Downloads |
Table Of Contents
Related Features and Technologies
Supported Standards, MIBs, and RFCs
ATM Primary Interface (Multipoint) Using aal5snap Encapsulation and Inverse ARP Example
ATM Point-to-Point Subinterface Using aa15snap Encapsulation Example
ATM Point-to-Point Subinterface Using aa15nlpid Encapsulation Example
ATM Point-to-Point Subinterface Using aa15mux PPP Encapsulation Example
DHCP Client on WAN Interfaces
Feature History
This document describes the DHCP Client on WAN Interfaces feature in Cisco IOS Release 12.2(8)T and includes the following sections:
•
Supported Standards, MIBs, and RFCs
Feature Overview
The DHCP Client on WAN Interfaces feature extends the Dynamic Host Configuration Protocol (DHCP) to allow a DHCP client to acquire an IP address over PPP over ATM (PPPoA) and certain ATM interfaces. By using DHCP rather than the IP Control Protocol (IPCP), a DHCP client can acquire other useful information such as DNS addresses, the DNS default domain name, and the default route.
The configuration of PPPoA and Classical IP and ARP over ATM already allows for a broadcast capability over the interface (using the broadcast keyword on the ATM interface). Most changes in this feature are directed at removing already existing restrictions on what types of interfaces are allowed to send out DHCP packets (previously, dialer interfaces have not been allowed). This feature also ensures that DHCP RELEASE messages are sent out the interface before a connection is allowed to be broken.
Benefits
DHCP is beneficial on WAN interfaces because it can be used to acquire information such as DNS server addresses, the DNS default domain name, and the default route.
Restrictions
This feature works with ATM point-to-point interfaces and will accept any encapsulation type. For ATM multipoint interfaces, this feature is only supported using the aal5snap encapsulation type combined with Inverse ARP. Inverse ARP, which builds an ATM map entry, is necessary to send unicast packets to the server (or relay agent) on the other end of the connection. Inverse ARP is only supported for the aal5snap encapsulation type.
For multipoint interfaces, an IP address can be acquired using other encapsulation types because broadcast packets are used. However, unicast packets to the other end will fail because there is no ATM map entry and thus DHCP renewals and releases also fail.
See the "Troubleshooting Tips" section of this document for more information.
Related Features and Technologies
•
•
Related Documents
•
•
•
•
•
•
Supported Platforms
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Determining Platform Support Through Cisco Feature Navigator
Cisco IOS software is packaged in feature sets that support specific platforms. To get updated information regarding platform support for this feature, access Cisco Feature Navigator. Cisco Feature Navigator dynamically updates the list of supported platforms as new platform support is added for the feature.
Cisco Feature Navigator is a web-based tool that enables you to quickly determine which Cisco IOS software images support a specific set of features and which features are supported in a specific Cisco IOS image. You can search by feature or release. Under the release section, you can compare releases side by side to display both the features unique to each software release and the features in common.
Cisco Feature Navigator is updated regularly when major Cisco IOS software releases and technology releases occur. For the most current information, go to the Cisco Feature Navigator home page at the following URL:
Supported Standards, MIBs, and RFCs
Standards
None
MIBs
None
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 the following URL:
http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml
RFCs
No new or modified RFCs are supported by this feature.
Configuration Tasks
This feature has no new configuration commands; however, the ip address dhcp interface configuration command can now be configured on PPPoA and certain ATM interfaces.
Troubleshooting Tips
•
•
•
•
•
Configuration Examples
This section provides the following configuration examples:
•
•
•
•
ATM Primary Interface (Multipoint) Using aal5snap Encapsulation and Inverse ARP Example
In the following example, the protocol ip 255.255.255.255 broadcast configuration is needed because there must be an ATM map entry to recognize the broadcast flag on the permanent virtual circuit (PVC). You can use any ATM map entry. The protocol ip inarp configuration is needed so the ATM Inverse ARP can operate on the interface such that the system on the other side can be pinged once an address is assigned by DHCP.
interface atm0ip address dhcppvc 1/100encapsulation aal5snapbroadcastprotocol ip 255.255.255.255 broadcastprotocol ip inarpATM Point-to-Point Subinterface Using aa15snap Encapsulation Example
The following example shows an ATM point-to-point subinterface configuration using aa15snap encapsulation:
interface atm0.1 point-to-pointip address dhcppvc 1/100encapsulation aal5snapbroadcastATM Point-to-Point Subinterface Using aa15nlpid Encapsulation Example
The following example shows an ATM point-to-point subinterface configuration using aa15nlpid encapsulation:
interface atm0.1 point-to-pointip address dhcppvc 1/100encapsulation aal5nlpidbroadcastATM Point-to-Point Subinterface Using aa15mux PPP Encapsulation Example
The following example shows an ATM point-to-point subinterface configuration using aa15mux PPP encapsulation:
interface atm0.1 point-to-pointpvc 1/100encapsulation aal5mux ppp virtual-template1broadcast!interface virtual-template1ip address dhcpCommand Reference
This section documents the modified ip address dhcp command. All other commands used with this feature are documented in the Cisco IOS Release 12.2 command reference publications.
ip address dhcp
To acquire an IP address on an interface from the Dynamic Host Configuration Protocol (DHCP), use the ip address dhcp command in interface configuration mode. To deconfigure any address that was acquired, use the no form of this command.
ip address dhcp [client-id interface-name] [hostname host-name]
no ip address dhcp [client-id interface-name] [hostname host-name]
Syntax Description
Defaults
The host name is the globally configured host name of the router.
The client identifier is an ASCII value.
Command Modes
Interface configuration
Command History
Usage Guidelines
The ip address dhcp command allows any interface to dynamically learn its IP address by using the DHCP protocol. It is especially useful on Ethernet interfaces that dynamically connect to an Internet serviceprovider (ISP). Once assigned a dynamic address, the interface can be used with the Port Address Translation (PAT) of Cisco IOS Network Address Translation (NAT) to provide Internet access to a privately addressed network attached to the router.
The ip address dhcp command also works with ATM point-to-point interfaces and will accept any encapsulation type. However, for ATM multipoint interfaces you must specify Inverse ARP via the protocol ip inarp interface configuration command and only use the aa15snap encapsulation type.
Some ISPs require that the DHCPDISCOVER message have a specific host name and client identifier that is the MAC address of the interface. The most typical usage of the ip address dhcp client-id interface-name hostname host-name command is when interface-name is the Ethernet interface where the command is configured and host-name is the host name provided by the ISP.
A client identifier (DHCP option 61) can be a hexadecimal or an ASCII value. By default, the client identifier is an ASCII value. The client-id interface-name option overrides the default and forces the use of the hexadecimal MAC address of the named interface.
Note
If a Cisco router is configured to obtain its IP address from a DHCP server, it sends a DHCPDISCOVER (broadcast) message to provide information about itself to the DHCP server on the network.
If you use the ip address dhcp command with or without any of the optional keywords, the DHCP option 12 field (host name option) is included in the DHCPDISCOVER message. By default, the host name specified in option 12 will be the globally configured host name of the router. However, you can use the ip address dhcp hostname host-name command to place a different name in the DHCP option 12 field than the globally configured host name of the router.
The no ip address dhcp command deconfigures any IP address that was acquired, thus sending a DHCPRELEASE message.
You might need to experiment with different configurations to determine the one required by your DHCP server. Table 1 shows the possible configuration methods and the information placed in the DHCPDISCOVER message for each method.
The following example shows a client acquiring an IP address over a multipoint ATM interface:
interface atm0ip address dhcppvc 1/100encapsulation aal5snapbroadcastprotocol ip 255.255.255.255 broadcastprotocol ip inarpThe following example shows a client acquiring an IP address over a point-to-point ATM subinterface:
interface atm0.1 point-to-pointip address dhcppvc 1/100encapsulation aa15nlpidbroadcastIn the examples that follow, the command ip address dhcp is entered for Ethernet interface 1. The DHCPDISCOVER message sent by a router configured as shown in the following example would contain "cisco- mac-address -Eth1" in the client ID field, and the value fresno in the option 12 field:
hostname fresno!interface Ethernet 1ip address dhcpThe DHCPDISCOVER message sent by a router configured as shown in the following example would contain "cisco- mac-address -Eth1" in the client ID field, and the value sanfran in the option 12 field:
hostname fresno!interface Ethernet 1ip address dhcp hostname sanfranThe DHCPDISCOVER message sent by a router configured as shown in the following example would contain the MAC address of Ethernet interface 1 in the client id field, and the value fresno in the option 12 field:
hostname fresno!interface Ethernet 1ip address dhcp client-id Ethernet 1The DHCPDISCOVER message sent by a router configured as shown in the following example would contain the MAC address of Ethernet interface 1 in the client id field, and the value sanfran in the option 12 field.
hostname fresno!interface Ethernet 1ip address dhcp client-id Ethernet 1 hostname sanfranRelated Commands
Glossary
ATM—Asynchronous Transfer Mode.
DHCP—Dynamic Host Configuration Protocol.
INARP—Inverse ARP.
PPP—Point-to-Point Protocol.
PPPoA—Point-to-Point Protocol over ATM.
