Thus, for connectivity between a set of N LSR,
just N LDP LSPs are required.
LSP Hierarchy One solution to improving network scaling is to use LSP hierarchy,
based on adding labels to the MPLS label stack. For example, one popular approach
is to run LDP ???over??? RSVP-TE. In this approach, LDP LSPs are used end to end, but
RSVP-TE LSPs are used to cross the backbone area. This enables service providers to
use the rapid restoration mechanisms of RSVP-TE and to traffic engineer within the
backbone (typically from city to city), but to scale the network to a similar size to that
achievable with a pure LDP design. In order for this approach to operate, the Area
Border Routers (ABRs) must be configured with targeted LDP sessions to each other,
so that the ABRs for each area can advertise labels to the ABRs for all other areas for
each edge router (Label Edge Router or LER) in the area. For a network of N LERs
and M areas with two ABRs per area, this results in N + 2M(M ??“ 1) LSPs for a network
where each LER has a single LSP to every other LER.
This ???LDP over RSVP-TE??? approach may be extended to use an RSVP-TE LSP from
the LER to the ABR, from ABR to ABR, and from ABR to LER.
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