1.
Split horizons should prevent routing loops between adjacent routers, but poison-reverse updates are necessary to defeat larger routing loops.
Increases in routing metrics generally indicate routing loops.
Poison-reverse updates then are sent to remove the route and place it in holddown.
On Distance-Vector protocols, poison-reverse updates are sent if a route metric has increased by a factor of 1.1 or greater.
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2. Let's take EIGRP for example:
EIGRP maintains a number of timers and variables containing time intervals.
These include an update timer, an invalid timer, a hold-time period, and a flush timer.
- The update timer specifies how frequently routing update messages should be sent. The EIGRP default for this variable is 90 seconds.
- The invalid timer specifies how long a router should wait in the absence of routing-update messages about a specific route before declaring that route invalid. The EIGRP default for this variable is three times the update period.
- The hold-time variable specifies the holddown period. The EIGRP default for this variable is three times the update timer period plus 10 seconds.
- Finally, the flush timer indicates how much time should pass before a route should be flushed from the routing table. The EIGRP default is seven times the routing update period.
To answer your question, if the route is received within the interval on the timers mentioned above, the process is reset and the route is installed back into the routing table
