[c-nsp] Temporarily disable all forwarding on ASR9K

[John Neiberger]

We need to upgrade some ASR9Ks that have a lot of connected devices with
complex interrelationships and we have to do a lot of work to make sure all
the correct redundancy is in place prior to the upgrade. Since the router
takes so long to reload, I'd like to find a way to essentially simulate the
loss of forwarding for a minute or so to verify that our redundancy
preparations were thorough, but I need to be able to back out of it
quickly. I thought about shutting down the linecards but that's still a
fairly long restart. I'm hoping to find some method much faster than that.

The simplest and most straightforward way is to shut down all the
interfaces manually and then rollback if necessary. We can take it out of
routing by setting the overload bit in ISIS, but that still leaves routing
and forwarding in place for locally connected interfaces, which is what we
want to stop. We were tossing around some ideas and wondered, probably just
academically, if there were a way to completely stop forwarding temporarily.

Is there a way to disable forwarding through an ASR9K that is easily and
quickly reversible? We'll probably do the interface shutdown method since
it's so simple, but now I'm curious what other options might be available.