Protocol Timers and Other IS-IS Database Parameters

IS-IS uses a suite of timers to control various events and to ensure routing stability without compromising the capability to converge fast enough in case of significant changes in the network. Various timers are used to ensure integrity of the Link-State database by enforcing periodic refresh of individual LSPs and aging out of stale information. This section discusses relevant timers for network operation purposes and provides insight into associated IOS commands that allow timer adjustments where possible.

Maxage

Maxage refers to the maximum life span of an LSP from the time it is generated by the source. ISO 10589 specifies maxage as a protocol constant with a value of 1200 seconds (20 minutes). The time lapse until expiration of an LSP is indicated in the Remaining Lifetime field of the LSP on Cisco routers. Cisco IOS adheres to the maxage specification in ISO 10589 by setting the Remaining Lifetime field of an LSP to 1200 seconds when it is first generated. Actually, the remaining lifetime is referred to as LSP holdtime on a Cisco router ( show isis database detail command). Usually, an LSP is refreshed by its source before its remaining lifetime reaches zero (see the next section, "LSP Generation Interval"). If a router is removed from the network, however, it is not available to refresh its LSP, and the LSP ages until it expires at a remaining lifetime value of zero.

Occasionally, another router in the network initiates a purge of a corrupted LSP from the network by setting the remaining lifetime of the LSP to zero and reflooding the LSP to its neighbors. The ZeroAgeLifetime is the time an LSP is retained after it expires, before it is purged from the database. Its value is specified as 60 seconds by ISO 10589.

You can use the IOS command max-lsp-lifetime to modify the default value of maxage, which is specified as 1200 seconds. All of the 2 bytes of the Remaining Lifetime fieldare leveraged by Cisco IOS to allow a much higher value (up to 65,535 seconds) to be configurable on Cisco routers; 65,535 (2 ¹⁶ “ 1) seconds is approximately 18.2 hours.

Maxage is considered a protocol constant and must have a consistent value on all IS-IS routers in the network. Therefore, if a router receives an LSP with a remaining lifetime higher than the expected value of maxage, the LSP is considered corrupted and is therefore discarded.

LSP Generation Interval

This timer has two varieties:

• The maximum LSP generation interval ” This interval is known also as the LSP refresh interval. It refers to the periodic replacement of an LSP by the originating routers before the LSP expires; 900 seconds (15 minutes) is specified (compared to 1200 seconds of maxage).

• The minimum LSP generation interval ” This interval defines the minimum interval between consecutive generations of an LSP by a router. A value of 30 seconds is specified for this parameter.

The LSP refresh interval needs to be less than maxage to require a router to periodically generate a newer copy of its LSP into the network, even when there are no changes to report, before the LSP expires. This procedure helps ensure the continued integrity of the LSP throughout the network. You can use the IOS command lsp-refresh-interval to modify the refresh interval. Setting this timer reasonably high helps cut down on bandwidth overhead and saves processing resources. Exercise caution when setting the refresh interval. If set too high, you might significantly inhibit expedient removal of incorrect link-state information from the network.

Minimum LSP Transmission Interval

The minimum LSP transmission interval specifies the minimum interval between consecutive transmissions of two LSPs. The default is 33 milliseconds . The related IOS command for modifying the set value is isis lsp-interval .

As discussed in the section on point-to-point flooding, the reliable flooding mechanism requires a router to retransmit LSPs onto point-to-point links until it receives an acknowledgment from the neighbor. The interval between retransmissions is the retransmission interval and is set at a default value of 5 seconds. You can use the IOS command isis retransmit-interval to modify the LSP retransmission interval to a more desirable value.

CSNP Interval

The CSNP interval relates to the periodic transmission of CSNPs by the DIS on a broadcast medium. The default value is 10 seconds. The IOS command isis csnp-interval enables you to modify this default. If the routers on a broadcast link are fairly stable, an operator might decide to increase the CSNP interval so that CSNPs are advertised less frequently, thereby conserving link bandwidth.

Table 5-7 lists the timers discussed in this section, providing default values and the relevant Cisco IOS commands for modifying them.

Table 5-7. IS-IS Database Timers

Timer	Default Value	IOS Command
Maxage	1200 seconds	isis max-lsp-interval
LSP refresh interval	900 seconds	isis lsp-refresh-interval
LSP transmission interval	33 milliseconds	isis lsp-interval
LSP retransmit interval	5 seconds	isis retransmit-interval
CSNP interval	10 seconds	isis csnp-interval