Routing Loop, Split Horizon and Route poisoning

Routing Loops

A routing loop is a situation where a packet keeps getting routed between two or more routers because of problems in the routing table. In case of distance vector protocols, the fact that these protocols route by rumor and have a slow convergence time can cause routing loops.

Split Horizon

solution to the routing loop problem is called split horizon. This reduces incorrect routing

information and routing overhead in a distance-vector network by enforcing the rule that

routing information cannot be sent back in the direction from which it was received.

In other words, the routing protocol differentiates which interface a network route was

learned on, and once this is determined, it won’t advertise the route back out that same interface.

Route Poisoning

Another way to avoid problems caused by inconsistent updates and stop network loops is route

poisoning. For example, when Network 5 goes down, Router E initiates route poisoning by

advertising Network 5 as 16, or unreachable (sometimes referred to as infinite).

This poisoning of the route to Network 5 keeps Router C from being susceptible to incorrect

updates about the route to Network 5. When Router C receives a route poisoning from Router E,

it sends an update, called a poison reverse, back to Router E. This ensures all routes on the segment

have received the poisoned route information.

Routing Information Protocol .

Route poisoning and split horizon create a much more resilient and dependable distancevector

network than we’d have without them, and they serve us well in preventing network

loops.

Hold-down

A hold down prevents regular update messages from reinstating a route that is going up and

down (called flapping). Hold down  prevent routes from changing too rapidly by allowing time for either the downed route to come back up or the network to stabilize somewhat before changing to

the next best route. These also tell routers to restrict, for a specific time period, changes that

might affect recently removed routes.

When a router receives an update from a neighbor indicating that a previously accessible network

isn’t working and is inaccessible, the hold down timer will start. If a new update arrives

from a neighbor with a better metric than the original network entry, the hold down is removed

and data is passed. But if an update is received from a neighbor router before the hold down

timer expires and it has an equal or lower metric than the previous route, the update is ignored

and the hold down timer keeps ticking. This allows more time for the network to stabilize before

trying to converge.

Hold downs use triggered updates that reset the hold down timer to alert the neighbor routers

of a change in the network. Unlike update messages from neighbor routers, triggered updates

create a new routing update that is sent immediately to neighbor routers because a change was

detected in the internetwork.

There are three instances when triggered updates will reset the hold down timer:

The hold down timer expires.

Another update is received with a better metric.

A flush time, which is the time a route would be held before being removed, removes the

route from the routing table when the timer expires.