Table Of Contents
DHCP Option 82 Support for Routed Bridge Encapsulation
Supported Standards, MIBs, and RFCs
Configuring DHCP Option 82 for RBE
Verifying DHCP Option 82 for RBE Configuration
DHCP Option 82 for RBE Configuration Example
DHCP Option 82 Support for Routed Bridge Encapsulation
This document describes the DHCP Option 82 Support for Routed Bridge Encapsulation feature and includes the following sections:
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Supported Standards, MIBs, and RFCs
Feature History for the DHCP Option 82 Support for Routed Bridge Encapsulation Feature
12.2(2)T
This feature was introduced.
12.2(28)SB
This feature was integrated into Cisco IOS Release 12.2(28)SB.
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.
Feature Overview
The DHCP relay agent information option (option 82) enables a Dynamic Host Configuration Protocol (DHCP) relay agent to include information about itself when forwarding client-originated DHCP packets to a DHCP server. The DHCP server can use this information to implement IP address or other parameter-assignment policies.
The DHCP Option 82 Support for Routed Bridge Encapsulation feature provides support for the DHCP relay agent information option when ATM routed bridge encapsulation (RBE) is used. Figure 1 shows a typical network topology in which ATM RBE and DHCP are used. The aggregation router that is using ATM RBE is also serving as the DHCP relay agent.
Figure 1 Network Topology Using ATM RBE and DHCP
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 ATM interface and the PVC 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 format of the agent remote ID suboption.
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
Port Type
Port type. The value 0x01 indicates RBE. (1 byte)
Version
Option 82 version. The value 0x01 specifies the RBE version of Option 82. (1 byte)
Reserved
Reserved. (2 bytes)
NAS IP Address
IP address of one of the interfaces on the DHCP relay agent. The rbe nasip command can be used to specify which IP address will be used. (4 bytes)
NAS Port
RBE-enabled virtual circuit on which the DHCP request has come in. See Figure 3 for the format of this field. (4 bytes)
Figure 3 shows the format of the network access server (NAS) port field in the agent remote ID suboption.
Figure 3 Format of the NAS Port Field
Figure 4 shows the format of the interface field. If there is no module, the value of the module bit is 0.
Figure 4 Format of the Interface Field
Benefits
Service providers are increasingly using ATM routed bridge encapsulation to configure digital subscriber line (DSL) access. The DHCP Option 82 Support for Routed Bridge Encapsulation feature enables those service providers to use DHCP to assign IP addresses and DHCP option 82 to implement security and IP address assignment policies.
Related Documents
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Supported Platforms
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Supported Standards, MIBs, and RFCs
Standards
No new or modified standards are supported by this feature.
MIBs
No new or modified MIBs are supported by this feature.
To obtain lists of supported MIBs by platform and Cisco IOS release, and to download MIB modules, go to the Cisco MIB website on Cisco.com at http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml.
RFCs
No new or modified standards are supported by this feature.
Prerequisites
DHCP option 82 support must be configured on the DHCP relay agent using the ip dhcp relay information option command before you can use the DHCP Option 82 Support for Routed Bridge Encapsulation feature.
Configuration Tasks
See the following sections for configuration tasks for the DHCP Option 82 Support for Routed Bridge Encapsulation feature. Each task in the list is identified as either optional or required.
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Configuring DHCP Option 82 for RBE
To configure DHCP option 82 support for RBE, use the following commands in global configuration mode:
Verifying DHCP Option 82 for RBE Configuration
To verify that the DHCP Option 82 Support for Routed Bridge Encapsulation feature is configured correctly, use the following command in privileged EXEC mode:
Configuration Examples
This section provides the following configuration example:
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DHCP Option 82 for RBE Configuration Example
In the following example, DHCP option 82 support is enabled on the DHCP relay agent using the ip dhcp relay agent information option command. The rbe nasip command configures the router to forward the IP address for Loopback0 to the DHCP server.
ip dhcp-server 172.16.1.2!ip dhcp relay information option!interface Loopback0ip address 10.1.1.129 255.255.255.192!interface ATM4/0no ip address!interface ATM4/0.1 point-to-pointip unnumbered Loopback0ip helper-address 172.16.1.2atm route-bridged ippvc 88/800encapsulation aal5snap!!interface Ethernet5/1ip address 172.16.1.1 255.255.0.0!router eigrp 100network 10.1.0.0network 172.16.0.0!rbe nasip Loopback0For the configuration example above, the value (in hexadecimal) of the agent remote ID suboption would be 010100000B01018140580320. Table 2 shows the value of each field within the agent remote ID suboption.
Command Reference
This section documents the new command that configures the DHCP Option 82 Support for Routed Bridge Encapsulation feature.
rbe nasip
To specify the IP address of an interface on the Dynamic Host Configuration Protocol (DHCP) relay agent that will be sent to the DHCP server via the agent remote ID option, use the rbe nasip command in global configuration mode. To remove this specification, use the no form of this command.
rbe nasip source-interface
no rbe nasip source-interface
Syntax Description
Defaults
No IP address is specified.
Command Modes
Global configuration
Command History
12.2(2)T
This command was introduced.
12.2(28)SB
This command was integrated into Cisco IOS Release 12.2(28)SB.
Usage Guidelines
The rbe nasip command is used to configure support for the DHCP relay agent information option (option 82) for ATM routed bridge encapsulation (RBE).
Support for the DHCP relay agent information option must be configured on the DHCP relay agent using the ip dhcp relay information option command in order for the rbe nasip command to be effective.
Examples
In the following example, support for DHCP option 82 is enabled on the DHCP relay agent by the use of the ip dhcp relay agent information option command. The rbe nasip command configures the router to forward the IP address for Loopback0 to the DHCP server. ATM routed bridge encapsulation is configured on ATM subinterface 4/0.1.
ip dhcp-server 10.1.1.1!ip dhcp relay information option!interface Loopback0ip address 10.5.1.1 255.255.255.0!interface ATM4/0no ip address!interface ATM4/0.1 point-to-pointip unnumbered Loopback0ip helper-address 10.1.1.1atm route-bridged ippvc 88/800encapsulation aal5snap!router eigrp 100network 10.0.0.0!rbe nasip loopback0Related Commands
ip dhcp relay information option
Enables the system to insert the DHCP relay agent information option in forwarded BOOT REQUEST messages to a Cisco IOS DHCP server.
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.
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