Step 5. Triggered Updates¶
We will demonstrate the benefits of triggered updates on speeding up the routing table convergence after a router detects a topology change.
In distance-vector routing algorithm, routers periodically (by default every 30 seconds) send update messages to their neighbours (i.e. other routers directly attached to them). This means that if a topology change occurs, the other router will not become aware of this on average for 15 seconds. The propagation of topology changes from router to router can be significantly delayed and may impact on the network performance.
To alleviate the effects of this problem, a second mechanism called triggered updates is used. A triggered update happens whenever a node notices a link failure or receives an update from one of its neighbors that causes it to change one of the routes in its routing table. Whenever a node’s routing table changes, it immediately sends an update to its neighbors, which may lead to a change in their tables, causing them to send an update to their neighbors quickly.
Our network configuration is the same as the one we used in the earlier steps.
The configuration in
omnetpp.ini is shown below:
[Config Step5] description = "RIP triggered updates" extends = Step3 *.router*.rip.triggeredUpdate = true
We break the link between
switch2 at t = 50s. (The scenario
step doing that is inherited from Step 3, see the
extends = Step3 line under
[Config Step5].) Observe that, since triggered updates are enabled,
router2 notifies its directly attached neighbours about the link change
immediately after it breaks (around t = 52s), not after the next scheduled
update at t = 60s.