RIP Special Features For Resolving RIP Algorithm Problems
(Page 4 of 4)
Split horizon tries to solve the counting to infinity problem by suppressing the transmission of invalid information about routes that fail. For extra insurance, we can implement a feature that changes how devices receiving route information process it in the case of a failed route. The hold down feature works by having each router start a timer when they first receive information about a network that is unreachable. Until the timer expires, the router will discard any subsequent route messages that indicate the route is in fact reachable. A typical hold-down timer runs for 60 or 120 seconds.
The main advantage of this technique is that a router won't be confused by receiving spurious information about a route being accessible when it was just recently told that the route was no longer valid. It provides a period of time for out-of-date information to be flushed from the system, which is valuable especially on complex internetworks. The addition of hold-down to split horizon can also help in situations where split horizon alone is not sufficient to prevent counting to infinity, such as when a trio of routers are linked in a triangle, as discussed earlier.
The main disadvantage of hold-down is that it forces a delay in a router responding to a route once it is fixed. Suppose that a network hiccup causes a route to go down for five seconds. After the network is up again, routers will want to again know about this. However, the hold-down timer must expire before the router will try to use that network again. This makes internetworks using hold-down relatively slow to respond to corrected routes, and may lead to delays in accessing networks that fail intermittently.
Again, while I called the items above features, at least some of them are really necessary to ensure proper RIP functionality. Therefore, they are generally considered standard parts of RIP, and most were described even in the earliest RIP documents. However, sometimes performance or stability issues may arise when these techniques are used, especially in combination. For this reason different RIP implementations may omit some features. For example, hold-down slows down route recovery and may not be needed when other features such as split horizon are used. As always, care must be taken to ensure that all routers are using the same features, or even greater problems may arise.
Also see the specific section on RIP-2 for a description of the Next Hop feature that helps reduce convergence and routing problems when RIP is used.
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