Preventing Routing Loops with Holddown timers
Let's imagine that a particular network is unstable. The interface resets as up, then down, then up again in rapid succession. The route is flapping. Using triggered updates, the routers might react too quickly and unknowingly create a routing loop. A routing loop could also be created by a periodic update that is sent by the routers during the instability. Holddown timers prevent routing loops from being created by these conditions. Holddown timers also help prevent the count to infinity condition.
Holddown timers are used to prevent regular update messages from inappropriately reinstating a route that may have gone bad. Holddown timers instruct routers to hold any changes that might affect routes for a specified period of time. If a route is identified as down or possibly down, any other information for that route containing the same status, or worse, is ignored for a predetermined amount of time (the holddown period). This means that routers will leave a route marked as unreachable in that state for a period of time that is long enough for updates to propagate the routing tables with the most current information.
Holddown timers work in the following way:
1. A router receives an update from a neighbor indicating that a network that previously was accessible is now no longer accessible.
2. The router marks the network as possibly down and starts the holddown timer.
3. If an update with a better metric for that network is received from any neighboring router during the holddown period, the network is reinstated and the holddown timer is removed.
4. If an update from any other neighbor is received during the holddown period with the same or worse metric for that network, that update is ignored. Thus, more time is allowed for the information about the change to be propagated.
5. Routers still forward packets to destination networks that are marked as possibly down. This allows the router to overcome any issues associated with intermittent connectivity. If the destination network truly is unavailable and the packets are forwarded, black hole routing is created and lasts until the holddown timer expires.
Split HoriZon Rules
Split Horizon :-Routing Technique in which information about routes is prevented from exiting the router interface through which that information was received split-horizon updates are useful in preventing routing loops.
Another method used to prevent routing loops caused by slow convergence of a distance vector routing protocol is split horizon. The split horizon rule says that a router should not advertise a network through the interface from which the update came.
Applying split horizon to the previous example of route 10.4.0.0 produces the following actions:
R3 advertises the 10.4.0.0 network to R2.
R2 receives the information and updates its routing table.
R2 then advertises the 10.4.0.0 network to R1 out S0/0/0. R2 does not advertise 10.4.0.0 to R3 out S0/0/1, because the route originated from that interface.
R1 receives the information and updates its routing table.
Because of split horizon, R1 also does not advertise the information about network 10.4.0.0 back to R2.
Complete routing updates are exchanged, with the exception of routes that violate the split horizon rule. The results look like this:
R2 advertises networks 10.3.0.0 and 10.4.0.0 to R1.
R2 advertises networks 10.1.0.0 and 10.2.0.0 to R3.
R1 advertises network 10.1.0.0 to R2.
R3 advertises network 10.4.0.0 to R2.
Note: Split horizon can be disabled by an administrator. Under certain conditions, this has to be done to achieve the proper routing.
Route Poisoning
Route poisoning is yet another method employed by distance vector routing protocols to prevent routing loops. Route poisoning is used to mark the route as unreachable in a routing update that is sent to other routers. Unreachable is interpreted as a metric that is set to the maximum. For RIP, a poisoned route has a metric of 16.
The following process occurs:
Network 10.4.0.0 becomes unavailable due to a link failure.
R3 poisons the metric with a value of 16 and then sends out a triggered update stating that 10.4.0.0 is unavailable.
R2 processes that update. Because the metric is 16, R2 invalidates the routing entry in its routing table.
R2 then sends the poison update to R1, indicating that route is unavailable, again by setting the metric value to 16.
R1 processes the update and invalidates the routing entry for 10.4.0.0 in its routing table.
Route poisoning speeds up the convergence process as the information about 10.4.0.0 spreads through the network more quickly than waiting for the hop count to reach "infinity".
Split Horizon with Poison Reverse
Poison reverse can be combined with the split horizon technique. The method is called split horizon with poison reverse. The rule for split horizon with poison reverse states when sending updates out a specific interface, designate any networks that were learned on that interface as unreachable.
The concept of split horizon with poison reverse is that explicitly telling a router to ignore a route is better than not telling it about the route in the first place.
The following process occurs:
Network 10.4.0.0 becomes unavailable due to a link failure.
R3 poisons the metric with a value of 16 and then sends out a triggered update stating that 10.4.0.0 is unavailable.
R2 processes that update, invalidates the routing entry in its routing table, and immediately sends a poison reverse back to R3.
Poison reverse is a specific circumstance that overrides split horizon. It occurs to ensure that R3 is not susceptible to incorrect updates about network 10.4.0.0.
Note: Split horizon is enabled by default. However split horizon with poison reverse may not be the default on all IOS implementations.
Let's imagine that a particular network is unstable. The interface resets as up, then down, then up again in rapid succession. The route is flapping. Using triggered updates, the routers might react too quickly and unknowingly create a routing loop. A routing loop could also be created by a periodic update that is sent by the routers during the instability. Holddown timers prevent routing loops from being created by these conditions. Holddown timers also help prevent the count to infinity condition.
Holddown timers are used to prevent regular update messages from inappropriately reinstating a route that may have gone bad. Holddown timers instruct routers to hold any changes that might affect routes for a specified period of time. If a route is identified as down or possibly down, any other information for that route containing the same status, or worse, is ignored for a predetermined amount of time (the holddown period). This means that routers will leave a route marked as unreachable in that state for a period of time that is long enough for updates to propagate the routing tables with the most current information.
Holddown timers work in the following way:
1. A router receives an update from a neighbor indicating that a network that previously was accessible is now no longer accessible.
2. The router marks the network as possibly down and starts the holddown timer.
3. If an update with a better metric for that network is received from any neighboring router during the holddown period, the network is reinstated and the holddown timer is removed.
4. If an update from any other neighbor is received during the holddown period with the same or worse metric for that network, that update is ignored. Thus, more time is allowed for the information about the change to be propagated.
5. Routers still forward packets to destination networks that are marked as possibly down. This allows the router to overcome any issues associated with intermittent connectivity. If the destination network truly is unavailable and the packets are forwarded, black hole routing is created and lasts until the holddown timer expires.
Split HoriZon Rules
Split Horizon :-Routing Technique in which information about routes is prevented from exiting the router interface through which that information was received split-horizon updates are useful in preventing routing loops.
Another method used to prevent routing loops caused by slow convergence of a distance vector routing protocol is split horizon. The split horizon rule says that a router should not advertise a network through the interface from which the update came.
Applying split horizon to the previous example of route 10.4.0.0 produces the following actions:
R3 advertises the 10.4.0.0 network to R2.
R2 receives the information and updates its routing table.
R2 then advertises the 10.4.0.0 network to R1 out S0/0/0. R2 does not advertise 10.4.0.0 to R3 out S0/0/1, because the route originated from that interface.
R1 receives the information and updates its routing table.
Because of split horizon, R1 also does not advertise the information about network 10.4.0.0 back to R2.
Complete routing updates are exchanged, with the exception of routes that violate the split horizon rule. The results look like this:
R2 advertises networks 10.3.0.0 and 10.4.0.0 to R1.
R2 advertises networks 10.1.0.0 and 10.2.0.0 to R3.
R1 advertises network 10.1.0.0 to R2.
R3 advertises network 10.4.0.0 to R2.
Note: Split horizon can be disabled by an administrator. Under certain conditions, this has to be done to achieve the proper routing.
Route Poisoning
Route poisoning is yet another method employed by distance vector routing protocols to prevent routing loops. Route poisoning is used to mark the route as unreachable in a routing update that is sent to other routers. Unreachable is interpreted as a metric that is set to the maximum. For RIP, a poisoned route has a metric of 16.
The following process occurs:
Network 10.4.0.0 becomes unavailable due to a link failure.
R3 poisons the metric with a value of 16 and then sends out a triggered update stating that 10.4.0.0 is unavailable.
R2 processes that update. Because the metric is 16, R2 invalidates the routing entry in its routing table.
R2 then sends the poison update to R1, indicating that route is unavailable, again by setting the metric value to 16.
R1 processes the update and invalidates the routing entry for 10.4.0.0 in its routing table.
Route poisoning speeds up the convergence process as the information about 10.4.0.0 spreads through the network more quickly than waiting for the hop count to reach "infinity".
Split Horizon with Poison Reverse
Poison reverse can be combined with the split horizon technique. The method is called split horizon with poison reverse. The rule for split horizon with poison reverse states when sending updates out a specific interface, designate any networks that were learned on that interface as unreachable.
The concept of split horizon with poison reverse is that explicitly telling a router to ignore a route is better than not telling it about the route in the first place.
The following process occurs:
Network 10.4.0.0 becomes unavailable due to a link failure.
R3 poisons the metric with a value of 16 and then sends out a triggered update stating that 10.4.0.0 is unavailable.
R2 processes that update, invalidates the routing entry in its routing table, and immediately sends a poison reverse back to R3.
Poison reverse is a specific circumstance that overrides split horizon. It occurs to ensure that R3 is not susceptible to incorrect updates about network 10.4.0.0.
Note: Split horizon is enabled by default. However split horizon with poison reverse may not be the default on all IOS implementations.
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