Customizing IS-IS for Your Network Design

This module describes optional tasks that you can perform to customize Intermediate System-to-Intermediate System (IS-IS) for your network design. You can optimize network traffic flow by setting metrics, specifying an IS-IS system type, summarizing addresses, generating a default route, and configuring a global default metric.

Prerequisites for Customizing IS-IS for Your Network Design

  • Before performing the tasks in this module, you should be familiar with the concepts described in the "Integrated IS-IS Routing Protocol Overview" module.

  • You should understand the concept of IP addressing. For more information on IP addressing, see the "Configuring IPv4 Addresses" chapter of the Cisco IOS XE IP Addressing Services Configuration Guide, Release 2.

  • You should know your network design and how you want traffic to flow through it before configuring IS-IS. Define areas, prepare an addressing plan for the routers (including defining the network entity titles [NETs]), and determine the interfaces that will run Integrated IS-IS.

  • IS-IS must be enabled.

Information About Customizing IS-IS for Your Network Design

You can enhance network traffic flow by configuring IS-IS metric values for Level-1 or Level-2 routing, in order to prioritize traffic through certain paths. You can customize network traffic flow by changing the metric cost for a specified interface. All IS-IS links use the metric of 10 by default. The protocol does not automatically incorporate link attributes such as bandwidth or delay when metric values are assigned. The total cost to a destination is the sum of the costs on all outgoing interfaces along a particular path from the source to the destination. The least-cost paths are preferred.

On multi-access networks, IS-IS elects a router to act as a pseudo-node representing the multi-access circuit. The elected router is known as the designated intermediate system (DIS). The DIS issues pseudo-node LSPs listing all of the routers which are reachable on the network. Each router on the network advertises in its non-pseudonode LSPs reachability to the DIS. This reduces the amount of information that needs to be advertised. A DIS is elected for each level that is operating on the network, for example both Level 1 and Level 2. By default, all routers have the same priority for being elected DIS. The MAC address of each router’s interface onto the network is used as the tiebreaker. When all routers have the same priority, the addition or removal of a router onto the network can result in a chance in the DIS. This churn can be prevented by assigning a higher priority to the router which you wish to act as the DIS. Priorities can be configured individually for Level 1 and Level 2. By default the priority is 64. You can configure the priority in the range from 0 to 127.

You can configure a summary address to represent summarized (aggregate) addresses within the IS-IS routing table. This process is called route summarization. Using a summary address can enhance scalability and network stability because it reduces the amount of information that needs to be advertised and reduces the frequency of updates required. For example, a single route flap may not cause the summary advertisement to flap. The disadvantage of using the summary addresses is that routing may be sub-optimal, for example, the path to a specific destination covered by the summary address may be longer than it would have been, had all the individual addresses been advertised. Summary addresses are most commonly used to summarize routes from one Level-one area into the Level-2 subdomain. One summary address can include multiple groups of addresses for a given level. Routes learned from other routing protocols can also be summarized. The metric used to advertise the summary is the smallest metric of all the more-specific routes.

Whenever you specifically configure redistribution of routes into an IS-IS routing domain, the Cisco IOS XE software does not, by default, redistribute the default route into the IS-IS routing domain. If you wish to advertise a default route you must use the default-information originate command. This command causes a default route to be advertised by the router. Advertisement of the default route can be made conditional by using a route map. You can use the route map to identify the level into which the default route is to be announced, whether a particular non-default prefix must be reachable, etc.

In Cisco IOS XE software, IS-IS has a default metric value of 10 for all active interfaces. If the interface is passive, the default value is zero. Rather than change the metric values for the active interfaces one by one, you can configure a different default metric value to be used by all interfaces. All interfaces that had the original IS-IS default metric 10 will be configured with the new default value. Besides offering the user the convenience of being able to globally configure the value for all IS-IS interfaces, the feature helps prevent errors that may occur when interfaces are individually configured to change the metric value. For example the user may remove configured metrics from an interface, thereby restoring the default metric value of 10--perhaps unintentionally making that interface a highly preferred one in the network. Such an occurrence on the wrong interface could mean the rerouting of traffic across the network on an undesirable path.


Note

The MTU size (1418 bytes) in GRE tunnel is less than default ISIS LSP-MTU of 1492 bytes. To configure ISIS IPv6 over GRE tunnels, you must update the ISIS LSP-MTU size in all routers when overlay MTU size is less than 1492 bytes in GRE tunnel.

Enhancing Your IS-IS Network Design at the Interface Level

Setting the IS-IS Link-State Metrics

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. interface type name
  4. isis metric default-metric [level-1 | level-2 ]
  5. end
  6. show isis [process-tag ] database [level-1 ] [level-2 ] [l1 ] [l2 ] [detail ] [lspid ]

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 name

Example:

Router(config)# interface gigabitethernet 0/0/0

Enters interface configuration mode.

Step 4

isis metric default-metric [level-1 | level-2 ]

Example:

Router(config-if)# isis metric 15 level-1

Configures the metric for an interface.

Note

 

We highly recommend that you configure the metrics on all interfaces. If you do not do so, all links will have the same cost and the cost to reach any node in the network will be logically equivalent to the number of hops.

Step 5

end

Example:

Router(config-if)# end

Exits interface configuration mode and returns to privileged EXEC mode.

Step 6

show isis [process-tag ] database [level-1 ] [level-2 ] [l1 ] [l2 ] [detail ] [lspid ]

Example:

Router# show isis database detail

(Optional) Displays the IS-IS link-state database.

  • To display information about each LSP and the link-state database, enter the detail keyword.

Prioritizing Designated Intermediate Systems for IS-IS

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. interface type name
  4. isis priority number-value [level-1 | level-2 ]
  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

interface type name

Example:

Router(config)# interface gigabitethernet 0/3/0

Enters interface configuration mode.

Step 4

isis priority number-value [level-1 | level-2 ]

Example:

Router(config-if)# isis priority 2 level-1

Configures the priority used in the designated router election.

Step 5

end

Example:

Router(config-if)# end

Exits interface configuration mode and returns to privileged EXEC mode.

Enhancing Your IS-IS Network Design at the Router Level

Limiting Level 1 and Level 2 Operations on the IS-IS Router

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. router isis area-tag
  4. is-type {level-1 | level-1-2 | level-2-only }
  5. end
  6. show isis [ipv6 ] [*] topology [level-1 ] [level-2 ]

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 1

Enables IS-IS as an IP routing protocol.

  • Enters router configuration mode.

Step 4

is-type {level-1 | level-1-2 | level-2-only }

Example:

Router(config-router)# is-type level-1

Configures the routing level for an instance of the IS-IS routing process.

Note

 

By default, Cisco IOS XE software enables both Level 1 and Level 2 operations on IS-IS routers. To specify that a router is to operate only as an area router (Level 1) or only as a backbone router (Level 2), use the is-type command. Specifying routers to act as Level 1, Level 2, or Level 1 and 2 can streamline your network design.

Step 5

end

Example:

Router(config-router)# end

Exits router configuration mode and returns to privileged EXEC mode.

Step 6

show isis [ipv6 ] [*] topology [level-1 ] [level-2 ]

Example:

Router# show isis topology level-1

(Optional) Displays a list of all connected routers in all areas.

  • To confirm paths to all Level 1 or Level 2 routers in the area or areas in which this router resides, enter the level-1 or level-2 keywords, respectively.

Examples

The following example shows output from the show isis topology command for a router within a dual CLNS-IP network. In this example, because neither the level-1 nor level-2 optional keywords were entered, information is displayed for both Level 1 and Level 2 routers. 


Router# show isis topology
Tag L2BB:
IS-IS paths to level-2 routers
System Id       Metric  Next-Hop        Interface       SNPA
0000.0000.0005  --
0000.0000.0009  10      0000.0000.0009  Tu529           *Tunnel*        
0000.0000.0017  20      0000.0000.0009  Tu529           *Tunnel*        
0000.0000.0053  30      0000.0000.0009  Tu529           *Tunnel*        
0000.0000.0068  20      0000.0000.0009  Tu529           *Tunnel* 
Tag A3253-01:
IS-IS paths to level-1 routers
System Id       Metric  Next-Hop        Interface       SNPA
0000.0000.0003  10      0000.0000.0003  FE1/0/0         0000.0c03.6944 
0000.0000.0005  --
0000.0000.0053  10      0000.0000.0053  FE1 /0/0        0060.3e58.ccdb

Summarizing Address Ranges in the IS-IS Routing Table

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. router isis area-tag
  4. summary-address address mask {level-1 | level-1-2 | level-2 }[tag tag-number ] [metric metric-value ]
  5. end
  6. show isis database verbose

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 1

Enables IS-IS as an IP routing protocol.

  • Enters router configuration mode.

Step 4

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

Example:

Router(config-router)# summary-address 10.1.0.0 255.255.0.0 level-2

Creates aggregate addresses for IS-IS.

Note

 

Multiple groups of addresses can be summarized for a given level. Routes learned from other routing protocols can also be summarized. The metric used to advertise the summary is the smallest metric of all the more-specific routes. This command helps reduce the size of the routing table.

Step 5

end

Example:

Router(config-router)# end

Exits router configuration mode and returns to privileged EXEC mode.

Step 6

show isis database verbose

Example:

Router# show isis database verbose

(Optional) Displays detailed information about the IS-IS database.

Generating an IS-IS Default Route

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. router isis area-tag
  4. default-information originate [route-map map-name ]
  5. end
  6. show ip route

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 1

Enables IS-IS as an IP routing protocol.

  • Enters router configuration mode.

Step 4

default-information originate [route-map map-name ]

Example:

Router(config-router)# default-information originate

Generates a default route into an IS-IS routing domain.

Step 5

end

Example:

Router(config-router)# end

Exits router configuration mode and returns to privileged EXEC mode.

Step 6

show ip route

Example:

Router# show ip route

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

Configuring an IS-IS Default Metric


Note

If you have already configured a metric for a specific interface by entering the isis metric command, the metric that has been configured for that specific interface will take precedence over any default set by the metric command.

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. router isis area-tag
  4. metric default-value [level-1 | level-2 ]
  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 1

Enables IS-IS as an IP routing protocol.

  • Enters router configuration mode.

Step 4

metric default-value [level-1 | level-2 ]

Example:

Router(config-router)# metric 25 level-2

Globally sets a new default metric value for all IS-IS interfaces.

  • The value 25 shown in the example will apply only to Level 2 IS-IS interfaces. If you do not enter the level-1 or level-2 keyword, the metric will be applied to both Level 1 and Level 2 IS-IS interfaces.

Step 5

end

Example:

Router(config-router)# end

Exits router configuration mode and returns to privileged EXEC mode.

Configuration Examples for Customizing IS-IS for Your Network Design

Example Configuring a Global Default Metric for IPv4

The following configuration example for an IS-IS routing process called area1 sets a global default metric of 111 for the IS-IS interfaces: 


interface gigabitethernet3/1/0
 ip address 172.16.10.2 255.255.0.0
 ip router isis area1
 no ip route-cache
 duplex half
!
interface gigabitethernet3/2/0
 ip address 192.168.242.2 255.255.255.0
 ip router isis area1
 no ip route-cache
 duplex half
router isis area1
 net 01.0000.0309.1234.00
 metric-style wide
 metric 111

In the following example, the show clns interface command confirms that the IS-IS IPv4 interface metric for both Level 1 and Level 2 interfaces is assigned the new default metric value 111: 


Router# show clns interface
GigabitEthernet3/1/0 is up, line protocol is up
  Checksums enabled, MTU 1497, Encapsulation SAP
  ERPDUs enabled, min. interval 10 msec.
  CLNS fast switching enabled
  CLNS SSE switching disabled
  DEC compatibility mode OFF for this interface
  Next ESH/ISH in 39 seconds
  Routing Protocol: IS-IS
    Circuit Type: level-1-2
    Interface number 0x0, local circuit ID 0x1
    Level-1 Metric: 111, Priority: 64, Circuit ID: mekong.01
    Level-1 IPv6 Metric: 10
    Number of active level-1 adjacencies: 0
    Level-2 Metric: 111, Priority: 64, Circuit ID: mekong.01
    Level-2 IPv6 Metric: 10
    Number of active level-2 adjacencies: 0
    Next IS-IS LAN Level-1 Hello in 922 milliseconds
    Next IS-IS LAN Level-2 Hello in 1 seconds
GigabitEthernet3/2/0 is up, line protocol is up
  Checksums enabled, MTU 1497, Encapsulation SAP
  ERPDUs enabled, min. interval 10 msec.
  CLNS fast switching enabled
  CLNS SSE switching disabled
  DEC compatibility mode OFF for this interface
  Next ESH/ISH in 20 seconds
  Routing Protocol: IS-IS
    Circuit Type: level-1-2
    Interface number 0x1, local circuit ID 0x2
    Level-1 Metric: 111, Priority: 64, Circuit ID: mekong.02
    Level-1 IPv6 Metric: 10
    Number of active level-1 adjacencies: 1
    Level-2 Metric: 111, Priority: 64, Circuit ID: mekong.02
    Level-2 IPv6 Metric: 10
    Number of active level-2 adjacencies: 1
    Next IS-IS LAN Level-1 Hello in 2 seconds
    Next IS-IS LAN Level-2 Hello in 1 seconds

In the following example, the isis metric command is entered so that it will assign a metric value of 10. The metric value that is set with the isis metric command for GigabitEthernet interface 3/1/0 will take precedence over the metric value that was previously set with the metric command. 


interface GigabitEthernet3/1/0
 ip address 172.30.10.2 255.255.0.0
 ip router isis area1
 no ip route-cache
 duplex half
 isis metric 10
!
interface GigabitEthernet3/2/0
 ip address 192.168.224.2 255.255.255.0
 ip router isis area1
 no ip route-cache
 duplex half
router isis area1
 net 01.0000.0309.1234.00
 metric-style wide
 metric 111

When the show clns interface command is entered, the router output confirms that the interface has an assigned IS-IS IPv4 metric value of 10: 


Router# show clns interface
GigabitEthernet3/1/0 is up, line protocol is up
  Checksums enabled, MTU 1497, Encapsulation SAP
  ERPDUs enabled, min. interval 10 msec.
  CLNS fast switching enabled
  CLNS SSE switching disabled
  DEC compatibility mode OFF for this interface
  Next ESH/ISH in 53 seconds
  Routing Protocol: IS-IS
    Circuit Type: level-1-2
    Interface number 0x0, local circuit ID 0x1
    Level-1 Metric: 10, Priority: 64, Circuit ID: mekong.01
    Level-1 IPv6 Metric: 10
    Number of active level-1 adjacencies: 0
    Level-2 Metric: 10, Priority: 64, Circuit ID: mekong.01
    Level-2 IPv6 Metric: 10
    Number of active level-2 adjacencies: 0
    Next IS-IS LAN Level-1 Hello in 4 seconds
    Next IS-IS LAN Level-2 Hello in 4 seconds
GigabitEthernet3/2/0 is up, line protocol is up
  Checksums enabled, MTU 1497, Encapsulation SAP
  ERPDUs enabled, min. interval 10 msec.
  CLNS fast switching enabled
  CLNS SSE switching disabled
  DEC compatibility mode OFF for this interface
  Next ESH/ISH in 30 seconds
  Routing Protocol: IS-IS
    Circuit Type: level-1-2
    Interface number 0x1, local circuit ID 0x2
    Level-1 Metric: 111, Priority: 64, Circuit ID: mekong.02
    Level-1 IPv6 Metric: 10
    Number of active level-1 adjacencies: 1
    Level-2 Metric: 111, Priority: 64, Circuit ID: mekong.02
    Level-2 IPv6 Metric: 10
    Number of active level-2 adjacencies: 1
    Next IS-IS LAN Level-1 Hello in 2 seconds
    Next IS-IS LAN Level-2 Hello in 922 milliseconds

Additional References

Related Documents

Related Topic

Document Title

IS-IS commands: complete command syntax, command mode, defaults, command history, usage guidelines, and examples

Cisco IOS IP Routing: ISIS Command Reference

Overview of Integrated IS-IS conceptual information with links to all the individual IS-IS modules

"Integrated IS-IS Routing Protocol Overview" module

Configuring IPv6

"Implementing IPv6 Addressing and Basic Connectivity" chapter in the Cisco IOS IPv6 XE Configuration Guide, Release 2

Configuring the IS-IS protocol for IPv6 networks

"Implementing IS-IS for IPv6" module in the Cisco IOS XE IPv6 Configuration Guide , Release 2

Customizing IS-IS for fast convergence and scalability

"Overview of IS-IS Fast Convergence" module

Enhancing IS-IS network security

"Enhancing Security in an IS-IS Network" module

Cisco IOS master command list, all releases

Cisco IOS Master Command List, All Releases

Standards

Standard

Title

No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.

--

MIBs

MIB

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:

http://www.cisco.com/go/mibs

RFCs

RFCs

Title

RFC 1195

s

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 Customizing IS-IS for Your Network Design

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.