.108.6.6 255.255.255.0

!

interface serial 0

 ip unnumbered ethernet 0

In the following example, Ethernet VLAN subinterface 3/0.2 is configured as an IP unnumbered subinterface: 

interface ethernet 3/0.2

 encapsulation dot1q 200

 ip unnumbered ethernet 3/1

In the following example, Fast Ethernet subinterfaces in the range from 5/1.1 to 5/1.4 are configured as IP unnumbered subinterfaces: 

interface range fastethernet5/1.1 - fastethernet5/1.4 

 ip unnumbered ethernet 3/1

Any Internet Protocol (IP) addresses used in this document are not intended to be actual addresses. Any examples, command display output, and figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses in illustrative content is unintentional and coincidental.

Copyright © 2003, 2005-2006 Cisco Systems, Inc. All rights reserved.