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Routing loops can occur when inconsistent
routing tables are not updated due to slow convergence in a changing
network.
- Just before the failure of Network
1, all routers have consistent knowledge and correct routing tables.
The network is said to have converged. Assume for the remainder of
this example that Router C's preferred path to Network 1 is by way
of Router B, and the distance from Router C to Network 1 is 3.
- When Network 1 fails, Router E sends
an update to Router A. Router A stops routing packets to Network 1,
but Routers B, C, and D continue to do so because they have not yet
been informed of the failure. When Router A sends out its update,
Routers B and D stop routing to Network 1. However, Router C has not
received an update. To Router C, Network 1 is still reachable via
Router B.
- Now Router C sends a periodic update
to Router D, indicating a path to Network 1 by way of Router B.
Router D changes its routing table to reflect this good, but
incorrect, information, and propagates the information to Router A.
Router A propagates the information to Routers B and E, and so on.
Any packet destined for Network 1 will now loop from Router C to B
to A to D and back to again to C.
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