Split Horizon, Route Poisoning and Holddown Explained.
Split Horizon
RIP as a distance vector protocol is also susceptible to routing loops (network problem which enables a data packet to continually be routed through the same routers over and over. The data packets continue to be routed within the network in an endless circle.).
Split horizon is one of the features of distance vector routing protocols that prevents routing loops. This feature prevents a router from advertising a route back to the interface from which it was learned or received.
Illustrated network topology.
Using the above diagram, R2 has a route to the subnet 192.168.1.0/24 that is advertised to router R1 by using RIP.
R1 receives the update and stores the route on its routing table.
R1 knows that the routing update for that route has come from R2, so it won’t advertise the route back to router R2, because, if the route to the network goes down, router R1 could receive a route to the subnet 192.168.1.0/24 from R2.
Router R1 now believes that R2 has the route to reach the subnet, and uses that route. R2 receives update packets from R1 and sends them back to R2 because both routers think that each has a route to reach the subnet, this will go on forever; this is what is called a routing loop.
Route Poisoning
Route poisoning is another method distance vector routing protocols use to prevent routing loops.
When a router detects that one of its directly connected routes has failed, the router sends the advertisement for that route with an infinite metric of 16 (“poisoning the route”).
Any router on the network that receives the update will realize that the route has failed and doesn’t use it anymore.
Consider the following example.
Note this; R1 is directly connected to the 192.168.1.0/24 subnet.
R1 has RIP enabled and the subnet is advertised to R2.
When the R1’s Fa0/1 interface fails, a route update advertisement is sent by R1 to R2, indicating that the route has failed.
The route will be labelled with a metric of 16, which is more than the RIP’s maximum hop count of 15, so R1 considers the route to be unreachable.
Hold-down
Hold-down is another loop-prevention mechanism used by distance vector routing protocol.
This feature prevents a router from learning new information about a failed route. When a router receives updated information about an unreachable route, a hold-down timer is instantly started.
The router overlooks all routing updates for that route until the timer expires (180 seconds is the default for RIP).
The only routing updates permitted during that period are updates sent from the router that initially advertised the route.
If the R1 router advertises the update, the hold-down timer is stopped and the routing information is processed.
Let’s use the following network topology below as an example:
The hold-down time starts by R2 after it receives a failed (unreachable) route update advertisement from R1.
During that time all updates from any other routers about that route are ignored to prevent routing loops.
If interface Fa0/1 on R1 comes back up, R1 will advertise the route once more.
The r2 process updates even while the hold-down timer is still running, because the update is sent by the same router that originally advertised the route.
In Summary
A routing loop is a common issue in a computer network. This happens where data packets become trapped, endlessly circulating between routers without reaching their intended destination.
Split Horizon:
Function: Split Horizon prevents a router from advertising a route back out the same interface where it originally learned the route.
Purpose: To eliminate routing loops.
How It Works:
If a network is reachable through interface FastEthernet 0/0, the router does not advertise that network to other routers connected to the same interface. This technique ensures deterministic routing and avoids loops. It propagates data packets forward to all attached nodes except the router that sent the new update.
Route Poisoning
Purpose: To reduce routing loops.
Technique: When a link is down, the router immediately starts a hold-down timer.
Effect: The poisoned route becomes unusable, breaking the loop instantly.
Example: In RIP, poisoned updates indicate unreachable routes, preventing loops3.
Holddown:
Role: Holddown timers complement Split Horizon and Route Poisoning.
Scenario: In the event of a routing loop without poison reverse, waiting for timers to expire would be necessary.
Holddown Timer: Starts when a router learns that an attached link is down; this enable the router to temporarily avoids accepting updates for that route.
Use Case: Prevents premature route reinstatement during network instability.
Remember, these mechanisms contribute to stable and efficient routing in network environments
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