BGP Route Servers

[ipspace]

I wanted to implement BGP route servers as a separate plugin, but it looks like we could squeeze the baseline functionality into bgp.session plugin (ignoring the idea of having multiple RIB views, if someone wants to have that, they're more than welcome to implement it in a bgp.rs plugin).

It seems like we need:

• bgp.as_check nerd knob on the RS clients (default is True, which means it would have to be set to False). It could be set manually or by the plugin whenever a BGP neighbor is a route server.
• bgp.rs node parameter on the server that would then set bgp.rs_client on the BGP neighbors (to be used in the templates).

Other considerations:

• EBGP sessions between route servers are a bad idea as they can result either in unwanted AS insertion or routing loops. The plugin should trim the EBGP neighbors to remove them.
• I don't think it makes sense to have IBGP sessions on route servers, but we could make this a configurable behavior.

Any thoughts @steffann @ssasso @jbemmel?

[ipspace]

Implemented in #1196