Downloads |
 Feedback
|
Table Of Contents
Prerequisites for IS-IS Incremental SPF
Information About IS-IS Incremental SPF
Benefits of IS-IS Incremental SPF
How to Enable IS-IS Incremental SPF
Configuration Examples for IS-IS Incremental SPF
IS-IS Incremental SPF
Integrated Intermediate System-to-Intermediate System (IS-IS) can be configured to use an incremental SPF algorithm for calculating the shortest path first routes. Incremental SPF is more efficient than the full SPF algorithm, thereby allowing IS-IS to converge faster on a new routing topology in reaction to a network event.
History for the IS-IS Incremental SPF Feature
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.
Contents
•
Prerequisites for IS-IS Incremental SPF
•
•
•
Prerequisites for IS-IS Incremental SPF
It is presumed that you have IS-IS configured in your network.
Information About IS-IS Incremental SPF
Before you enable the IS-IS Incremental SPF feature, you should understand the concept described in this section.
•
Benefits of IS-IS Incremental SPF
IS-IS uses Dijkstra's SPF algorithm to compute the shortest path tree (SPT). During the computation of the SPT, the shortest path to each node is discovered. The topology tree is used to populate the routing table with routes to IP networks. When changes occur, the entire SPT is recomputed. In many cases, the entire SPT need not be recomputed because most of the tree remains unchanged. Incremental SPF allows the system to recompute only the affected part of the tree. Recomputing only a portion of the tree rather than the entire tree results in faster IS-IS convergence and saves CPU resources.
Incremental SPF computes only the steps needed to apply the changes in the network topology diagram. That process requires that the system keep more information about the topology in order to apply the incremental changes. Also, more processing must be done on each node for which the system receives a new LSP. However, incremental SPF typically reduces demand on CPU.
How to Enable IS-IS Incremental SPF
This section contains the following procedure:
Enabling Incremental SPF
This section describes how to enable incremental SPF.
SUMMARY STEPS
1.
2.
3.
4.
5.
DETAILED STEPS
Configuration Examples for IS-IS Incremental SPF
This section contains an example of configuring IS-IS incremental SPF.
Incremental SPF: Example
This example enables incremental SPF:
router isisispf level-1 60Additional References
The following sections provide references related to IS-IS.
Related Documents
Standards
No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.
—
MIBs
None
To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:
RFCs
No new or modified RFCs are supported by this feature, and support for existing RFCs has not been modified by this feature.
—
Technical Assistance
Command Reference
This section documents a new command only.
•
ispf
To enable incremental shortest path first (SPF), use the ispf command in router configuration mode. To disable incremental SPF, use the no form of this command.
ispf {level-1 | level-2 | level-1-2} [seconds]
no ispf
Syntax Description
Defaults
Incremental SPF is disabled.
seconds: 120Command Modes
Router configuration
Command History
Usage Guidelines
Intermediate System-to-Intermediate System (IS-IS) and Open Shortest Path First (OSPF) use Dijkstra's SPF algorithm to compute the shortest path tree (SPT). During the computation of the SPT, the shortest path to each node is discovered. The topology tree is used to populate the routing table with routes to IP networks. When changes to a Type 1 or Type 2 link-state advertisement (LSA) occur in an area, the entire SPT is recomputed. In many cases, the entire SPT need not be recomputed because most of the tree remains unchanged. Incremental SPF allows the system to recompute only the affected part of the tree. Recomputing only a portion of the tree rather than the entire tree results in faster OSPF convergence and saves CPU resources. Note that if the change to a Type 1 or Type 2 LSA occurs in the calculating router itself, then the full SPT is performed.
Incremental SPF computes only the steps needed to apply the changes in the network topology diagram. That process requires that the system keep more information about the topology in order to apply the incremental changes. Also, more processing must be done on each node for which the system receives a new link-state packet (LSP). However, incremental SPF typically reduces demand on CPU.
Incremental SPF is scheduled in the same way as the full SPF. Routers enabled with incremental SPF and routers not enabled with incremental SPF can function in the same internetwork.
Incremental SPF works only for IPv4.
Even if incremental SPF is configured, there are some cases where full SPF is executed; for example, periodic SPF, a calculation change for the routing calculation (such as a change in metric, is-type, and so on), the configuration of the clear ip route or clear isis commands, or adjacency changes.
Examples
The following example enables OSPF incremental SPF:
Router(config)# router ospf 1Router(config-router)# ispf level-1The following examples enables IS-IS incremental SPF for Level 1 and Level 2 packets:
Router(config)# router isisRouter(config-router)# ispf level-1-2
© 2003-2005 Cisco Systems, Inc. All rights reserved.
