Catalyst 4500 Series Switch Software Configuration Guide, IOS XE 3.9.xE and IOS 15.2(5)Ex

Configuring IP Unnumbered Interface

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This chapter discusses the IP Unnumbered Interface feature, which allows you to enable IP processing on an interface without assigning an explicit IP address.

This chapter contains these sections:

• About IP Unnumbered Interface Support
• IP Unnumbered Configuration Guidelines and Restrictions
• Configuring IP Unnumbered Interface Support with DHCP Server
• Configuring IP Unnumbered Interface Support with Connected Host Polling
• Displaying IP Unnumbered Interface Settings
• Troubleshooting IP Unnumbered Interface
• Related Documents

Note For complete syntax and usage information for the switch commands used in this chapter, see the
Cisco IOS Command Reference Guides for the Catalyst 4500 Series Switch.

If a command is not in the Cisco Catalyst 4500 Series Switch Command Reference , you can locate it in the Cisco IOS Master Command List, All Releases.

About IP Unnumbered Interface Support

Before you configure VLANs and LAN interfaces with IP unnumbered interfaces, you should understand the following concepts:

• IP Unnumbered Interface Support with DHCP Server and Relay Agent
• DHCP Option 82
• IP Unnumbered Interface with Connected Host Polling

IP Unnumbered Interface Support with DHCP Server and Relay Agent

The 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 from another interface that is already configured on the Catalyst 4500 series switch, which conserves network and address space. When used with the DHCP server/relay agent, this feature allows a host address assigned by the DHCP server to be learned dynamically at the DHCP relay agent.

Figure 18-1 shows a sample network topology implementing the IP Unnumbered Interface feature. In this topology, IP routes are dynamically established by the aggregation switch when the DHCP server assigns IP addresses to the hosts.

Figure 18-1 Sample Network Topology Using the VLANs over IP Unnumbered Interfaces Feature

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.

The IP Unnumbered Interface 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 entered. The DHCP server can use this information to make IP address assignments and security policy decisions.

Figure 18-2 shows the agent remote ID suboption format that is used with the IP Unnumbered Interfaces feature.

Figure 18-2 Format of the Agent Remote ID Suboption

 

Table 18-1 describes the agent remote ID suboption fields displayed in Figure 18-2.

 

Table 18-1 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 interface. (2 bytes)

IP Unnumbered Interface with Connected Host Polling

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Note This feature option is applicable to LAN and VLAN interfaces only.

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In some cases, the host IP address is assigned statically. The IP Unnumbered Interfaces feature can learn the static host IP address dynamically.

IP Unnumbered Configuration Guidelines and Restrictions

When using (or configuring) IP Unnumbered Interface, consider these guidelines and restrictions:

• For IP Unnumbered Interfaces, the following features are not supported:

– Dynamic routing protocols

– HSRP/VRRP

– Static ARP

– Unnumbered Interface and Numbered Interface in different VRFs

• The option to add dhcp host routes as connected routes is available in Cisco IOS. When using connected mode, however, the clear ip route * command deletes the dhcp host connected routes permanently.

Workarounds:

– For a layer 3 interface (SVI), enter shut then no shut.

– To enable IP unnumbered to use static routes, enter the ip dhcp route static command.

• IP Redirect is not sent by an interface configured with IP Unnumbered Interface.
• IP Unnumbered Interface is unable to forward multicast source packets.

Configuring IP Unnumbered Interface Support with DHCP Server

Note DHCP must be configured and operational before you perform this task.

This section contains the following procedures:

• Configuring IP Unnumbered Interface Support on LAN and VLAN Interfaces
• Configuring IP Unnumbered Interface Support on a Range of Ethernet VLANs

Configuring IP Unnumbered Interface Support with Connected Host Polling

To configure IP unnumbered interface support with connected host polling, perform this task:

The following example shows how to enable IP processing and connected host polling on Fast Ethernet interface 6/2. It also shows how to set the global backlog queue to 2000 and the maximum number of ARP requests to 500:

Displaying IP Unnumbered Interface Settings

Use the show ip interface unnumbered command to display status of an unnumbered interface with connected host polling for the switch.

To display status of an unnumbered interface, enter this command:

The following example shows how to display the status of unnumbered interfaces with connected host polling:

To display key statistic for the backlog of unnumbered interfaces with connected host polling for the switch, perform this task:

The following example shows how to display key statistic for the backlog of unnumbered interfaces with connected host polling:

To clear the key statistic for the backlog of unnumbered interfaces, use the clear ip arp poll statistic command, as follows:

Troubleshooting IP Unnumbered Interface

To understand how to debug connect host polling, see the Cisco IOS documentation of the debug arp command on cisco.com.

When an IP unnumbered interface shares the IP address of a loopback interface whose prefix is advertised in an OSPF network, you must modify the loopback interface as a point-to-point interface. Otherwise, only the loopback interface host route is advertised to an OSPF neighbor.

Related Documents