Configuring IS-IS Interface-Specific Parameters

You can change IS-IS interface-specific parameters; most can be configured independently of other attached routers. You are not required to alter any interface parameters; however, some parameters must be consistent across all routers in your network. If you change certain values from the defaults, you must configure them on multiple interfaces and routers.

You can configure IS-IS interfaces as follows:

• Set an HMAC MD5 authentication password on an interface.
• Configure the IS-IS link state metrics.
• Set the advertised complete sequence number PDU (CSNP) interval.
• Set the advertised hello interval and hello multiplier.
• Set the LSP transmission interval.
• Set the retransmission interval.
• Set the retransmission throttle interval.
• Specify the designated router election.
• Specify the interface circuit type.
• Configure a passive interface.

In the following command guidelines, notice that many parameters are preset to a default value. If that parameter has been modified from its default, use the no version of the command to restore its default value.

    isis authentication-key

• Use to specify a password associated with an interface for authentication of IS-IS hellos.
• You can specify whether the password is for level 1 or level 2 hellos.
• Example

host1(config-if)#isis-authentication-key 0 red5flower6 

• Use the no version to delete the password.

    isis circuit-type

• Use to specify adjacency levels on a specified interface; however, normally, you do not need to use this command.
• Configure a router as a level 1-only, a level 1-level 2 system, or a level 2-only system.
• Configure some interfaces to be level 2-only for routers that are between areas. This prevents wasting bandwidth by sending out unused level 1 hellos.
• On point-to-point interfaces, the level 1 and level 2 hellos are in the same packet.
• Level 1-2 is the default.
• Example

host1(config-if)#isis circuit-type level-2-only

• Use the no version to restore the default value, level-1-2.

    isis csnp-interval

• Configure the isis csnp-interval level for a specified interface. The level can be configured independently for level 1 and level 2.
• For LAN interfaces: the default value is 10 seconds, which you probably do not need to change. For WAN interfaces: the default value is 0 seconds or disabled.
• On point-to-point subinterfaces use isis csnp-interval in conjunction with the isis mesh-group command.
• Completed sequence number PDUs are sent by the designated router to maintain database synchronization.
• Example

host1(config-if)#isis csnp-interval 30 level-1

• Use the no version to restore the default value.

    isis hello-interval
    isis hello-multiplier

• isis hello-interval sets the length of time (in seconds) between hello packets sent on a specific interface. Configure independently for level 1 and level 2, except on point-to-point interfaces because only a single type of hello packet is sent on serial links. For this reason, it is independent of levels 1 and 2. For example, you can specify an optional level for Frame Relay multiaccess networks.
• isis hello-multiplier sets a number by which to multiply the hello interval seconds. This number determines the total holding time transmitted in the IS-IS hello packet. The default is 3. Use when hello packets are frequently lost and IS-IS adjacencies are failing unnecessarily.

• Holding time - the time a neighbor waits for another hello packet before declaring the neighbor is down. It determines how quickly a failed link or neighbor is identified so that routes can be recalculated.

• Raise the hello multiplier and lower the hello interval simultaneously to make the hello protocol more reliable without increasing the time required to detect a link failure.
• Example

host1(config-if)#isis hello-interval 6 level-1

host1(config-if)#isis hello-multiplier 10 level-1

• Use the no version to restore a default value.

    isis lsp-interval

• Use to configure the delay between successive IS-IS link state packet (LSP) transmissions.
• You can choose an interval in the range 1-4294967295 milliseconds. For example, setting 100 milliseconds allows 10 packets per second.
• The default value is 33 milliseconds.
• If your network has many IS-IS neighbors and interfaces, a particular router may have difficulty with the CPU load imposed by LSP transmission and reception. If this is the case, you can reduce the LSP transmission rate by issuing this command.
• Example

host1(config-if)#isis lsp-interval 100

• Use the no version to restore the default value, 33.

    isis message-digest-key

• Use to configure HMAC MD5 authentication for an interface.
• Generates a secure, encrypted message digest of level 1 or level 2 hello packets and inserts the digest into the packet from which it is created. Level 1 is the default.
• You can specify whether the key is entered in unencrypted or encrypted format. If you do not specify which, the string is assumed to be unencrypted.
• Example

host1(config-if)#isis message-digest-key 3 hmac-md5 
wdi6c3s39n level-2 

• For point-to-point interfaces, configure keys only for level 1, because only one hello packet is sent (at level 1), not one at level 1 and one at level 2. Keys configured at level 2 are ignored for point-to-point interfaces.
• Use the no version to delete the MD5 key, specified by the key-id, from the interface.

    isis metric

• Use to configure a cost for a specified interface.
• You can select a number in the range 0-63 if you configured the system with the metric-style narrow command. You can select a number in the range 0-16277215 if you configured the system with the metric-style transition or the metric-style wide command.
• The default value is 10. The default metric is the value assigned when no quality of service (QoS) routing is performed.
• You can configure the default metric for a specified interface by selecting level 1 or level 2 routing. This resets the metric only for level 1 or level 2 routing, respectively.
• We recommend that you configure metrics on all interfaces that are related to link speed. If you do not, the IS-IS metrics are simply hop-count-like metrics.
• Example

host1(config-if)#isis metric 20 level-2

• Use the no version to restore the default value, 10.

    isis priority

• Use to set the priority of use for your designated router.
• You can configure an individual priority for level 1 and level 2 by choosing a priority level in the range 0-127.
• The default priority level is 64.
• Specifying the level 1 or level 2 keywords resets the priority only for level 1 or level 2 routing, respectively.
• Priorities are used to determine which router in the network is the designated intermediate system (DIS); the router with the highest priority becomes the DIS. Priorities are advertised in hellos.
• IS-IS has no backup designated router. Setting the priority to 0 reduces the chance of this router becoming the DIS, but does not prevent it. If a router with a higher priority is identified, it takes over the role from the current DIS. When priorities are equal, the highest MAC address breaks the tie and becomes the DIS.
• Example

host1(config-if)#isis priority 80 level-1

• Use the no version to restore the default value, 64.

    isis retransmit-interval

• Use to configure the number of seconds between the retransmission of IS-IS LSPs with the same lsp-id for point-to-point links.
• You can select an interval in the range 1-65535 seconds.
• The default value is 5 seconds.
• Specify a number greater than the expected round-trip delay between any two routers on your network.
• Always specify conservatively; otherwise, excessive retransmission will result.
• Because retransmissions occur only when LSPs are dropped, when you set isis retransmit-interval to a higher value, it has little effect on reconvergence.
• You should set to a higher value when routers have many neighbors and/or more paths over which LSPs can be flooded.
• Use a large value for serial lines.
• Example

host1(config-if)#isis retransmit-interval 60

• Use the no version to restore the default value, 5.

    isis retransmit-throttle-interval

• Use to configure the maximum rate at which IS-IS LSPs are retransmitted on point-to-point links. The interval is in terms of the number of milliseconds between packets.
• You can choose an interval in the range 0-65535 milliseconds.
• The default delay value is 33 milliseconds.
• The isis retransmit-throttle-interval is the maximum rate at which IS-IS LSPs are retransmitted. It is different from isis lsp-interval, which is the rate at which LSPs are transmitted on the interface, and it is different from isis retransmit-interval, which is the period between successive retransmissions of the same LSP. Use all three commands in conjunction with each other to control the load of routing traffic from one router to its neighbors.
• Typically, you can set this interval for very large networks with many LSPs and many interfaces as a way of controlling LSP retransmission traffic.
• Example

host1(config-if)#isis retransmit-throttle-interval 300

• Use the no version to restore the default value, 33.

    passive-interface

• Use to configure an IS-IS interface such that its IP address is advertised in its link state PDUs, but no IS-IS packets are sent from or received on the interface.
• Example

The following commands configure loopback 0 as a passive interface and enable IS-IS on subinterfaces atm 2/0.1 and atm 2/1.1. IS-IS advertises the IP address of loopback 0 in its link state PDUs, but runs only on atm 2/0.1 and atm 2/1.1:

host1(config)#router isis floor12

host1(config-router)#net 
47.0010.0000.0000.0000.0001.0001.1111.1111.1111.00

host1(config-router)#passive-interface loopback 0

host1(config-router)#exit

host1(config)#interface atm 2/0.1

host1(config-if)#ip router isis floor12

host1(config-if)#exit

host1(config)#interface atm 2/1.1

host1(config-if)#ip router isis floor12

You can override the passive-interface configuration simply by issuing the complementary command. For example, suppose you issue the following commands after the previous configuration:

host1(config-router)#passive-interface atm 2/0.1

host1(config-router)#exit

host1(config)#interface loopback 0

host1(config-if)#ip router isis floor12

Now IS-IS advertises the IP address of atm 2/0.1 in its link state PDUs, but runs only on loopback 0 and atm 2/1.1.

• You can also accomplish the equivalent of the passive-interface command by using the redistribute command to redistribute a connected route to level 1.
• Use the no version to delete the passive interface.

Summary Example

host1(config-router)#passive-interface loopback 0

host1(config-if)#interface atm 8/0.500

host1(config-if)#isis metric 20 level-2

host1(config-if)#isis csnp-interval 30 level-1

host1(config-if)#isis hello-interval 6 level-1

host1(config-if)#isis hello-multiplier 10 level-1

host1(config-if)#isis lsp-interval 100

host1(config-if)#isis retransmit-interval 60

host1(config-if)#isis retransmit-throttle-interval 300

host1(config-if)#isis priority 80 level-1

host1(config-if)#isis circuit-type level-2-only