Chapter 2 Advanced Routing for FortiOS 5.0 : Intermediate System to Intermediate System Protocol (IS-IS) : Troubleshooting IS-IS : Routing loops : Routing loop effect on the network
  
Routing loop effect on the network
In addition to this “traffic jam” of routed packets, every time the routing table for a router changes that router sends an update out to all of the IS-IS routers connected to it. In a network loop, its possible for a router to change its routes very quickly as it tries and fails along these new routes. This can quickly result in a flood of updates being sent out, which can effectively grind the network to a halt until the problem is fixed.