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> Deleting the filter caused all possible routes to the internet to pass through the routers, resulting in several of the devices exceeding their memory and pro
by perryizgr8 4y ago
> Deleting the filter caused all possible routes to the internet to pass through the routers, resulting in several of the devices exceeding their memory and processing capacities. This caused the core network to shut down.
I don't get this. If a switch can handle only x number of routes, it won't get "overwhelmed" if you try to add more, it would just refuse to accept any more routes. Why would the network go down completely? These things are designed with extremely robust software, you simply start getting error logs saying that the device has run out of memory, but the operation continues. Most of the heavy lifting is done in hardware chips, so the software just can't program new routes, but the routing itself should continue unaffected.
- citrin_ru 4y ago> it won't get "overwhelmed" if you try to add more, it would just refuse to accept any more routes It not how it worked in old Cisco routers - when more and more routes are added at some point it runs out of fast memory (TCAM) and becomes either too slow or not operational at all. Unless you manually configure route number limit it would install more routes into FIB than it can handle. So one have to monitor TCAM utilization and based on this configure various limits to prevent its exhaustion. May be TCAM exhaustion is still a problem even in modern Cisco equipment - I've moved to another field and don't know much about it.
- perryizgr8 4y agoI've personally tested this scenario on relatively newer Cisco devices and it literally just prints a message on the console to the tune of "TCAM exhausted, refusing to add route A.B.C.D/X" and continues working without problems. I think there is a config on some models to install the route in the slow path, but by default it just doesn't use the route.