What is the configuration and function of each timer in ISIS network?
(1) Configure Hello packet transmit interval:
isis timer hello seconds [ level-1 | level-2 ]
There are two kinds of Hello packets (Level-1 and Level-2) on broadcast link; different kinds of packets can be set different value. There is no difference of Level-1 and Level-2 on point-to-point link. There is no need to set the level.
(2) Configure Hello packet invalid number:
isis timer holding-multiplier value [ level-1 | level-2 ]
IS-IS maintains neighbor relation with neighboring router through the reception and transmission of Hello packet. When local router does not receive hello packet sent by the peer with some periods, i.e. it does not receive hello packet of designated count continuously, it regards neighboring router as invalid.
In IS-IS, holding time is adjusted by setting Hello packet invalid number.
(3) Configure CSNP packet transmit interval:
isis timer csnp seconds [level-1 | level-2]
CSNP packet is sent when DIS (Designated IS) synchronizes LSDB on broadcast network.
(4) Configure LSP retransmit interval of the interface:
isis timer lsp-retransmit seconds
In point-to-point link, if LSP of local end does not receive the response within the period, it regards previously transmitted LSP lost or dropped; to ensure the reliability, local router will re-transmit former LSP.
LSP transmitted on broadcast link has no need to reply.
(5) Configure the minimum interval of the interface transmitting LSP:
isis timer lsp-throttle seconds
Set the minimum interval of LSP packet transmitted by ISIS on the interface, i.e. the duration of two consecutive LSPs.
(6) Configure LSP refresh period:
timer lsp-refresh seconds
To ensure that LSP of the whole area can keep synchronous, IS-IS periodically transmits current LSP. The LSP refresh period should be smaller than LSP valid time.
(7) Configure LSP valid time:
timer lsp-max-age seconds
When the router creates system LSP, it will fill living time in LSP. When LSP is received by other routers, the living time decreases with the time. If the router does not receive updated LSP and the living time of LSP is 0, such LSP will be deleted from LSDB.
(8) enable LSP flash-flood:
flash-flood [ lsp-count ] [ max-timer-interval timer ] [ level-1 | level-2 ]
Using flash-flood command can accelerate the flooding of LSP. The user can designate the amount of flooding LSP every time and the amount is in allusion to all interfaces. If required LSP is larger than the amount, lsp-count LSP is transmitted. If the timer is configured and is not timeout before routing calculation, it floods at once. Otherwise it is transmitted when the timer is timeout.
(9) configure SPF intelligent timer:
timer spf max-interval initial-interval incremental-interval
In IS-IS, when LSDB changes, routing calculation is required. Frequent routing calculation will occupy much system resource and the performance is lowered. Delaying SPF calculation can improve the efficiency of routing calculation to some degree and reduce the exhaustion of system resource. But too long delay will reduce the convergence speed. SPF intelligent timer is a good solution. It can automatically adjust delay time according to the change of LSDB. For initial SPF calculation, required interval is initial-interval; and then one incremental-interval is increased for the change once until max-interval. After it stabilizes max-interval three times, the interval returns initial-interval.
(10) Configure duration of SPF calculation:
If there are too many routes (more than 150,000) in routing table, SPF calculation of IS-IS may occupy CPU for long. The duration of SPF calculation can be set to prevent such situation. After setting SPF subsection calculation, the route that is not handled once can continue to be calculated one second later.