Contents

Reducing Alternate-Path Calculation Times in IS-IS Networks
Finding Feature Information
Prerequisites for Reducing Alternate-Path Calculation Times in IS-IS Networks
Information About Reducing Alternate-Path Calculation Times in IS-IS Networks
Route Redistribution
IS-IS Caching of Redistributed Routes
Prioritizing the Update of IP Prefixes in the RIB to Reduce Alternate-Path Calculation Time
IS-IS Priority-Driven IP Prefix RIB Installation
Tagging IS-IS Routes to Control Their Redistribution
How Route Summarization Can Enhance Scalability in IS-IS Networks
Benefits of IS-IS Route Tags
IS-IS Route Tag Characteristics
IS-IS Route Leaking Based on a Route Tag
Limiting the Number of Routes That Are Redistributed into IS-IS
Streamlining the Routing Table Update Process by Excluding Connected IP Prefixes from LSP Advertisements
Small-Scale Method to Reduce IS-IS Convergence Time
Large-Scale Method to Reduce IS-IS Convergence Time
Benefit of Excluding IP Prefixes of Connected Networks in LSP Advertisements
How to Reduce Alternate-Path Calculation Times in IS-IS Networks
Configuring Incremental SPF
Assigning a High Priority Tag to an IS-IS IP Prefix
Troubleshooting Tips
Tagging Routes for Networks Directly Connected to an Interface
What to Do Next
Tagging Routes Using a Route Map
What to Do Next
Tagging a Summary Address
What to Do Next
Using the Tag to Set Values and or Redistribute Routes
Limiting the Number of IS-IS Redistributed Routes
Requesting a Warning About the Number of Prefixes Redistributed into IS-IS
Excluding Connected IP Prefixes on a Small Scale
Excluding Connected IP Prefixes on a Large Scale
Monitoring IS-IS Network Convergence Time
Configuration Examples for Reducing Alternate-Path Calculation Times in IS-IS Networks
Example Assigning a High Priority Tag Value to an IS-IS IP Prefix
Example Tagging Routes for Networks Directly Connected to an Interface and Redistributing Them
Example Redistributing IS-IS Routes Using a Route Map
Example Tagging a Summary Address and Applying a Route Map
Example Filtering and Redistributing IS-IS Routes Using an Access List and a Route Map
Example IS-IS Limit on Number of Redistributed Routes
Example Requesting a Warning About the Number of Redistributed Routes
Example Excluding Connected IP Prefixes on a Small Scale
Example Excluding Connected IP Prefixes on a Large Scale
Where to Go Next
Additional References
Feature Information for Reducing Alternate-Path Calculation Times in IS-IS Networks

Reducing Alternate-Path Calculation Times in IS-IS Networks

Last Updated: January 30, 2012

The tasks in this module explain how to tune IS-IS and enable routers to more quickly respond to topology changes and network failures when they receive routing updates that advertise topology changes.

Finding Feature Information
Prerequisites for Reducing Alternate-Path Calculation Times in IS-IS Networks
Information About Reducing Alternate-Path Calculation Times in IS-IS Networks
How to Reduce Alternate-Path Calculation Times in IS-IS Networks
Configuration Examples for Reducing Alternate-Path Calculation Times in IS-IS Networks
Where to Go Next
Additional References
Feature Information for Reducing Alternate-Path Calculation Times in IS-IS Networks

Finding Feature Information

Your software release may not support all the features documented in this module. 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 Table at the end of this document.

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

Prerequisites for Reducing Alternate-Path Calculation Times in IS-IS Networks

You should be familiar with the concepts described in the "Overview of IS-IS Fast Convergence" module.

Information About Reducing Alternate-Path Calculation Times in IS-IS Networks

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.

Route Redistribution
IS-IS Caching of Redistributed Routes
Prioritizing the Update of IP Prefixes in the RIB to Reduce Alternate-Path Calculation Time
Tagging IS-IS Routes to Control Their Redistribution
Limiting the Number of Routes That Are Redistributed into IS-IS
Streamlining the Routing Table Update Process by Excluding Connected IP Prefixes from LSP Advertisements

Route Redistribution

Routers are allowed to redistribute external prefixes, or routes, that are learned from any other routing protocol, static configuration, or connected interfaces. The redistributed routes are allowed in either a Level 1 router or a Level 2 router. When Level 2 routes are injected as Level 1 routes, this is called route leaking.

IS-IS Caching of Redistributed Routes

Beginning with Cisco IOS Release 12.0(27)S, and later releases, IS-IS caches routes that are redistributed from other routing protocols or from another IS-IS level into a local redistribution cache that is maintained by IS-IS. Caching occurs automatically and requires no configuration. The caching of redistributed routes improves IS-IS convergence time when routes are being redistributed into IS-IS. IS-IS caching of redistributed routes increases the performance of LSP generation, significantly improving network scalability.

Prioritizing the Update of IP Prefixes in the RIB to Reduce Alternate-Path Calculation Time

The time needed for the IS-IS Routing Information Base (RIB) or routing table to update depends on the number of changed IS-IS prefixes or routes that must be updated. You can tag important IS-IS IP prefixes and configure the router to give priority to the tagged prefixes so that high-priority prefixes are updated first in the RIB. For example, the loopback addresses for the routers in an MPLS VPN environment are considered high priority prefixes.

IS-IS Priority-Driven IP Prefix RIB Installation

IS-IS Priority-Driven IP Prefix RIB Installation

In a network where routers run the IS-IS protocol, convergence is achieved by distributing a consistent view of the topology to all routers in the network. When a network event causes a topology change, a number of steps must occur in order for convergence to occur. The router that initially detects the topology change (for example, an interface state change) must inform other routers of the topology change by flooding updated routing information (in the form of link-state PDUs) to other routers. All routers, including the router that detected the topology change, must utilize the updated topology information to recompute shortest paths (run an SPF), providing the updated output of the SPF calculation to the router's RIB, which will eventually cause the updated routing information to be used to forward packets. Until all routers have performed these basic steps, some destinations may be temporarily unreachable. Faster convergence benefits the network performance by minimizing the period of time during which stale topology information--the previous routing information that will be obsoleted by the updated routing information--is used to forward packets.

After performing an SPF, IS-IS must install updated routes in the RIB. If the number of prefixes advertised by IS-IS is large, the time between the installation of the first prefix and the last prefix is significant. Priority-driven IP prefix RIB installation allows a subset of the prefixes advertised by IS-IS to be designated as having a higher priority. Updates to the paths to these prefixes will be installed before updates to prefixes that do not have this designation. This reduces the convergence time for the important IS-IS IP prefixes and results in faster updating for routes that are dependent on these prefixes. This in turn shortens the time during which stale information is used for forwarding packets to these destinations.

Prefixes are characterized as having one of three levels of importance:

High priority prefixes--prefixes that have been tagged with a tag designated for fast convergence.
Medium priority prefixes--any /32 prefixes that have not been designated as high priority prefixes.
Low priority prefixes--all other prefixes.

When IS-IS updates the RIB, prefixes are updated in the order based on the associated level of importance.

When you assign a high priority tag to some IS-IS IP prefixes, those prefixes with the higher priority are updated in the routing tables before prefixes with lower priority. In some networks, the high priority prefixes will be the provider edge (PE) loopback addresses. The convergence time is reduced for the important IS-IS IP prefixes and results in reduced convergence time for the update processes that occur in the global RIB and Cisco Express Forwarding (CEF).

Tagging IS-IS Routes to Control Their Redistribution

You can control the redistribution of IS-IS routes by tagging them. The term "route leaking" refers to controlling distribution through tagging of routes.

How Route Summarization Can Enhance Scalability in IS-IS Networks
Benefits of IS-IS Route Tags
IS-IS Route Tag Characteristics
IS-IS Route Leaking Based on a Route Tag

How Route Summarization Can Enhance Scalability in IS-IS Networks

Summarization is a key factor that affects the scalability of a routing protocol. Summarization reduces the number of routing updates that are flooded across areas or routing domains. Especially for multi-area IS-IS, a good addressing scheme can optimize summarization by not allowing an overly large Level 2 database that is unnecessarily populated with updates that have come from Level 1 areas.

A router can summarize prefixes on redistribution whether the prefixes have come from internal prefixes, local redistribution, or Level 1 router redistribution. Routes that have been leaked from Level 2 to Level 1 and routes that are advertised into Level 2 from Level 1 can also be summarized.

Benefits of IS-IS Route Tags

The IS-IS Support for Route Tags feature allows you to tag IP addresses of an interface and use the tag to apply administrative policy with a route map.
You can tag IS-IS routes to control their redistribution. You can configure a route map to set a tag for an IS-IS IP prefix (route) and/or match on the tag (perhaps on a different router) to redistribute IS-IS routes. Although the match tag and set tag commands existed for other protocols before this feature, they were not implemented for IS-IS, so they did nothing when specified in an IS-IS network until now.
You can tag a summary route and then use a route map to match the tag and set one or more attributes for the route.

IS-IS Route Tag Characteristics

An IS-IS route tag number can be up to 4 bytes long. The tag value is set into a sub-TLV 1 for TLV (Type Length Value) Type 135. For more information about TLV Type 135, see the Intermediate System-to-Intermediate System (IS-IS) TLVs document referenced in the " Related Documents " section.

Only one tag can be set to an IS-IS IP route (prefix). The tag is sent out in link-state PDUs (LSPs) advertising the route. Setting a tag to a route alone does nothing for your network. You can use the route tag at area or Level 1/Level 2 boundaries by matching on the tag and then applying administrative policies such as redistribution, route summarization, or route leaking.

Configuring a tag for an interface (with the isis tag command) triggers the generation of new LSPs from the router because the tag is new information for the PDUs.

IS-IS Route Leaking Based on a Route Tag

You can tag IS-IS routes to configure route leaking (redistribution). Since only the appropriate routes are redistributed--or leaked--the results is network scalability and faster convergence for the router update. If you configure route leaking and you want to match on a tag, use a route map (not a distribute list). For more information on route leaking, see the IS-IS Route Leaking document referenced in the IS-IS Route Leaking Based on a Route Tag section.

There are two general steps to using IS-IS route tags: tagging routes and referencing the tag to set values for the routes and/or redistribute routes.

There are three ways to tag IS-IS routes: tag routes for networks directly connected to an interface, set a tag in a route map, or tag a summary route. All three methods are described in this section. The tagging method is independent of how you use the tag.

After you tag the routes, you can use the tag to set values (such as a metric, or next hop, and so on) and/or redistribute routes. You might tag routes on one router, but reference the tag on other routers, depending on what you want to achieve. For example, you could tag the interface on Router A with a tag, match the tag on Router B to set values, and redistribute routes on Router C based on values using a route map.

Limiting the Number of Routes That Are Redistributed into IS-IS

If someone mistakenly injects a large number of IP routes into IS-IS, perhaps by redistributing Border Gateway Protocol (BGP) into IS-IS, the network can be severely flooded. Limiting the number of redistributed routes prevents this potential problem. You can either configure IS-IS to stop allowing routes to be redistributed once your maximum configured value has been reached or configure the software to generate a system warning once the number of redistributed prefixes has reached the maximum value. Before configuring the tasks in this section, you should be familiar with the following concept.

LSP Full State

In some cases when a limit is not placed on the number of redistributed routes, the LSP may become full and routes may be dropped. A user can specify which routes should be suppressed in that event so that the consequence of an LSP full state is handled in a graceful and predictable manner.

Redistribution is usually the cause of the LSP full state. By default, external routes redistributed into IS-IS are suppressed if the LSP full state occurs. IS-IS can have 255 fragments for an LSP in a level. When there is no space left in any of the fragments, an LSPFULL error message is generated.

Once the problem that caused the LSP full state is resolved, a user can clear the LSPFULL state.

This section contains the following procedures, which are mutually exclusive. That is, you cannot both limit redistributed prefixes and also choose to be warned only.

Streamlining the Routing Table Update Process by Excluding Connected IP Prefixes from LSP Advertisements

In order to speed up IS-IS convergence, the number of IP prefixes carried in LSPs needs to be limited. Configuring interfaces as unnumbered would limit the prefixes. However, for network management reasons, you might want to have numbered interfaces and also want to prevent advertising interface addresses into IS-IS. There are two alternative methods to avoid the overpopulation of routing tables and thereby reduce IS-IS convergence time. In order to choose the method that will work best for your network, you should become familiar with the following concepts.

Small-Scale Method to Reduce IS-IS Convergence Time
Large-Scale Method to Reduce IS-IS Convergence Time
Benefit of Excluding IP Prefixes of Connected Networks in LSP Advertisements

Small-Scale Method to Reduce IS-IS Convergence Time

You can explicitly configure an IS-IS interface not to advertise its IP network to the neighbors (by using the no isis advertise-prefixcommand).This method is feasible for a small network; it does not scale well. If you have dozens or hundreds of routers in your network, with possibly ten times as many physical interfaces involved, it would be difficult to add this command to each router's configuration.

Large-Scale Method to Reduce IS-IS Convergence Time

An easier way to reduce IS-IS convergence is to configure the IS-IS instance on a router to advertise only passive interfaces (by using the advertise-passive-only command). This command relies on the fact that when enabling IS-IS on a loopback interface, you usually configure the loopback as passive (to prevent sending unnecessary hello PDUs out through it because there is no chance of finding a neighbor behind it). Thus, if you want to advertise only the loopback and if it has already been configured as passive, configuring the advertise-passive-only command per IS-IS instance would prevent the overpopulation of the routing tables.

Benefit of Excluding IP Prefixes of Connected Networks in LSP Advertisements

Whether you choose to prevent the advertising of IS-IS interface subnetworks or to advertise only the IS-IS prefixes that belong to passive (loopback) interfaces, you will reduce IS-IS convergence time. The IS-IS Mechanisms to Exclude Connected IP Prefixes from LSP Advertisements feature is recommended in any case where fast convergence is required.

How to Reduce Alternate-Path Calculation Times in IS-IS Networks

Configuring Incremental SPF
Assigning a High Priority Tag to an IS-IS IP Prefix
Tagging Routes for Networks Directly Connected to an Interface
Tagging Routes Using a Route Map
Tagging a Summary Address
Using the Tag to Set Values and or Redistribute Routes
Limiting the Number of IS-IS Redistributed Routes
Requesting a Warning About the Number of Prefixes Redistributed into IS-IS
Excluding Connected IP Prefixes on a Small Scale
Excluding Connected IP Prefixes on a Large Scale
Monitoring IS-IS Network Convergence Time

Configuring Incremental SPF

SUMMARY STEPS

1. enable

2. configure terminal

3. router isis area-tag

4. ispf [level-1 | level-2 | level-1-2][seconds]

5. end

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Router> enable	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: Router# configure terminal	Enters global configuration mode.
Step 3	router isis area-tag Example: Router(config)# router isis	Enables IS-IS as an IP routing protocol and assigns a tag to a process, if required. Enters router configuration mode.
Step 4	ispf [level-1 \| level-2 \| level-1-2][seconds] Example: Router(config-router)# ispf level-1-2 60	Enables incremental SPF. The seconds argument represents the number of seconds after configuring this command that incremental SPF is activated. Value can be in the range from 1 to 600. The default value is 120 seconds. The seconds argument applies only when you have enabled IS-IS.
Step 5	end Example: Router(config-router)# end	Returns to privileged EXEC mode.

Assigning a High Priority Tag to an IS-IS IP Prefix

SUMMARY STEPS

1. enable

2. configure terminal

3. interface type number

4. ip router isis [area-tag]

5. isis tag tag-value

6. exit

7. router isis [area-tag]

8. ip route priority high tag tag-value

9. end

10. show isis rib [ip-address | ip-address-mask]

DETAILED STEPS

Command or Action

Purpose

Step 1

enable

Example:

Router> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3

interface type number

Example:

Router(config)# interface Ethernet 0

Configures an interface type and enters interface configuration mode.

Step 4

ip router isis [area-tag]

Example:

Router(config-if)# ip router isis tag13

Enables IS-IS as an IP routing protocol, and assigns a tag to a process, if required.

Note

If the area-tag argument is not specified, a null tag is assumed and the process is referenced with a null tag. This name must be unique among all IP or Connectionless Network Service (CLNS) router processes for a given router.

Step 5

isis tag tag-value

Example:

Router(config-if)# isis tag 17

Sets a tag on the IP address configured for an interface when this IP prefix is put into an IS-IS LSP.

The tag-value argument requires an interger in a range from 1 to 4294967295 and serves as a tag on an IS-IS route.

Step 6

exit

Example:

Router(config-if)# exit

Returns to global configuration mode.

Step 7

router isis [area-tag]

Example:

Router(config)# router isis marketing

Enables the IS-IS routing protocol and specifies an IS-IS process. Enters router configuration mode.

Note

If the area-tag argument is not specified, a null tag is assumed and the process is referenced with a null tag. This name must be unique among all IP or CLNS router processes for a given router.

Step 8

ip route priority high tag tag-value

Example:

Router(config-router)# ip route priority high tag 17

Assigns a high priority to prefixes associated with the specified tag value.

Assigns a high priority to IS-IS IP prefixes with a specific route tag in a range from 1 to 4294967295 that you specify for the tag-value argument.

Step 9

end

Example:

Router(config-router)# end

(Optional) Saves configuration commands to the running configuration file and returns to privileged EXEC mode.

Step 10

show isis rib [ip-address | ip-address-mask]

Example:

Router# show isis rib 255.255.255.0

Displays paths for a specific route in the IP Version 4 IS-IS local RIB.

IS-IS maintains a local database for all IS-IS routing information. This local database is referred to as the IS-IS local RIB. It contains additional attributes that are not maintained in the global IP routing table. Access to the contents of the local RIB is used to support the show isis rib command, which is used here to verify routing information related to the Priority-Driven IP Prefix RIB Installation feature.

Troubleshooting Tips

Troubleshooting Tips

You can enter the debug isis rib local command to verify whether the IP prefixes that are advertised by IS-IS link-state PDUs (LSPs) are being updated correctly in the IS-IS local RIB.

Tagging Routes for Networks Directly Connected to an Interface

Before You Begin

Because the IS-IS route tag will be used in a route map, you must understand how to configure a route map.
In order to use the route tag, you must configure the metric-style wide command. (The metric-style narrowcommand is configured by default). The tag value is set into sub-TLV 1 for TLV (Type Length Value) Type 135.
You must understand the task for which you are using the route tag, such as route redistribution, route summarization, or route leaking.

Before you tag any IS-IS routes, you need to decide on the following:

Your goal to set values for routes or redistribute routes (or both).
Where in your network you want to tag routes.
Where in your network you want to reference the tags.
Which tagging method you will use, which determines which task in this section to perform.

SUMMARY STEPS

1. enable

2. configure terminal

3. interface type number

4. ip address ip-address mask

5. ip address ip-address mask secondary

6. isis tag tag-value

7. end

8. show isis database verbose

9. show ip route [ip-address [mask] [longer-prefixes] | protocol [process-id] | list [access-list-number | access-list-name]]

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Router> enable	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: Router# configure terminal	Enters global configuration mode.
Step 3	interface type number Example: Router(config)# interface ethernet 0	Configures an interface.
Step 4	ip address ip-address mask Example: Router(config-if)# ip address 10.1.1.1 255.255.255.0	Sets a primary IP address for an interface. In this example, the network 10.1.1.0 will be tagged.
Step 5	ip address ip-address mask secondary Example: Router(config-if)# ip address 10.2.2.1 255.255.255.0 secondary	(Optional) Sets a secondary IP address for an interface. In this example, the network 10.2.2.0 will be tagged.
Step 6	isis tag tag-value Example: Router(config-if)# isis tag 120	Sets a tag on the IP addresses configured under this interface when those IP prefixes are put into an IS-IS LSP. The tag must be an integer.
Step 7	end Example: Router(config-if)# end	(Optional) Exits configuration mode and returns to privileged EXEC mode.
Step 8	show isis database verbose Example: Router# show isis database verbose	(Optional) Displays details about the IS-IS link-state database, including the route tag. Perform this step if you want to verify the tag.
Step 9	show ip route [ip-address [mask] [longer-prefixes] \| protocol [process-id] \| list [access-list-number \| access-list-name]] Example: Router# show ip route 10.1.1.1 255.255.255.0	(Optional) Displays the current state of the routing table. Perform this step if you want to verify the tag.

What to Do Next

What to Do Next

Applying the tag does nothing of value for your network until you use the tag by referencing it in a route map, either to set values, to redistribute routes, or to do both. Proceed to the section, "Using the Tag to Set Values and or Redistribute Routes."

Tagging Routes Using a Route Map

SUMMARY STEPS

1. enable

2. configure terminal

3. route-map map-tag [permit | deny] [sequence-number]

4. match tag tag-value [...tag-value]

5. Use an additional match command for each match criterion that you want.

6. set tag tag-value

7. Set another value, depending on what else you want to do with the tagged routes.

8. Repeat Step 7 for each value that you want to set.

9. Repeat Steps 3 through 8 for each route-map statement that you want.

10. end

11. show isis database verbose

12. show ip route [ip-address [mask] [longer-prefixes] | protocol [process-id] | [list access-list-number | [access-list-name]]

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Router> enable	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: Router# configure terminal	Enters global configuration mode.
Step 3	route-map map-tag [permit \| deny] [sequence-number] Example: Router(config)# route-map static-color permit 15	Defines the conditions for redistributing routes from one routing protocol into another or from one IS-IS level to another. This command causes the router to enter route-map configuration mode.
Step 4	match tag tag-value [...tag-value] Example: Router(config-route-map)# match tag 15	(Optional) Matches routes tagged with the specified tag numbers. If you are setting a tag for the first time, you cannot match on tag; this step is an option if you are changing tags.
Step 5	Use an additional match command for each match criterion that you want.	(Optional) See the appropriate match commands in the Cisco IOS IP Routing: Protocol-Independent Command Reference Repeat this step for each match criterion you that want.
Step 6	set tag tag-value Example: Router(config-route-map)# set tag 10	Specifies the tag number to set.
Step 7	Set another value, depending on what else you want to do with the tagged routes.	(Optional) See the following set commands in the Cisco IOS IP Routing: Protocol-Independent Command Reference set level set metric set metric-type
Step 8	Repeat Step 7 for each value that you want to set.	(Optional)
Step 9	Repeat Steps 3 through 8 for each route-map statement that you want.	(Optional)
Step 10	end Example: Router(config-route-map)# end	(Optional) Exits configuration mode and returns to privileged EXEC mode.
Step 11	show isis database verbose Example: Router# show isis database verbose	(Optional) Displays details about the IS-IS link-state database, including the route tag. Perform this step if you want to verify the tag.
Step 12	show ip route [ip-address [mask] [longer-prefixes] \| protocol [process-id] \| [list access-list-number \| [access-list-name]] Example: Router# show ip route 10.1.1.1 255.255.255.0	(Optional) Displays the current state of the routing table. Perform this step if you want to verify the tag.

What to Do Next

What to Do Next

Applying the tag does nothing of value for your network until you use the tag by referencing it in a route map, either to set values, to redistribute routes, or to do both. Proceed to the Using the Tag to Set Values and or Redistribute Routes.

Tagging a Summary Address

SUMMARY STEPS

1. enable

2. configure terminal

3. router isis [ area-tag ]

4. metric-style wide

5. summary-address address mask {level-1 | level-1-2 | level-2} [tag tag-value] [metric metric-value]

6. end

7. show isis database verbose

8. show ip route [ip-address [mask] [longer-prefixes] | protocol [process-id] | [list access-list-number | [access-list-name]]

DETAILED STEPS

Command or Action

Purpose

Step 1

enable

Example:

Router> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3

router isis [ area-tag ]

Example:

Router(config)# router isis

Enables IS-IS as an IP routing protocol and assigns a tag to a process, if required.

Enters router configuration mode.

Step 4

metric-style wide

Example:

Router(config-router)# metric-style wide

Configures a router running IS-IS so that it generates and accepts type, length, and value object (TLV) 135 for IP addresses.

Step 5

summary-address address mask {level-1 | level-1-2 | level-2} [tag tag-value] [metric metric-value]

Example:

Router(config-router)# summary-address 192.168.0.0 255.255.0.0 tag 12345 metric 321

Creates aggregate addresses for IS-IS.

Note

If a tagged route is summarized and the tag is not explicitly configured in the summary-address command, then the tag is lost.

Step 6

end

Example:

Router(config-router)# end

(Optional) Exits configuration mode and returns to privileged EXEC mode.

Step 7

show isis database verbose

Example:

Router# show isis database verbose

(Optional) Displays details about the IS-IS link-state database, including the route tag.

Perform this step if you want to verify the tag.

Step 8

show ip route [ip-address [mask] [longer-prefixes] | protocol [process-id] | [list access-list-number | [access-list-name]]

Example:

Router# show ip route 10.1.1.1 255.255.255.0

(Optional) Displays the current state of the routing table.

Perform this step if you want to verify the tag.

What to Do Next

What to Do Next

Applying the tag does nothing of value for your network until you use the tag by referencing it in a route map to set values. It is unlikely that you will redistribute summary routes. Proceed to the "Using the Tag to Set Values and/or Redistribute Routes" section.

Using the Tag to Set Values and or Redistribute Routes

Before You Begin

You must have already applied a tag on the interface, in a route map, or on a summary route. See the Tagging IS-IS Routes to Control Their Redistribution.

SUMMARY STEPS

1. enable

2. configure terminal

3. route-map map-tag [permit | deny] [sequence-number]

4. match tag tag-value

5. Specify a match command for each match criterion that you want.

6. Set a value, depending on what you want to do with the tagged routes.

7. Repeat Step 6 for each value that you want to set.

8. Repeat Steps 3 through 7 for each route-map statement that you want.

9. exit

10. router isis

11. metric-style wide

12. redistribute protocol [process-id] [level-1| level-1-2 | level-2] [metric metric-value] [metric-type type-value] [route-map map-tag]

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Router> enable	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: Router# configure terminal	Enters global configuration mode.
Step 3	route-map map-tag [permit \| deny] [sequence-number] Example: Router(config)# route-map static-color permit 15	Defines the conditions for redistributing routes from one routing protocol into another or from one IS-IS level to another. This command causes you to enter route-map configuration mode.
Step 4	match tag tag-value Example: Router(config-route-map)# match tag 120	(Optional) Applies the subsequent set commands to routes that match routes tagged with this tag number.
Step 5	Specify a match command for each match criterion that you want.	(Optional) Reference the appropriate match commands in the Cisco IOS IP Routing: Protocol-Independent Command Reference.
Step 6	Set a value, depending on what you want to do with the tagged routes.	(Optional) See the following set commands in the Cisco IOS IP Routing: Protocol-Independent Command Reference. set level set metric set metric-type
Step 7	Repeat Step 6 for each value that you want to set.	(Optional)
Step 8	Repeat Steps 3 through 7 for each route-map statement that you want.	(Optional)
Step 9	exit Example: Router(config-route-map)# exit	(Optional) Returns to global configuration mode.
Step 10	router isis Example: Router(config)# router isis	(Optional) Enables the IS-IS routing protocol and specifies an IS-IS process.
Step 11	metric-style wide Example: Router(config-router)# metric-style wide	Configures a router running IS-IS so that it generates and accepts type, length, and value object (TLV) 135 for IP addresses.
Step 12	redistribute protocol [process-id] [level-1\| level-1-2 \| level-2] [metric metric-value] [metric-type type-value] [route-map map-tag] Example: Router(config-router)# redistribute static ip metric 2 route-map static-color	(Optional) Redistributes routes from one routing domain into another routing domain.

Limiting the Number of IS-IS Redistributed Routes

SUMMARY STEPS

1. enable

2. configure terminal

3. router isis [ area-tag ]

4. redistribute protocol [process-id]{level-1| level-1-2| level-2} [as-number] [metric metric-value] [metric-type type-value] [match{internal| external 1| external 2}][tag tag-value] [route-map map-tag]

5. redistribute maximum-prefix maximum [percentage] [warning-only | withdraw]

6. end

DETAILED STEPS

Command or Action

Purpose

Step 1

enable

Example:

Router> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3

router isis [ area-tag ]

Example:

Router(config)# router isis

Enables IS-IS as an IP routing protocol and assigns a tag to a process, if required.

Enters router configuration mode.

Step 4

redistribute protocol [process-id]{level-1| level-1-2| level-2} [as-number] [metric metric-value] [metric-type type-value] [match{internal| external 1| external 2}][tag tag-value] [route-map map-tag]

Example:

Router(config-router)# redistribute eigrp 10 level-1

Redistributes routes from one routing domain into another routing domain.

Step 5

redistribute maximum-prefix maximum [percentage] [warning-only | withdraw]

Example:

Router(config-router)# redistribute maximum-prefix 1000 80

Sets a maximum number of IP prefixes that are allowed to be redistributed into IS-IS.

There is no default value for the maximum argument.
The percentage value defaults to 75 percent.
If the withdraw keyword is specified and the maximum number of prefixes is exceeded, IS-IS rebuilds the link-state protocol data unit (PDU) fragments without the external IP prefixes. That is, the redistributed prefixes are removed from the PDUs.

Note

If the warning-only keyword had been configured in this command, no limit would be enforced; a warning message is simply logged.

Step 6

end

Example:

Router(config-router)# end

Exits router configuration mode.

Requesting a Warning About the Number of Prefixes Redistributed into IS-IS

SUMMARY STEPS

1. enable

2. configure terminal

3. router isis [area- tag ]

5. redistribute maximum-prefix maximum [percentage] [warning-only | withdraw]

6. lsp-full suppress {[external] [interlevel] | none}

7. end

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Router> enable	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: Router# configure terminal	Enters global configuration mode.
Step 3	router isis [area- tag ] Example: Router(config)# router isis	Enables IS-IS as an IP routing protocol and assigns a tag to a process, if required. Enters router configuration mode.
Step 4	redistribute protocol [process-id]{level-1\| level-1-2\| level-2} [as-number] [metric metric-value] [metric-type type-value] [match{internal\| external 1\| external 2}][tag tag-value] [route-map map-tag] Example: Router(config-router)# redistribute eigrp 10 level-1	Redistributes routes from one routing domain into another routing domain.
Step 5	redistribute maximum-prefix maximum [percentage] [warning-only \| withdraw] Example: Router(config-router)# redistribute maximum-prefix 1000 80 warning-only	Causes a warning message to be logged when the maximum number of IP prefixes has been redistributed into IS-IS. Because the warning-only keyword is included, no limit is imposed on the number of redistributed prefixes into IS-IS. There is no default value for the maximum argument. The percentage value defaults to 75 percent. In this example configuration, two warnings are generated: one at 80 percent of 1000 (800 prefixes redistributed) and another at 1000 prefixes redistributed.
Step 6	lsp-full suppress {[external] [interlevel] \| none} Example: Router(config-router)# lsp-full suppress external interlevel	(Optional) Controls which routes are suppressed when the link-state PDU becomes full. The default is external(redistributed routes are suppressed). The interlevel keyword causes routes from another level to be suppressed. The external and interval keywords can be specified together or separately. See the Requesting a Warning About the Number of Prefixes Redistributed into IS-IS".
Step 7	end Example: Router(config-router)# end	Exits router configuration mode.

Excluding Connected IP Prefixes on a Small Scale

SUMMARY STEPS

1. enable

2. configure terminal

3. interface type number

4. ip address ip-address netmask

5. no ip directed-broadcast

6. ip router isis [area- tag ]

7. no isis advertise-prefix

8. exit

9. Repeat Steps 3 through 8 for each interface on which you do not want to advertise IP prefixes.

10. router isis [area- tag ]

11. net network-entity-title

12. end

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Router> enable	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: Router# configure terminal	Enters global configuration mode.
Step 3	interface type number Example: Router(config)# interface Ethernet 0	Configures an interface type and enters interface configuration mode.
Step 4	ip address ip-address netmask Example: Router(config-if)# ip address 192.168.20.1 255.255.255.0	Sets a primary IP address for an interface. The network mask can be indicated as a 4-part dotted decimal address or as a prefix. This example uses a 4-part dotted decimal number.
Step 5	no ip directed-broadcast Example: Router(config-if)# no ip directed-broadcast	(Optional) Disables the translation of a directed broadcast to physical broadcasts.
Step 6	ip router isis [area- tag ] Example: Router(config-if)# ip router isis	Configures an IS-IS routing process for IP on an interface and attaches an area designator to the routing process.
Step 7	no isis advertise-prefix Example: Router(config-if)# no isis advertise-prefix	Prevents the advertising of IP prefixes of connected networks in LSP advertisements per IS-IS interface.
Step 8	exit Example: Router(config-if)# exit	Returns to global configuration mode.
Step 9	Repeat Steps 3 through 8 for each interface on which you do not want to advertise IP prefixes.	(Optional)
Step 10	router isis [area- tag ] Example: Router(config)# router isis	Enables IS-IS as an IP routing protocol and assigns a tag to a process, if required. Enters router configuration mode.
Step 11	net network-entity-title Example: Router(config-router)# net 47.0004.004d.0001.0001.0c11.1111.00	Configures an IS-IS network entity title (NET) for the routing process.
Step 12	end Example: Router(config-router)# end	(Optional) Saves configuration commands to the running configuration file, exits configuration mode, and returns to privileged EXEC mode.

Excluding Connected IP Prefixes on a Large Scale

SUMMARY STEPS

1. enable

2. configure terminal

3. interface loopback number

4. ip address ip-address netmask

5. no ip directed-broadcast

6. exit

7. interface type number

8. ip address ip-address netmask

9. no ip directed-broadcast

10. ip router isis [area- tag ]

11. exit

12. router isis [area- tag ]

13. passive-interface [default] type number

14. net network-entity-title

15. advertise-passive-only

16. end

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Router> enable	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: Router# configure terminal	Enters global configuration mode.
Step 3	interface loopback number Example: Router(config)# interface loopback 0	Configures a loopback interface and enters interface configuration mode.
Step 4	ip address ip-address netmask Example: Router(config-if)# ip address 192.168.10.1 255.255.255.255	Sets a primary IP address for an interface. The network mask can be indicated as a 4-part dotted decimal address or as a prefix. This example uses a 4-part dotted decimal number.
Step 5	no ip directed-broadcast Example: Router(config-if)# no ip directed-broadcast	(Optional) Disables the translation of a directed broadcast to physical broadcasts.
Step 6	exit Example: Router(config-if)# exit	Returns to global configuration mode.
Step 7	interface type number Example: Router(config)# interface Ethernet 0	Configures an interface type and enters interface configuration mode.
Step 8	ip address ip-address netmask Example: Router(config-if)# ip address 192.168.20.1 255.255.255.0	Sets a primary IP address for an interface. The network mask can be indicated as a 4-part dotted decimal address or as a prefix. This example uses a 4-part dotted decimal number.
Step 9	no ip directed-broadcast Example: Router(config-if)# no ip directed-broadcast	(Optional) Disables the translation of a directed broadcast to physical broadcasts.
Step 10	ip router isis [area- tag ] Example: Router(config-if)# ip router isis	Configures an IS-IS routing process for IP on an interface and attaches an area designator to the routing process.
Step 11	exit Example: Router(config-if)# exit	Returns to global configuration mode.
Step 12	router isis [area- tag ] Example: Router(config)# router isis	Enables IS-IS as an IP routing protocol and assigns a tag to a process, if required. Enters router configuration mode.
Step 13	passive-interface [default] type number Example: Router(config-router)# passive-interface loopback 0	Disables sending routing updates on an interface.
Step 14	net network-entity-title Example: Router(config-router)# net 47.0004.004d.0001.0001.0c11.1111.00	Configures an IS-IS NET for the routing process.
Step 15	advertise-passive-only Example: Router(config-router)# advertise-passive-only	Configures IS-IS to advertise only prefixes that belong to passive interfaces.
Step 16	end Example: Router(config-router)# end	(Optional) Saves configuration commands to the running configuration file, exits configuration mode, and returns to privileged EXEC mode.

Monitoring IS-IS Network Convergence Time

SUMMARY STEPS

1. enable

2. configure terminal

3. isis display delimiter [return count | character count]

4. exit

5. show isis database [level-1] [level-2] [l1] [l2] [detail] [lspid]

6. show isis [area-tag] routes

7. show isis [area-tag] [ipv6 | *] spf-log

8. show isis [process-tag] topology

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Router> enable	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: Router# configure terminal	Enters global configuration mode.
Step 3	isis display delimiter [return count \| character count] Example: Router(config)# isis display delimiter return 2	Makes output from multiarea displays easier to read by specifying the delimiter to use to separate displays of information.
Step 4	exit Example: Router(config)# exit	Returns to privileged EXEC mode.
Step 5	show isis database [level-1] [level-2] [l1] [l2] [detail] [lspid] Example: Router# show isis database detail	Displays the IS-IS link-state database.
Step 6	show isis [area-tag] routes Example: Router# show isis financetag routes	Displays the IS-IS Level 1 forwarding table for IS-IS learned routes.
Step 7	show isis [area-tag] [ipv6 \| ] spf-log Example:* Router# show isis spf-log	Displays how often and why the router has run a full SPF calculation.
Step 8	show isis [process-tag] topology Example: Router# show isis financetag topology	Displays a list of all connected routers in all areas. If a process tag is specified, output is limited to the specified routing process. When "null" is specified for the process tag, output is displayed only for the router process that has no tag specified. If a process tag is not specified, output is displayed for all processes.

Examples

The following sample output from the show isis spf-log command displays the following important information:

At what time the SPFs have been executed
Total elapsed time for the SPT computation, LRIB update, and RIB and FIB update
Number of nodes in the graph
Number of triggers that caused the SPF calculation
Information regarding what triggered the SPF calculation

Router# show isis spf-log

   Level 1 SPF log

  When   Duration  Nodes  Count     Last trigger LSP   Triggers

00:15:46    3124     40      1          milles.00-00  TLVCODE

00:15:24    3216     41      5          milles.00-00  TLVCODE NEWLSP

00:15:19    3096     41      1          deurze.00-00  TLVCODE

00:14:54    3004     41      2          milles.00-00  ATTACHFLAG LSPHEADER

00:14:49    3384     41      1          milles.00-01  TLVCODE

00:14:23    2932     41      3          milles.00-00  TLVCODE

00:05:18    3140     41      1                        PERIODIC

00:03:54    3144     41      1          milles.01-00  TLVCODE

00:03:49    2908     41      1          milles.01-00  TLVCODE

00:03:28    3148     41      3           bakel.00-00  TLVCODE TLVCONTENT

00:03:15    3054     41      1          milles.00-00  TLVCODE

00:02:53    2958     41      1          mortel.00-00  TLVCODE

Configuration Examples for Reducing Alternate-Path Calculation Times in IS-IS Networks

Example Assigning a High Priority Tag Value to an IS-IS IP Prefix
Example Tagging Routes for Networks Directly Connected to an Interface and Redistributing Them
Example Redistributing IS-IS Routes Using a Route Map
Example Tagging a Summary Address and Applying a Route Map
Example Filtering and Redistributing IS-IS Routes Using an Access List and a Route Map
Example IS-IS Limit on Number of Redistributed Routes
Example Requesting a Warning About the Number of Redistributed Routes
Example Excluding Connected IP Prefixes on a Small Scale
Example Excluding Connected IP Prefixes on a Large Scale

Example Assigning a High Priority Tag Value to an IS-IS IP Prefix

The following example uses the ip route priority high command to assign a tag value of 200 to the IS-IS IP prefix:

interface Ethernet 0
 ip router isis
 isis tag 200
!
router isis
 ip route priority high tag 200

Example Tagging Routes for Networks Directly Connected to an Interface and Redistributing Them

In this example, two interfaces are tagged with different tag values. By default, these two IP addresses would have been put into the IS-IS Level 1 and Level 2 database. However, by using the redistribute command with a route map to match tag 110, only IP address 172.16.10.5 255.255.255.0 is put into the Level 2 database.

interface ethernet 1/0
 ip address 192.168.129.1 255.255.255.0
 ip router isis
 isis tag 120
interface ethernet 1/1
 ip address 172.16.10.5 255.255.255.0
 ip router isis
 isis tag 110
router isis
 net 49.0001.0001.0001.0001.00
 redistribute isis ip level-1 into level-2 route-map match-tag
route-map match-tag permit 10
 match tag 110

Example Redistributing IS-IS Routes Using a Route Map

In a scenario using route tags, you might configure some commands on one router and other commands on another router. For example, you might have a route map that matches on a tag and sets a different tag on a router at the edge of a network, and on different routers you might configure the redistribution of routes based on a tag in a different route map.

Figure 1

Example of Redistributing IS-IS Routes Using a Route Map

The figure above illustrates a flat Level 2 IS-IS area. On the left edge are static routes from Router A to reach some IP prefixes. Router A redistributes the static routes into IS-IS. Router B runs BGP and redistributes IS-IS routes into BGP and then uses the tag to apply different administrative policy based on different tag values.

Router A

router isis
 net 49.0000.0000.0001.00
 metric-style wide
 redistribute static ip route-map set-tag
!
route-map set-tag permit 5
 set tag 10

Router B

router bgp 100
 redistribute isis level-2 route-map tag-policy
route-map tag-policy permit 20
 match tag 10
 set metric 1000

Example Tagging a Summary Address and Applying a Route Map

The figure below illustrates two Level 1 areas and one Level 2 area between them. Router A and Router B are Level 1/Level 2 edge routers in the Level 2 area. On edge Router A, a summary address is configured to reduce the number of IP addresses put into the Level 2 IS-IS database. Also, a tag value of 100 is set to the summary address.

On Router B, the summary address is leaked into the Level 1 area, and administrative policy is applied based on the tag value.

Figure 2

Tag on a Summary Address

Router A

router isis
 net 49.0001.0001.0001.00
 metric-style wide
 summary-address 10.0.0.0 255.0.0.0 tag 100

Router B

router isis
 net 49.0002.0002.0002.0002.0
 metric-style wide
 redistribute isis ip level-2 into level-1 route-map match-tag
route-map match-tag permit 10
 match tag 100

Example Filtering and Redistributing IS-IS Routes Using an Access List and a Route Map

In this example, the first redistribute isis ip command controls the redistribution of Level 1 routes into Level 2. Only the routes with the tag of 90 and whose IP prefix is not 192.168.130.5/24 will be redistributed from Level 1 into Level 2.

The second redistribute isis ip command controls the route leaking from Level 2 into the Level 1 domain. Only the routes tagged with 60 or 50 will be redistributed from Level 2 into Level 1.

interface ethernet 1
 ip address 192.168.130.5 255.255.255.0
 ip router isis
 isis tag 60
!
interface ethernet 2
 ip address 192.168.130.15 255.255.255.0
 ip router isis
 isis tag 90
!
interface ethernet 3
 ip address 192.168.130.25 5 255.255.255.0
 ip router isis
 isis tag 50
!
router isis
 net 49.0001.0001.0001.0001.00
 metric-style wide
 redistribute isis ip level-1 into level-2 route-map redist1-2
 redistribute isis ip level-2 into level-1 route-map leak2-1
!
access-list 102 deny ip host 192.168.130.5 host 255.255.255.255
access-list 102 permit ip any any
!
route-map leak2-1 permit 10
 match tag 60
!
route-map leak2-1 permit 20
 match tag 50
!
route-map redist1-2 permit 10
 match ip address 102
 match tag 90

Example IS-IS Limit on Number of Redistributed Routes

This example shows how to set a maximum of 1200 prefixes that can be redistributed into IS-IS. When the number of prefixes redistributed reaches 80 percent of 1200 (960 prefixes), a warning message is logged. When 1200 prefixes are redistributed, IS-IS rebuilds the LSP fragments without external prefixes and no redistribution occurs.

router isis 1
 redistribute maximum-prefix 1200 80 withdraw

Example Requesting a Warning About the Number of Redistributed Routes

This example shows how to allow two warning messages to be logged. The first message is generated if the number of prefixes redistributed reaches 85 percent of 600 (510 prefixes), and the second message is generated if the number of redistributed prefixes reaches 600. However, the number of redistributed prefixes is not limited. If the LSPFULL state occurs, external prefixes will be suppressed.

router isis 1
 redistribute maximum-prefix 600 85 warning-only
 lsp-full suppress external

Example Excluding Connected IP Prefixes on a Small Scale

The following example uses the no isis advertise-prefix command on Ethernet interface 0. Only the IP address of loopback interface 0 is advertised.

!
interface loopback 0
 ip address 192.168.10.1 255.255.255.255
 no ip directed-broadcast
!
interface Ethernet 0
 ip address 192.168.20.1 255.255.255.0
 no ip directed-broadcast
 ip router isis 
 no isis advertise-prefix
.
.
.
router isis 
 passive-interface loopback 0
 net 47.0004.004d.0001.0001.0c11.1111.00
 log-adjacency-changes
!

Example Excluding Connected IP Prefixes on a Large Scale

The following example uses the advertise-passive-only command, which applies to the entire IS-IS instance, thereby preventing IS-IS from advertising the IP network of Ethernet interface 0. Only the IP address of loopback interface 0 is advertised.

!
interface loopback 0
 ip address 192.168.10.1 255.255.255.255
 no ip directed-broadcast
!
interface Ethernet0
 ip address 192.168.20.1 255.255.255.0
 no ip directed-broadcast
 ip router isis 
.
.
.
router isis 
 passive-interface Loopback0
 net 47.0004.004d.0001.0001.0c11.1111.00
 advertise-passive-only
 log-adjacency-changes
!

Where to Go Next

To configure features to improve IS-IS network convergence times, complete the optional tasks in one or more of the following modules:

"Overview of IS-IS Fast Convergence"
"Reducing Failure Detection Times in IS-IS Networks"
"Reducing Link Failure and Topology Change Notification Times in IS-IS Networks"

Additional References

RFCs

RFC	Title
No new or modified RFCs are supported, and support for existing RFCs has not been modified.	--

Technical Assistance

Description	Link
The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.	http://www.cisco.com/cisco/web/support/index.html

Feature Information for Reducing Alternate-Path Calculation Times in IS-IS Networks

The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

Table 1

Feature Information for Reducing Alternate-Path Calculation Times in IS-IS Networks

Feature Name	Releases	Feature Information
IS-IS Caching of Redistributed Routes	12.0(27)S 12.2(25)S 12.3(7)T Cisco IOS XE 3.1.0 SG	The IS-IS Caching of Redistributed Routes feature improves Intermediate System-to-Intermediate System (IS-IS) convergence time when routes are being redistributed into IS-IS. This document introduces new commands for monitoring and maintaining IS-IS redistributed routes.
IS-IS Incremental SPF	12.0(24)S 12.2(18)S 12.3(2)T Cisco IOS XE 3.1.0 SG	Integrated 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.
IS-IS Limit on Number of Redistributed Routes	12.0(25)S 12.2(18)S 12.3(4)T Cisco IOS XE 3.1.0 SG	The IS-IS Limit on Number of Redistributed Routes feature provides for a user-defined maximum number of prefixes that are allowed to be redistributed into IS-IS from other protocols or other IS-IS processes. Such a limit can help prevent the router from being flooded by too many redistributed routes.
IS-IS Mechanisms to Exclude Connected IP Prefixes from LSP Advertisements	12.0(22)S 12.2(18)S 12.3(2)T Cisco IOS XE 3.1.0 SG	This document describes two Integrated Intermediate System-to-Intermediate System (IS-IS) mechanisms to exclude IP prefixes of connected networks from link-state PDU (LSP) advertisements, thereby reducing IS-IS convergence time.
IS-IS Support for Priority-Driven IP Prefix RIB Installation	12.0(26)S 12.2(18)SXE 12.2(25)S 12.3(4)T Cisco IOS XE 3.1.0 SG	The IS-IS Support for Priority-Driven Prefix RIB Installation feature allows customers to designate a subset of IP prefixes advertised by IS-IS for faster processing and installation in the global routing table as one way to achieve faster convergence. For example, Voice over IP (VoIP) gateway addresses may need to be processed first to help VoIP traffic get updated faster than other types of packets.
IS-IS Support for Route Tags	12.2(18)S 12.2(27)SBC 12.3(2)T Cisco IOS XE 3.1.0 SG	The IS-IS Support for Route Tags feature provides the capability to tag IS-IS route prefixes and use those tags in a route map to control IS-IS route redistribution or route leaking.

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Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.