ets. They can support authentication of routing updates. As a result, OSPF-TE and IS-IS are conceptually similar. While OSPF-TE is natively built to route IP and is itself a Layer 3 protocol that runs on top of IP, SO that OSPF-TE packets are a particular type of IP packets, IS-IS is natively an OSI network layer protocol (it is at the same layer as CLNS [35-37]), a fact that may have allowed OSPF to be more widely used. IS-IS does not use IP to carry routing information messages and its packets are not IP packets. In line of principle IS-IS does not need IP addressing, even if, since some form of address- ing is needed to route GLSPs through the network, in practical applications IP addressing is almost always used in conjunction with IS-IS. Routing entities of a network implementing IS-IS build a topological representation of the network. This map indicates the subnetworks that each IS-IS enabled equipment can reach, and the lowest cost (shortest) path to any subnetwork is used to forward IS-IS pack- ets and to route GLSPs. Due to its structure, IS-IS is also more scalable that OSPF-TE: given the same set of resources, IS-IS can support more routing entities in an area with respect to OSPF-TE. IS-IS differs from OSPF-TE also in the way that "routing areas" are defined and interarea routing is performed. No hierarchy similar to that present in IS-IS is present in OSPF-TE, whose routing entities are all on the same ground, capable of interarea or intraarea routing when it is needed. In OSPF, areas are delineated on the interface such that an area border router (ABR) is actually in two or more areas at once, effectively creating the borders between areas inside the ABR, whereas in IS-IS area borders are in between routers, designated as Level 2 or Level 1-2. The result is that an IS-IS router is only ever a part of a single area. IS-IS also does not require the backbone (or Area Zero as it is sometimes called) through which all interarea traffic must pass, while OSPF-TE needs this definition to manage multiarea ro