Arrcus & Internet Initiative Japan Arrcus, Inc Arrcus In Massive Data Centers, BGP-SPF and similar routing protocols are used to build topology and reachability databases. These protocols need to discover IP Layer-3 attributes of links, such as neighbor IP addressing, logical link IP encapsulation abilities, and link liveness. This Layer-3 Discovery and Liveness protocol collects these data, which may then be disseminated using BGP-SPF and similar protocols.