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Maximum hop count , Hold down timers , split horizon & route poisioning.
In a link-state routing protocol, such as OSPF or IS-IS, a routing loop disappears as soon as the new network topology is flooded to all the routers within the routing area. Assuming a sufficiently reliable network, this happens within a few seconds.
Newer distance-vector routing protocols (BGP, EIGRP, DSDV, Babel) have built-in loop prevention: they use algorithms that assure that routing loops can never happen, not even transiently. Older routing protocols (RIP) do not implement the newest forms of loop prevention and only implement mitigations such as split horizon, route poisoning and holddown timers.
There are loop prevention mechanisms in Every Routing Protocol, Loop mitigation mechanism have already been mentioned here. which is the case with RIP.
EIGRP, Feasible and Advertise Distance do the trick for Loop Prevention.
OSPF, For Loop Prevention Multi-Area OSPF scenario requires all Inter-Area traffic to traverse over backbone area.
BGP makes use of AS Path for Loop Prevention
A routing loop is a serious network problem which happens when a data packet is continually routed through the same routers over and over. Routing Loop can happen in large internetworks when a second topology change emerges before the network is able to converge on the first change.
The following methods are used to avoid Routing Loops.
Maximum hop count mechanism can be used to prevent Routing Loops. Distance Vector protocols use the TTL (Time-to-Live) value in the IP datagram header to avoid Routing Loops. When an IP datagram move from router to router, a router keeps track of the hops in the TTL field in the IP datagram header. For each hop a packet goes through, the packet’s TTL field is decremented by one. If this value reaches0, the packet is dropped by the router that decremented the value from1 to0.
A split horizon is a routing configuration that stops a route from being advertised back in the direction from which it came. Split Horizon mechanism states that if a neighbouring router sends a route to a router, the receiving router will not propagate this route back to the advertising router on the same interface.
Route Poisoning is another method for avoiding routing loops. When a router detects that one of its connected routes has failed, the router will poison the route by assigning an infinite metric to it.
Hold-down timer is another mechanism used to prevent bad routes from being restored and propagated by mistake. When a route is placed in a hold-down state, routers will neither advertise the route nor accept advertisements about it for a specific interval called the hold-down period.
using STP protocol to avoid loops or stop loop in routing.
As per the Question presented, I believe the following to be helpful.Maximum hop count, Hold down timers, split horizon & route poisoning are the machanisms that different distance vector routing protocols use to avoide loops in a network. However, some routing protocols have specific features to avoide loops such as EIGRP, as it uses Advertised distance and Feasible distance to prevent loops. OSPF however, is a protocol itself which doesnt provide loop prevention mecahanism by the characterstics of the link state routing protocols, but it relies on the LSA types and which LSA is allowed to pass in whichever area. BGP uses two rules that are rule of Synchronization and rule of split horizon. The stated is used to avoide loops.Hope this will be helpful.Thanks.
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