Routing protocol optimizations:
Distance vector protocols suffer from "count to infinity"
problems. Several protocol features exist to prevent or reduce
the possibility of its occurrence. We describe each in turn.
- Triggered updates:
Triggered updates are an attempt to speed up convergence. To get
triggered updates, we simply add a rule that whenever a node changes
the metric for a route, it is required to send update messages almost
immediately, even if it is not yet time for one of the regular update
message. (The timing details will differ from protocol to protocol.
Some distance vector protocols, including RIP, specify a small time
delay, in order to avoid having triggered updates generate excessive
- Split-horizon with poision
reverse: It is never useful to claim reachability for a
destination network to the neighbor(s) from which the route was
learned. "Split horizon" is a scheme for avoiding problems caused by
including routes in updates sent to the node from which they were
learned. The "simple split horizon" scheme omits routes learned from
one neighbor in updates sent to that neighbor. "Split horizon with
poisoned reverse" includes such routes in updates, but sets their
metrics to infinity.
- Route poisoning: Advertise cost as infinity when cost from next
hop has been increasing and continue to stay with current next hop for
- Path hold-down: Do not switch route for n update periods after
cost goes up. This reduces the likelihood of switching to a route that
is invalid due to the count to infinity problem.