Các biện pháp chống routing loop trong distance vector

Routing loop trong distance vector xảy ra khi có sự cố về đường mạng hay khi có sự thay đổi đường mạng làm cho bảng định tuýên của router không kịp cập nhật thông tin do độ hội tụ chậm nên vịêc truyền gói tin đi bị loop. Có 5 cách để chống loopefining a maximum count ,split horizon, Route poisoning,triggered updates ,holddown timers
1)Defining a maximum count:

- Distance vector routing algorithms are self-correcting, but a routing loop problem can require a count to infinity. To avoid this prolonged problem, distance vector protocols define infinity as a specific maximum number. This number refers to a routing metric which may simply be the hop count.
- With this approach, the routing protocol permits the routing loop to continue until the metric exceeds its maximum allowed value.*The graphic shows the metric value as 16 hops. This exceeds the distance vector default maximum of 15 hops so the packet is discarded by the router. In any case, when the metric value exceeds the maximum value, Network 1 is considered unreachable.
2) split horizon

- Another possible source for a routing loop occurs when incorrect information that has been sent back to a router contradicts the correct information that the router originally distributed. Here is how this problem occurs:
- Router A passes an update to Router B and Router D, indicating that Network 1 is down. Router C, however, transmits an update to Router B, indicating that Network 1 is available at a distance of 4, by way of Router D. This does not violate split-horizon rules.
- Router B concludes, incorrectly, that Router C still has a valid path to Network 1, although at a much less favorable metric. Router B sends an update to Router A advising Router A of the new route to Network 1.
- Router A now determines that it can send to Network 1 by way of Router B; Router B determines that it can send to Network 1 by way of Router C; and Router C determines that it can send to Network 1 by way of Router D. Any packet introduced into this environment will loop between routers.
- Split-horizon attempts to avoid this situation. If a routing update about Network 1 arrives from Router A, Router B or Router D cannot send information about Network 1 back to Router A. Split-horizon thus reduces incorrect routing information and reduces routing overhead
3) Route poisoning
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- Route poisoning is used by various distance vector protocols in order to overcome large routing loops and offer explicit information when a subnet or network is not accessible. This is usually accomplished by setting the hop count to one more than the maximum.
One way to avoid inconsistent updates is route poisoning. When Network 5 goes down, Router E initiates route poisoning by making a table entry for Network 5 as 16, or unreachable. By this poisoning of the route to Network 5, Router C is not susceptible to incorrect updates about the route to Network 5. When Router C receives a router poisoning from Router E, it sends an update, called a poison reverse, back to Router E. This makes sure all routes on the segment have received the poisoned route information.
When route poisoning is used with triggered updates it will speed up convergence time because neighboring routers do not have to wait 30 seconds before advertising the poisoned route.
Route poisoning causes a routing protocol to advertise infinite-metric routes for a failed route. Route poisoning does not break split horizon rules. Split horizon with poison reverse is essentially route poisoning, but specifically placed on links that split horizon would not normally allow routing information to flow across. In either case, the result is that failed routes are advertised with infinite metrics.
4)triggered updates :

-New routing tables are sent to neighboring routers on a regular basis. For example, RIP updates occur every 30 seconds. However a triggered update is sent immediately in response to some change in the routing table. The router that detects a topology change immediately sends an update message to adjacent routers that, in turn, generate triggered updates notifying their adjacent neighbors of the change. When a route fails, an update is sent immediately rather than waiting on the update timer to expire. Triggered updates, used in conjunction with route poisoning, ensure that all routers know of failed routes before any holddown timers can expire. Triggered updates go ahead and send updates because routing information has changed not waiting for the timer to expire. The router sends another routing update on its other interfaces rather than waiting on the routing update timer to expire. This causes the information about the status of the route that has changed, to be forwarded and starts the holddown timers more rapidly on the neighboring routers. The wave of updates propagates throughout the network.
Issuing a triggered update Router C announces that network 10.4.0.0 is unreachable. Upon receipt of this information, Router B announces through interface S0/1 that network 10.4.0.0 is down. In turn, Router A sends an update out interface Fa0/0.
5) holddown timers:
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-A count to infinity problem can be avoided by using holddown timers:
When a router receives an update from a neighbor indicating that a previously accessible network is now inaccessible, the router marks the route as inaccessible and starts a hold-down timer.
If at any time during hold-down timer an update is received from
The same neighbor indicating that the network is again accessible, the router marks the network as accessible and removes the hold-down timer.
A different neighboring router with a better metric than originally recorded for the network, the router marks the network as accessible and removes the hold-down timer.*
A different neighboring router with a poorer metric, the update is ignored.