Routing Loop Avoidance Routing Loop Avoidance Consider the above diagram, If no loop avoidance mechanism has been configured and if the network 192.168.3.0 /24 goes down, Router C will update Router B within the update times limits ( 30 seconds) mentioning that the route is unreachable i.e. a metric of 16. But it could be possible that Router B, sends an update to Router C first. So, as Router B knows it can reach network 192.168.3.0 /24 in one hop via Router C. This creates an impression for Router C, that it can reach the network 192.168.3.0 /24 in two hops, through Router B. Thus, each router continues to increment the metric of the network till it reaches the hop count of 16. This is known as counting to infinity. Such issues can be prevented by using loop avoidance mechanisms. Split Horizon: This is a mechanism in which , if a Router learns something from a neighbor router, it will NOT advertise that same information to it. Considering our example, Router B actually learnt about the network 192.168.3.0 /24 from Router C, so it will never give information about the network 192.168.3.0 / 24 back to Router C. Cisco devices have split-horizon enabled by default on all routers. Route Poisoning: If a network goes down ( e.g. 192.168.3.0 /24) a triggered update is sent to all its neighbors. The triggered update contains the network that is down as an infinite metric (16). Hold-down Timer: When the other routers know that network 192.168.3.0 /24 is down, they start the hold-down timer which runs for 180 seconds and the below happens: - If any information about the 192.168.3.0 /24 network from another router with a same or poorer metric than existing in the router, then the information is ignored. - If information with better metric for network 192.168.3.0 /24 is received, then the hold-down timer is stopped and the routing table is updated with the new information. - If no information is received and the hold-down timer expires the network is removed from the routing table. Hope this information is of some help ;) !!
Posted on: Sat, 25 Oct 2014 12:35:52 +0000
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