Content

| 3.5 |  | Route Calculation |
| 3.5.2 |  | The initiation of routing updates |

Routing protocols can instruct a router to update neighbors after a specific amount of time has passed, after a certain event has occurred, or both. Time-driven routing protocols wait for the update timer to expire and then send an update. RIP will send a complete update every 30 seconds by default even if its routing table is unchanged since the last update. By contrast, protocols that are event-driven do not require the router to update neighbors until the router detects a change in the network topology. Link-state protocols and EIGRP send a partial update that includes only the changed information. Other protocols may send their entire table when triggered by an event.

Routing protocols that are exclusively time-driven react poorly to topology changes. If a router detects a change but has to wait 30 seconds before alerting neighbors, routing in that network could break down and it could take several minutes before convergence. In the meantime, routers unaware of the change may send packets the wrong way, leading to routing loops or loss of connectivity.

Routing protocols that are exclusively event-driven could go for extended periods of time without sending updates. If there is no other mechanism to ensure that routers regularly communicate, such as a Hello protocol, routers could base their routing decisions on outdated information.

For these reasons, most routing protocols use a combination of time-driven and event-driven updates. RIP is time-driven, but the Cisco implementation of RIP sends triggered updates whenever a change is detected. Topology changes also trigger immediate updates in IGRP routers, regardless of the update timer. Without triggered updates, RIP and IGRP will not perform.

Protocols that are primarily event-driven typically use timers as well. OSPF routers assign a MaxAge to the routing information. Once the information has reached its MaxAge it can no longer be used in the routing table and a new update must be requested.