Although there have been various efforts to define
solutions for inter-area traffic, engineering size limitations for a single IGP area (typically
several hundred nodes) are greater than the number of nodes that can be meshed
with RSVP-TE LSPs (the ???N squared??? problem discussed below).
For LDP LSPs, each LSR must have a host route for every other LSR to which it
wishes to build an LSP. This prevents these routes from being summarised at area
boundaries within the SP??™s network if LDP is to be used, but does not prevent other
routes (for example, those corresponding to the interfaces connecting routers) from being
summarised. An IGP can typically scale to several thousand routes, ensuring that
LDP LSPs can scale for most networks.
Label State Each MPLS LSR along the path of an LSP installs state for that LSP. In
the case of RSVP-TE, LSPs are point-to-point, and if connectivity is required between
a set of N LSR, then N(N ??“ 1) LSPs are required. This is known as the ???N squared???
problem and limits network scaling at the intermediate LSRs. LDP LSPs are generally
multipoint-to-point in nature since an LSR forwards all traffic to a given FEC to the
same next hop and using the same label.
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