Table Of Contents
ISSU and SSO—DHCP High Availability Features
First Published: December 4, 2006Last Updated: May 4, 2009
Cisco IOS XE Release 2.1 and 2.3 introduce the following series of Dynamic Host Configuration Protocol (DHCP) High Availability features:
•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
For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the "Feature Information for DHCP High Availability Features" section.
Use Cisco Feature Navigator to find information about platform support and Cisco IOS XE software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
Prerequisites for DHCP High Availability
•The Cisco IOS XE In-Service Software Upgrade (ISSU) process must be configured and working properly. See the "Cisco IOS XE 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:
The ISSU process allows Cisco IOS XE 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 XE software to be modified while packet forwarding continues, which increases network availability and reduces downtime caused by planned software upgrades.
SSO refers to the implementation of Cisco IOS XE 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 XE DHCP relay agent on unnumbered interfaces. See the "Configuring the Cisco IOS XE 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.
The following sections provide references related to DHCP high availability features.
Related Topic Document Title
DHCP commands: complete command syntax, command modes, command history, defaults, usage guidelines, and examples
DHCP conceptual and configuration information
Cisco IOS XE IP Addressing Services Configuration Guide
In-Service Software Upgrade process conceptual and configuration information
Nonstop Forwarding conceptual and configuration information
Cisco Nonstop Forwarding module
Stateful switchover conceptual and configuration information
Stateful Switchover module
MIB MIBs Link
No new or modified MIBs are supported by this feature.
To locate and download MIBs for selected platforms, Cisco IOS XE software releases, and feature sets, use Cisco MIB Locator found at the following URL:
Feature Information for DHCP High Availability Features
Table 1 lists the features in this module and provides links to specific configuration information.
Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which Cisco IOS XE 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 XE software release that introduced support for a given feature in a given Cisco IOS XE software release train. Unless noted otherwise, subsequent releases of that Cisco IOS XE software release train also support that feature.
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.
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