BGP Calculated LSP Next Hops

BGP can also calculate how to use LSPs to reach BGP next hops.

For example, an IBGP session is established across the OSPF/MPLS backbone, and the communicating routers run both OSPF and IBGP. When an IBGP route is installed, BGP determines whether a matching LSP next hop exists to the destination. If not, it checks for an LSP to the BGP next hop. If an LSP exists to the BGP next hop, that LSP is used as an LSP next hop for the IBGP route.

The recalculation requirements for BGP are similar to those for OSPF; when an LSP to a BGP next hop router changes state, the BGP routing table entries must be checked to ensure their LSP next hop information is still valid.