ISSU and SSO - DHCP High Availability Features

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Updated:October 27, 2006

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ISSU and SSO—DHCP High Availability Features

Table Of Contents

ISSU and SSO—DHCP High Availability Features

Contents

Prerequisites for DHCP High Availability

Restrictions for DHCP High Availability

Information About DHCP High Availability

ISSU

SSO

ISSU and SSO—DHCP Server

ISSU and SSO—DHCP Relay on Unnumbered Interface

ISSU and SSO—DHCP Proxy Client

ISSU and SSO—DHCP ODAP Client and Server

How to Configure DHCP High Availability

Configuration Examples for DHCP High Availability

Additional References

Related Documents

Standards

MIBs

RFCs

Technical Assistance

Command Reference

debug dhcp redundancy

debug ip dhcp server redundancy

Feature Information for DHCP High Availability Features

Glossary


ISSU and SSO—DHCP High Availability Features

First Published: December 4, 2006
Last Updated: February 19, 2007

Cisco IOS Release 12.2(31)SB2 introduces the following series of Dynamic Host Configuration Protocol (DHCP) High Availability features that support the Broadband Access Server (BRAS):

•ISSU—DHCP Server

•SSO—DHCP Server

•ISSU—DHCP Relay on Unnumbered Interface

•SSO—DHCP Relay on Unnumbered Interface

•ISSU—DHCP Proxy Client

•SSO—DHCP Proxy Client

•ISSU—DHCP ODAP Client and Server

•SSO—DHCP ODAP Client and Server

These features are enabled by default when the redundancy mode of operation is set to Stateful Switchover (SSO).

Finding Feature Information in This Module

Your Cisco IOS software release may not support all of the features documented in this module. To reach links to specific feature documentation in this module and to see a list of the releases in which each feature is supported, use the "Feature Information for DHCP High Availability Features" section.

Finding Support Information for Platforms and Cisco IOS and Catalyst OS Software Images

Use Cisco Feature Navigator to find information about platform support and Cisco IOS and Catalyst OS software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Contents

•Prerequisites for DHCP High Availability

•Restrictions for DHCP High Availability

•Information About DHCP High Availability

•How to Configure DHCP High Availability

•Configuration Examples for DHCP High Availability

•Additional References

•Command Reference

•Feature Information for DHCP High Availability Features

•Glossary

Prerequisites for DHCP High Availability

•The Cisco IOS In-Service Software Upgrade (ISSU) process must be configured and working properly. See the "Cisco IOS In-Service Software Upgrade Process" feature module for more information.

•Stateful Switchover (SSO) must be configured and working properly. See the "Stateful Switchover" feature module for more information.

•Nonstop Forwarding (NSF) must be configured and working properly. See the "Cisco Nonstop Forwarding" feature module for more information.

Restrictions for DHCP High Availability

The DHCP high availability features do not support DHCP accounting or DHCP authorized Address Resolution Protocol (ARP).

Information About DHCP High Availability

This section describes the following concepts:

•ISSU

•SSO

•ISSU and SSO—DHCP Server

•ISSU and SSO—DHCP Relay on Unnumbered Interface

•ISSU and SSO—DHCP Proxy Client

•ISSU and SSO—DHCP ODAP Client and Server

ISSU

The ISSU process allows Cisco IOS software to be updated or otherwise modified while packet forwarding continues. In most networks, planned software upgrades are a significant cause of downtime. ISSU allows Cisco IOS software to be modified while packet forwarding continues, which increases network availability and reduces downtime caused by planned software upgrades.

SSO

SSO refers to the implementation of Cisco IOS software that allows applications and features to maintain a defined state between an active and standby Route Processor (RP).

In specific Cisco networking devices that support dual RPs, SSO takes advantage of RP redundancy to increase network availability. The SSO feature takes advantage of RP redundancy by establishing one of the RPs as the active RP while the other RP is designated as the standby RP, and then synchronizing critical state information between them. Following an initial synchronization between the two processors, SSO dynamically maintains RP state information between them.

A switchover from the active to the standby processor occurs when the active RP fails, is removed from the networking device, or is manually taken down for maintenance.

ISSU and SSO—DHCP Server

The DHCP server that is ISSU and SSO aware is able to detect when a router is failing over to the standby RP and preserve the DHCP lease across a switchover event.

Each DHCP binding is synchronized and re-created from the active RP to the standby RP upon lease commit. Figure 1 illustrates this process. The lease extension and release are also synchronized to the standby RP.

Figure 1 DHCP Server Maintaining States Between the Active and Standby Route Processor

ISSU and SSO—DHCP Relay on Unnumbered Interface

The DHCP relay agent supports the use of unnumbered interfaces. For DHCP clients connected through the unnumbered interfaces, the DHCP relay agent automatically adds a static host route once the DHCP client obtains an address, specifying the unnumbered interface as the outbound interface. The route is automatically removed once the lease time expires or when the client releases the address.

The ip helper-address interface configuration command must be configured on the unnumbered interface to enable the Cisco IOS DHCP relay agent on unnumbered interfaces. See the "Configuring the Cisco IOS DHCP Relay Agent" configuration module for more information.

The ISSU and SSO DHCP relay on unnumbered interface functionality adds high availability support for host routes to clients connected through unnumbered interfaces. The DHCP relay agent can now detect when a router is failing over to the standby RP and keep the states related to unnumbered interfaces. Figure 2 illustrates the process.

Figure 2 DHCP Maintaining States with an IP Unnumbered Interface

ISSU and SSO—DHCP Proxy Client

The DHCP proxy client enables the router to obtain a lease for configuration parameters from a DHCP server for a remote Point-to-Point Protocol (PPP) client. The DHCP proxy client that is ISSU and SSO aware is able to request a lease from the DHCP server and the state of the lease is synchronized between the active and standby RP. Figure 3 illustrates the process.

Figure 3 DHCP Proxy Client Lease Synchronization

ISSU and SSO—DHCP ODAP Client and Server

The DHCP on-demand address pool (ODAP) client that is ISSU and SSO aware can request a lease for a subnet from the DHCP ODAP server. After the DHCP ODAP server allocates the subnet to the client, the state of the lease is synchronized between the active and standby RP through binding updates. Following a switchover event, the DHCP ODAP client can continue to allocate IP addresses from the same subnets and also continue to renew the subnets from the DHCP ODAP server. Figure 4 illustrates the process.

Figure 4 ODAP Subnet Lease Synchronization

How to Configure DHCP High Availability

There are no configuration tasks. The DHCP high availability features are enabled by default when the redundancy mode of operation is set to SSO.

Configuration Examples for DHCP High Availability

There are no configuration examples for DHCP high availability features.

Additional References

The following sections provide references related to DHCP high availability features.

Related Documents

Related Topic
Document Title

DHCP commands: complete command syntax, command modes, command history, defaults, usage guidelines, and examples

"DHCP Commands" chapter of the Cisco IOS IP Addressing Services Command Reference, Release 12.4T.

DHCP conceptual and configuration information

Cisco IOS IP Addressing Services Configuration Guide, Release 12.4T

In-Service Software Upgrade process conceptual and configuration information

Cisco IOS In-Service Software Upgrade Process feature module, Release 12.2(31)SB2

Nonstop Forwarding conceptual and configuration information

Cisco Nonstop Forwarding feature module, Release 12.2(31)SB2

Stateful switchover conceptual and configuration information

Stateful Switchover feature module, Release 12.2(31)SB2


Standards

Standard
Title

No new or modified standards are supported by this feature.

—


MIBs

MIB
MIBs Link

No new or modified MIBs are supported 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

RFC
Title

No new or modified RFCs are supported by this feature.

—


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, tools, and technical documentation. 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 new commands only.

•debug dhcp redundancy

•debug ip dhcp server redundancy

debug dhcp redundancy

To display debugging information about DHCP proxy client redundancy events, use the debug dhcp redundancy command in privileged EXEC mode. To disable the display of debugging output, use the no form of this command.

debug dhcp redundancy

no debug dhcp redundancy

Syntax Description

This command has no arguments or keywords.

Command Default

Debugging output is disabled for DHCP redundancy events.

Command Modes

Privileged EXEC

Command History

Release
Modification

12.2(31)SB2

This command was introduced.


Examples

The following example displays debug messages regarding DHCP redundancy events. The last line is output when the debug dhcp redundancy command is enabled. The line indicates that the active Route Processor has sent a dynamic lease synchronization message for IP address 10.1.1.1: 

Router# debug dhcp redundancy


*Mar 15 10:32:21: DHCPD: assigned IP address 10.1.1.1 to client

*Mar 15 10:32:21: DHCPD: dynamic sync sent for 10.1.1.1

Related Commands

Command
Description

debug ip dhcp server redundancy

Displays debugging information about DHCP server and relay agent redundancy events.



debug ip dhcp server redundancy

To display debugging information about DHCP server and relay agent redundancy events, use the debug ip dhcp server redundancy command in privileged EXEC mode. To disable the display of debugging output, use the no form of this command.

debug ip dhcp server redundancy

no debug ip dhcp server redundancy

Syntax Description

This command has no arguments or keywords.

Command Default

Debugging output is disabled for DHCP server and relay agent redundancy events.

Command Modes

Privileged EXEC

Command History

Release
Modification

12.2(33)ZW

This command was introduced.

12.2(31)SB2

This command was integrated into Cisco IOS release 12.2(31)SB2.


Usage Guidelines

Use this command with caution. Many bindings being synchronized between the active and standby Route Processor (RP) can trigger a large amount of debugging output.

Examples

The following example displays debug messages regarding DHCP server and relay agent redundancy events. The last line (and only that line) is output when the debug ip dhcp server redundancy command is enabled. The line indicates that a binding update message has been sent to the standby for the IP address 10.0.0.2 in the pool named "test." 

Router# debug ip dhcp server redundancy


*Mar 22 10:32:21: DHCPD: assigned IP address 10.0.0.2 to client 
0063.6973.636f.2d30.3030.342e.3465.6130.2e30.3831.632d.4661.312f.302e.31.

*Mar 22 10:32:21:  DHCPD: lease time = 3600

*Mar 22 10:32:21: DHCPD: dhcpd_lookup_route: host = 10.0.0.2

*Mar 22 10:32:21: DHCPD: dhcpd_lookup_route: index = 0

*Mar 22 10:32:21: DHCPD: dhcpd_create_and_hash_route: host = 10.0.0.2

*Mar 22 10:32:21: DHCPD: dhcpd_create_and_hash_route index = 0

*Mar 22 10:32:21: DHCPD: dhcpd_add_route: lease = 3600 

*Mar 22 10:32:21: DHCPD: dynamic sync completed for 10.0.0.2 in pool test

Related Commands

Command
Description

debug dhcp redundancy

Displays debugging information about DHCP proxy client redundancy events.


Feature Information for DHCP High Availability Features

Table 1 lists the release history for these features.

Not all commands may be available in your Cisco IOS software release. For release information about a specific command, see the command reference documentation.

Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which Cisco IOS and Catalyst OS software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Note Table 1 lists only the Cisco IOS software release that introduced support for a given feature in a given Cisco IOS software release train. Unless noted otherwise, subsequent releases of that Cisco IOS software release train also support that feature.

Table 1 Feature Information for DHCP High Availability Features 

Feature Name
Releases
Feature Information

ISSU—DHCP Server

12.2(31)SB2

The DHCP server has been enhanced to support ISSU.

SSO—DHCP Server

12.2(31)SB2

12.2(33)SRB

The DHCP server has been enhanced to support SSO.

ISSU—DHCP Relay on Unnumbered Interface

12.2(31)SB2

The DHCP relay on unnumbered interface has been enhanced to support ISSU.

SSO—DHCP Relay on Unnumbered Interface

12.2(31)SB2

12.2(33)SRB

The DHCP relay on unnumbered interface has been enhanced to support SSO.

ISSU—DHCP Proxy Client

12.2(31)SB2

The DHCP proxy client has been enhanced to support ISSU.

SSO—DHCP Proxy Client

12.2(31)SB2

The DHCP proxy client has been enhanced to support SSO.

ISSU—DHCP ODAP Client and Server

12.2(31)SB2

The DHCP client and server have been enhanced to support ISSU.

SSO—DHCP ODAP Client and Server

12.2(31)SB2

The DHCP ODAP client and server have been enhanced to support SSO.


Glossary

CPE—customer premises equipment. Terminating equipment, such as terminals, telephones, and modems, supplied by the service provider, installed at customer sites, and connected to the network.

DSLAM—digital subscriber line access multiplexer. A device that connects many digital subscriber lines to a network by multiplexing the DSL traffic onto one or more network trunk lines.

ISSU—In Service Software Upgrade. ISSU is a process that allows Cisco IOS software to be updated or otherwise modified while packet forwarding continues.

ODAP—On-Demand Address Pool. ODAPs enable pools of IP addresses to be dynamically increased or reduced in size depending on the address utilization level. Once configured, the ODAP is populated with one or more subnets leased from a source server and is ready to serve address requests from DHCP clients or from PPP sessions.

RP—Route Processor. A generic term for the centralized control unit in a chassis.

SSO—Stateful Switchover. SSO refers to the implementation of Cisco IOS software that allows applications and features to maintain a defined state between an active and standby RP. When a switching occurs, forwarding and sessions are maintained. SSO makes an RP failure undetectable to the network.

Note See Internetworking Terms and Acronyms for terms not included in this glossary.


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.

© 2006 Cisco Systems, Inc. All rights reserved.

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