[ixpmanager] BFD on Route Servers

[Richard Laager]

MICE has long enabled passive BFD (Bidirectional Forwarding Detection, 
RFC 5880) on our route servers. Here, "passive" means the route servers 
will speak BFD to a customer, but only if the customer initiates it. 
Accordingly, it is NOT necessary to track a per-customer setting about 
whether they want BFD.

We converted our route servers to use the configuration generated by IXP 
Manager. IXP Manager does not support BFD, so we added it using a skin. 
My question here is whether there is desire to see that upstreamed.

I've started a draft PR:
https://github.com/inex/IXP-Manager/pull/854

The first commit in there has been submitted as a separate PR and can be 
ignored for this discussion. The relevant changes are in the second 
commit, currently:
https://github.com/inex/IXP-Manager/pull/854/commits/1c684aa7a662480af2930c2e525a09a95c717e1c

As you can see, the changes themselves are trivial. However, if this is 
going to be merged into IXP Manager, there are some important design 
questions:

Is BFD going to be enabled everywhere? If not, it needs an option. Is 
that option on or off by default? If on, should it be set to off for 
existing Routers on migration?

Should the interval be customizable? Should the multiplier?

-- 
Richard