Distance vector routing protocols use four main techniques in attempting to prevent routing loops. These include:
Defining Infinity. Instead of allowing a packet to just loop around an internetwork endlessly, distance-vector protocols define what is considered to be infinity by specifying a maximum hop count. For example, RIP considers any route that takes more than 15 hops to reach to be unreachable. Remember that TTL value that gets decremented by 1 each time a router is crossed? Once the TTL reaches 0, the packet is discarded. While this technique doesn’t eliminate routing loops on its own, it does ensure that a packet doesn’t loop around an internetwork forever.
Split Horizon. In looking at the routing loop example, you should have noticed that Router A ultimately let Router B know about a route to network 172.16.0.0, even though it originally learned that route from Router B. Split horizon is a rule that specifies that a router can never send information about a route back to the router that originally supplied the information. Had split horizon been used in our example, Router A would not have included information about network 172.16.0.0 in its update to Router B.
Route Poisoning. Usually used in conjunction with split horizon, route poisoning involves explicitly poisoning a routing table entry for an unreachable network. In our example, once Router C learned that network 172.16.0.0 was unavailable it would have immediately poisoned the route to that network by setting its hop count to the routing protocol’s infinity value. In the case of RIP, that would mean a hop count of 16.
Triggered Updates. Obviously the interval between routing table updates is part of the problem that leads to routing loops. Instead of relying on regular periodic updates, distance vector protocols will send out a triggered update when a metric to reach a network increases. In our example, Router C would immediately send out an update about network 172.16.0.0 not being available to Router B, who would then immediately send out an update to Router A. While triggered updates cause a little more network traffic in the short term, they go a long way towards faster convergence on a distance-vector network.
Holddowns. Holddowns are a technique used to ensure that a route recently removed or changed is not reinstated by a routing table update from another router. Even with the use of triggered updates, it will still take some time for the information to reach all routers on the network. As such, there is still the possibility that a not-yet-current routing table update will attempt to reinstate a route that is unreachable. Holddowns make a router wait a period of time before accepting an update for a network whose status or metric has recently changed, giving a network the opportunity to converge.
