- Integrated IS-IS Routing Protocol Overview
- Configuring a Basic IS-IS Network
- Customizing IS-IS for Your Network Design
- IS-IS MIB
- IS-IS Support for an IS-IS Instance per VRF for IP
- Overview of IS-IS Fast Convergence
- Setting Best Practice Parameters for IS-IS Fast Convergence
- Reducing Failure Detection Times in IS-IS Networks
- Reducing Link Failure and Topology Change Notification Times in IS-IS Networks
- Reducing Alternate-Path Calculation Times in IS-IS Networks
- Enhancing Security in an IS-IS Network
- 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
- IS-IS Priority-Driven IP Prefix RIB Installation
- 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
- 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
- Configuring Incremental SPF
- 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
- Monitoring IS-IS Network Convergence Time
- 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
Reducing Alternate-Path Calculation Times in IS-IS Networks
The tasks in this module explain how to tune Intermediate System-to-Intermediate System (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
- 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
Before performing the tasks in this module, 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
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
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 link-state packet (LSP) protocol data unit (PDU) generation, significantly improving network scalability.
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 link-state PDU. However, incremental SPF typically reduces demand on CPU.
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
- 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
- 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
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 LSPs) 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).
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
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.
Only one tag can be set to an IS-IS IP route (prefix). The tag is sent out in link-state PDUs 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).
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.
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.
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
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
- 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
- Monitoring IS-IS Network Convergence Time
Configuring Incremental SPF
DETAILED STEPS
Prioritizing the Update of IP Prefixes in the RIB to Reduce Alternate-Path Calculation Time
DETAILED STEPS
Troubleshooting Tips
You can enter the debug isis rib local command to verify whether the IP prefixes that are advertised by IS-IS LSPs are being updated correctly in the IS-IS local RIB.
Tagging IS-IS Routes to Control Their Redistribution
- Prerequisites
- 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
Prerequisites
- 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.
Tagging Routes for Networks Directly Connected to an Interface
DETAILED STEPS
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, to redistribute routes, or to do both. Proceed to the Using the Tag to Set Values and or Redistribute Routes.
Tagging Routes Using a Route Map
DETAILED STEPS
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, to redistribute routes, or to do both. Proceed to the Using the Tag to Set Values and or Redistribute Routes.
Tagging a Summary Address
If a tagged route is summarized and the tag is not explicitly configured in the summary-address command, then the tag is lost.
DETAILED STEPS
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
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.
DETAILED STEPS
Limiting the Number of Routes That Are Redistributed into IS-IS
- Limiting the Number of IS-IS Redistributed Routes
- Requesting a Warning About the Number of Prefixes Redistributed into IS-IS
Limiting the Number of IS-IS Redistributed Routes
DETAILED STEPS
Requesting a Warning About the Number of Prefixes Redistributed into IS-IS
DETAILED STEPS
| Command or Action | Purpose | |
|---|---|---|
|
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Example: Router> enable |
Enables privileged EXEC mode.
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Example: Router# configure terminal |
Enters global configuration mode. |
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Example: Router(config)# router isis |
Enables IS-IS as an IP routing protocol and assigns a tag to a process, if required.
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Example: Router(config-router)# redistribute eigrp 10 level-1 |
Redistributes routes from one routing domain into another routing domain. |
|
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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.
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Example: Router(config-router)# lsp-full suppress external interlevel |
(Optional) Controls which routes are suppressed when the link-state PDU becomes full.
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Example: Router(config-router)# end |
Exits router configuration mode. |
Streamlining the Routing Table Update Process by Excluding Connected IP Prefixes from LSP Advertisements
Excluding Connected IP Prefixes on a Small Scale
DETAILED STEPS
| Command or Action | Purpose | |
|---|---|---|
|
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Example: Router> enable |
Enables privileged EXEC mode.
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Example: Router# configure terminal |
Enters global configuration mode. |
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Example: Router(config)# interface Gigabitethernet 0/0/0 |
Configures an interface type and enters interface configuration mode. |
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Example: Router(config-if)# ip address 192.168.20.1 255.255.255.0 |
Sets a primary IP address for an interface.
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Example: Router(config-if)# no ip directed-broadcast |
(Optional) Disables the translation of a directed broadcast to physical broadcasts. |
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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. |
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Example: Router(config-if)# no isis advertise-prefix |
Prevents the advertising of IP prefixes of connected networks in LSP advertisements per IS-IS interface. |
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Example: Router(config-if)# exit |
Returns to global configuration mode. |
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(Optional) |
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Example: Router(config)# router isis |
Enables IS-IS as an IP routing protocol and assigns a tag to a process, if required.
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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. |
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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
DETAILED STEPS
| Command or Action | Purpose | |
|---|---|---|
|
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Example: Router> enable |
Enables privileged EXEC mode.
|
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Example: Router# configure terminal |
Enters global configuration mode. |
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Example: Router(config)# interface loopback 0 |
Configures a loopback interface and enters interface configuration mode. |
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Example: Router(config-if)# ip address 192.168.10.1 255.255.255.255 |
Sets a primary IP address for an interface.
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Example: Router(config-if)# no ip directed-broadcast |
(Optional) Disables the translation of a directed broadcast to physical broadcasts. |
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Example: Router(config-if)# exit |
Returns to global configuration mode. |
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Example: Router(config)# interface Gigabitethernet 0/0/0 |
Configures an interface type and enters interface configuration mode. |
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Example: Router(config-if)# ip address 192.168.20.1 255.255.255.0 |
Sets a primary IP address for an interface.
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Example: Router(config-if)# no ip directed-broadcast |
(Optional) Disables the translation of a directed broadcast to physical broadcasts. |
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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. |
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Example: Router(config-if)# exit |
Returns to global configuration mode. |
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Example: Router(config)# router isis |
Enables IS-IS as an IP routing protocol and assigns a tag to a process, if required.
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Example: Router(config-router)# passive-interface loopback 0 |
Disables sending routing updates on an interface. |
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Example: Router(config-router)# net 47.0004.004d.0001.0001.0c11.1111.00 |
Configures an IS-IS NET for the routing process. |
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Example: Router(config-router)# advertise-passive-only |
Configures IS-IS to advertise only prefixes that belong to passive interfaces. |
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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
DETAILED STEPS
| Command or Action | Purpose | |
|---|---|---|
|
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Example: Router> enable |
Enables privileged EXEC mode. |
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Example: Router# configure terminal |
Enters global configuration mode. |
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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. |
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Example: Router(config)# exit |
Returns to privileged EXEC mode. |
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Example: Router# show isis database detail |
Displays the IS-IS link-state database. |
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Example: Router# show isis financetag routes |
Displays the IS-IS Level 1 forwarding table for IS-IS learned routes. |
|
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Example: Router# show isis spf-log |
Displays how often and why the router has run a full SPF calculation. |
|
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Example: Router# show isis financetag topology |
Displays a list of all connected routers in all areas. |
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 Gigabitethernet 0/0/0 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 Gigabitethernet 1/0/0 ip address 192.168.129.1 255.255.255.0 ip router isis isis tag 120 interface Gigabitethernet 1/1/0 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 Gigabitethernet 1/0/0 ip address 192.168.130.5 255.255.255.0 ip router isis isis tag 60 ! interface Gigabitethernet 2/0/0 ip address 192.168.130.15 255.255.255.0 ip router isis isis tag 90 ! interface Gigabitethernet 3/0/0 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 sets 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 routes 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 allows two warning messages to be logged the first if the number of prefixes redistributed reaches 85 percent of 600 (510 prefixes) and the second if the number of redistributed routes reaches 600. However, the number of redistributed routes 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 Gigabit Ethernet interface 0/0/0. 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 Gigabitethernet 0/0/0 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 Gigabit Ethernet interface 0/0/0. 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 Gigabitethernet 0/0/0 ip address 192.168.20.1 255.255.255.0 no ip directed-broadcast ip router isis . . . router isis passive-interface Loopback 0 net 47.0004.004d.0001.0001.0c11.1111.00 advertise-passive-only log-adjacency-changes !
Additional References
Related Documents
| Related Topic |
Document Title |
|---|---|
| Cisco IOS commands |
|
| IS-IS commands: complete command syntax, command mode, defaults, command history, usage guidelines, and examples |
Cisco IOS IP Routing: ISIS Command Reference |
| Cisco IS-IS conceptual information |
"Integrated IS-IS Routing Protocol Overview" |
| Improving IS-IS network convergence times |
Standards
| Standards |
Title |
|---|---|
| No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature. |
-- |
MIBs
| MIBs |
MIBs Link |
|---|---|
| No new or modified MIBs are supported by this feature, and support for existing MIBs has not been modified 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: |
RFCs
| RFC |
Title |
|---|---|
| No new or modified RFCs are supported by this feature, and support for existing RFCs has not been modified by this feature. |
-- |
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. |
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.
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.
| 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 |
Cisco IOS XE Release 2.1 |
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. This feature was introduced on the Cisco ASR 1000 Series Aggregation Services Routers. |
| IS-IS Incremental SPF |
Cisco IOS XE Release 2.1 |
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. This feature was introduced on the Cisco ASR 1000 Series Routers. The following commands were modified by this feature: router isis, ispf. |
| IS-IS Limit on Number of Redistributed Routes |
Cisco IOS XE Release 2.1 |
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. This feature was introduced on the Cisco ASR 1000 Series Routers. The following commands were modified by this feature: lsp-full suppress, redistribute maximum-prefix, router isis. |
| IS-IS Mechanisms to Exclude Connected IP Prefixes from LSP Advertisements |
Cisco IOS XE Release 2.1 |
This document describes two Integrated Intermediate System-to-Intermediate System (IS-IS) mechanisms to exclude IP prefixes of connected networks from LSP advertisements, thereby reducing IS-IS convergence time. This feature was introduced on the Cisco ASR 1000 Series Routers. The following commands were modified by this feature: advertise-passive-only, ip directed-broadcast, ip router isis, isis advertise-prefix, net, passive interface, router isis. |
| IS-IS Support for Priority-Driven IP Prefix RIB Installation |
Cisco IOS XE Release 2.1 |
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. This feature was introduced on the Cisco ASR 1000 Series Routers. The following commands were modified by this feature: debug isis rib local, ip route priority high tag, ip router isis, isis tag, show isis rib. |
| IS-IS Support for Route Tags |
Cisco IOS XE Release 2.1 |
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. This feature was introduced on the Cisco ASR 1000 Series Routers. The following commands were modified by this feature: isis tag, match tag, metric-style wide, router isis, route-map, set tag, show ip route, show isis database verbose, summary-address. |
Cisco and the Cisco logo are trademarks or registered trademarks of Cisco and/or its affiliates in the U.S. and other countries. To view a list of Cisco trademarks, go to this URL: www.cisco.com/go/trademarks. Third-party trademarks mentioned are the property of their respective owners. The use of the word partner does not imply a partnership relationship between Cisco and any other company. (1110R)
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.
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