Route limits and router protection filters
Settings that stop a router from accepting more routes than it can handle, whose absence or poor calibration can bring down an entire network.
A router keeps in memory the table of routes it learns from its neighbors; if a neighbor suddenly sends hundreds of thousands by mistake, memory fills up and the router fails. Two protections exist: the filter, which accepts only expected routes, and the limit, which shuts down the session when the number of routes exceeds a threshold. They pose a dilemma: without them, a neighbor's error floods the network; set too tight or all at the same threshold, they make every router isolate at once. For cyber and property insurers, these settings are one of the few points where a large non-malicious outage can be prevented. Investigation reports almost always cite them, and whether they are audited belongs in the assessment of a carrier's risk.
Two nationwide outages show both sides of the setting: on July 8, 2022, Rogers lost its core network after a filter was removed from distribution routers; on November 8, 2023, about 90 Optus routers isolated themselves together on reaching their route limit, set at the manufacturer's default values.
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