Chapter 2 Advanced Routing for FortiOS 5.0 : Intermediate System to Intermediate System Protocol (IS-IS) : IS-IS background and concepts : Parts and terminology of IS-IS : Timer options
  
Timer options
IS-IS uses various timers to regulate its performance including a garbage timer, update timer, and timeout timer. The FortiGate unit default timer settings (30, 180, and 120 seconds respectively) are effective in most configurations—if you change these settings, ensure that the new settings are compatible with local routers and access servers.
You can configure the three IS-IS timers in the CLI, using the following commands:
config router isis
set garbage-timer
set update-timer
set timeout-timer
end
You will find more information on each timer below.
Garbage timer
The garbage timer is the amount of time (in seconds) that the DIS will advertise a route as being unreachable before deleting the route from the routing table. If this timer is shorter, it will keep more up-to-date routes in the routing table and remove old ones faster. This results in a smaller routing table which is useful if you have a very large network, or if your network changes frequently.
Update timer
The update timer determines the interval between routing updates. Generally, this value is set to 30 seconds. There is some randomness added to help prevent network traffic congestion, which could result from all routers simultaneously attempting to update their neighbors. The update timer should be at least three times smaller than the timeout timer, otherwise you will experience an error.
If you are experiencing significant traffic on your network, you can increase this interval to send fewer updates per minute. However, ensure you increase the interval for all the routers on your network or you will experience timeouts that will degrade your network speed.
Timeout timer
The timeout timer is the maximum amount of time (in seconds) that a route is considered reachable while no updates are received for the route. This is the maximum time the DIS will keep a reachable route in the routing table while no updates for that route are received. If the DIS receives an update for the route before the timeout period expires, the timer is restarted. The timeout period should be at least three times longer than the update period, otherwise you will experience an error.
If you are experiencing problems with routers not responding in time to updates, increase this timer. However, remember that longer timeout intervals result in longer overall update periods — it may be considerable time before the DIS is done waiting for all the timers to expire on unresponsive routes.