mgd/ddm are still originating a tunnel endpoint prefixes when nexthops are physically down

[rcgoodfellow]

It's been observed that when we have a route to a given prefix, and all the routes to that prefix are on links that are physically down, we're still advertising the switch as a tunnel exit point for that prefix. That's problematic as we're essentially advertising a doomed route and this will cause traffic disruption.

I think we need to add a check in mg-lower that checks routes for active links before sending them to ddm to advertise.

[rcgoodfellow]

Somewhat related to

• mgd/ddm should advertise a lower tunnel exit metric when all BFD links are down #389