[rbridge] Max Network size / ARP servers

[Alper Yegin]

> RBridges are designed to replace bridges to improve routing, not to
> improve ARP security - the latter of which needs its own solution.
Given
> that solution, it can certainly be supported in an RBridge, but I
don't
> see why we would consider RBridges to uniqely enable or require it.

I agree. Use of an ARP server may be an additional optimization which
can be built on top of Rbridged network, much like other types of
networks. But I think we can and shall keep this separate unless there
is some specific Rbridge feature that requires presence of ARP servers
(which I can't see....)

Alper