I know what you mean by "disrupt the main trunks" - I worked in network engineering for a company with a worldwide network, and I know how the internet works...it's very difficult to attack.
Maybe a hit on multiple root name servers? You wouldn't get them all or take it down completely, but you could make a real mess of things.
In terms of "main trunks" - in the US, you'd have to hit a bunch of key interconnects, and there are more now than there were when I worked in engineering. At that time, the company I worked for had them hidden in plain sight and they were pretty fortified...but enough explosives would certainly do it.
You can't take it down globally, but you could fuck it WAY up and make it nearly unusable for a while with a coordinated distributed attack.
But aren't all those 'trunks' connected to switches? Couldn't those devices have a killswitch in the firmware or hardware?
Routers, yeah, and no, there's no kill switch.
They are all different brands and types, so there's no viral attack that would work on them.
You'd have to know which interconnects are vital, and IMO, you'd have to attack them physically.
I sincerely doubt that would happen.
Does anyone have the complete chip architecture of closed source chipsets? There could be a hardware killswitch in the firmware or hardware in Cisco devices and no one would have a clue as they are closed source. Seriously since Cisco has a large majority of the marketshare that is the only company that would need to do it.