FRR default route handling

[jbemmel]

In the current configs, FRR servers are announcing the default route they received back to the leaves, where it gets discarded

There are several ways to avoid this:

1. Add neighbor 192.168.99.1 sender-as-path-loop-detection to each FRR node
2. Add a BGP export policy to each leaf, allowing only a default route and marking it with a 'no-export' community
3. Other ideas?

[hellt]

I think opt1 is a good one