triggered update

In the context of the Routing Information Protocol (RIP), a "Triggered Update" is a specific type of message that is sent immediately after a change occurs in the network topology. The purpose of a triggered update is to quickly inform neighboring routers about the change so that they can update their routing tables accordingly. By promptly sharing information about the change, RIP aims to avoid potential routing issues such as loops or black holes.

Various events can trigger a triggered update in RIP. Some of the common triggers include:

1. Link Failure: When a link between two routers goes down, RIP detects the failure and sends a triggered update to inform other routers about the unreachable network.

2. New Link Establishment: If a new link is established, leading to a change in the network topology, a triggered update is sent to let other routers know about the new route.

3. Subnet Mask Change: A change in the subnet mask of a network can also trigger a triggered update, as it affects the routing information.

Unlike regular RIP updates that contain the entire routing table, a triggered update only includes information about the routes that have been affected by the recent change. This helps reduce the size of the message and minimizes the bandwidth consumption.

However, it's important to note that triggered updates can cause a burst of RIP messages in the network. When a router receives a triggered update, it may generate its own triggered update to propagate the change further. This can lead to network congestion and potentially delay the convergence time—the time it takes for all routers in the network to reach a consistent view of the network's topology.

To mitigate the issues caused by triggered updates, RIP uses several techniques:

1. Split Horizon: Split horizon is a technique used to prevent a router from advertising a route back to the interface through which it learned the route. By avoiding the advertisement of routes through the same interface, routing loops can be prevented.

2. Poison Reverse: Poison reverse is a mechanism that helps prevent routing loops when a link failure occurs. When a router detects a link failure, it advertises the route to the affected network with a metric of 16. This high metric indicates to other routers that the route is unreachable, effectively poisoning the route.

3. Hold-Down Timers: Hold-down timers are used to prevent routers from rapidly changing their routing tables in response to frequent triggered updates. When a router receives a triggered update, it enters a hold-down state for a specified period. During this time, the router ignores any other updates related to the same route. The hold-down timer helps stabilize the network and prevents routers from constantly adjusting their routes due to rapidly changing information.

By utilizing split horizon, poison reverse, and hold-down timers, RIP can effectively manage triggered updates and improve the overall convergence time of the network. The goal is to strike a balance between quickly updating routers about changes in the network and avoiding excessive update messages that can lead to inefficiencies and instability.