RIPv2: Convergence and Loop Prevention

RIPv2 does not include any loop prevention mechanisms in its algorithm to calculate best paths to remote networks.  As a result additional Loop Prevention mechanisms are built into RIPv2 as a result of the protocols inability to incorporate loop prevention in line with its routing decisions.

  • Counting to Infinity – if the next hop to a destination network suddenly advertises that same network with an increased metric, RIPv2 will accept the advertisement and update the metric accordingly.  If the updated metric reaches infinity (16) stop using that next hop
  • Split Horizon – Take routes learned and installed into the RIP routing table and advertise those routes out all participating RIPv2 interfaces except the interface for which the route is learned from.
  • Split Horizon with Poisoned Reverse – Same thing as split horizon except the routes that are advertised out all other ports are done so with an infinite metric.
  • Route Poisoning – the process of sending updates for failed routes with an infinite metric prompting removal of the route from routing tables.
  • Triggered Update – RIPv2 will immediately send an update when routing information changes, instead of waiting on the update timer to expire.  Only the updated network is sent in the update as opposed to the entire RIPv2 routing table.
  • Update Timer – A timer that specifies the time interval between routing updates (default 30 seconds)
  • Invalid After timer – A per route timer that is reset and begins after an update about a route is received.  If routes from the source start failing and updates are no longer coming in, this will prompt the invalid after timer (default 180 seconds) if no update received by the end of this timer, this triggers the holddown timer.
  • Holddown timer – a per route timer that begins after the route has been declared invalid.  The router starts advertising the route as unreachable, it will not accept any updated information and does not modify the routing entry until the holddown timer expires.
  • Flushed after Timer – a per route timer that is reset and starts after an update about a routed is received.  if the updates cease to be received and the flushed after timer reaches its limit the router removes the route from the routing table entirely.

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