Managing calculated LSP next hop entries is more involved.
The OSPF (Open Shortest Path First) Shortest Path First (SPF) algorithm checks the availability of LSPs to remote OSPF routers during a calculation. The intra-area SPF algorithm begins with the calculating router as the root of a graph. The graph is expanded by examining the networks connected to the root and then examining the routers connected to those networks. Continuing in this manner, the graph is built as a series of parent and child nodes. A check is made for a matching LSP next hop as each entry is added. A check is also made for an LSP next hop that can be inherited from the parent node. These inherited LSP next hops are referred to as calculated LSP next hops. Thus, for each route table entry, the modified SPF algorithm determines whether a matching LSP next hop is available and whether a calculated LSP next hop is available for use whenever a matching LSP next hop is not present.
The modification to the SPF algorithm described above is important, because it enables the capabilities provided by LDP or RVSP-TE LSPs to be fully utilized, while minimizing the resources devoted to label management.
For example, in a network where all the LERs/LSRs implement this feature (such as an all-Extreme MPLS (Multiprotocol Label Switching) network), labels only need to be advertised for the router IDs of the LERs/LSRs. Yet, LSPs can still be used to route traffic destined for any OSPF route.
More specifically, LSPs can be used for all routes advertised by OSPF, with the possible exception of LDP LSPs to routes summarized by OSPF area border routers (ABRs). The problem with using routes summarized by OSPF ABRs is that route summarization can prevent label mappings from being propagated for the links internal to the area being summarized, since an LSR only propagates LDP labels for FECs that exactly match a routing table entry.
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