Intermediate System to Intermediate System Protocol (IS-IS)

Default information originate option

Enabling default-information-originate generates and advertises a default route into the FortiGate unit’s IS-IS- enabled networks. The generated route may be based on routes learned through a dynamic routing protocol, routes in the routing table, or both. IS-IS does not create the default route unless you use the always option.

Select Disable if you experience any issues or if you wish to advertise your own static routes into IS-IS updates. The CLI commands associated with default information originate include:

config router isis

set default-originate end


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


You will find more information on each timer below.


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.


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.



In routing protocols, it is typically desireable to establish authentication rules that prevent malicious and otherwise unwanted information from being injected into the routing table. IS-IS routing protocol utilizes TLV 10 to establish authentication. For more information on TLVs, see TLV on page 420.

Initially, IS-IS used plain Clear Text to navigate the authentication rules, but this was found to be insecure since the Clear Text packets were unencrypted and could be exposed to packet sniffers. As per RFC 3567, HMAC-MD5 and Enhanced Clear Text authentication features were introduced to IS-IS, both of which encrypt authentication data, making them considerably more secure than using plain Clear Text authentication.

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