IP-VPNs One of the first applications for MPLS was to offer IP-VPNs. As mentioned
previously, the MPLS VPN architecture is described in RFC 4364 but is often known
as RFC 2547 after the initial RFC published to describe it. Most IP-VPN networks use
LDP-signaled LSPs, since a full mesh of tunnels is required between the LERs and
these can most easily be created using LDP.
???BGP Free??? Core Networks Some service providers have deployed MPLS as a means
of keeping their core networks free from external routes. Edge routers still use BGP to
learn external routes; however, they reach each other using LSPs across an MPLS core.
Since the core routers now switch packets over LSPs, each core router only needs to know
how to reach the edge routers. This also enhances security since external customers are
unable to communicate with the core routers. Multiple sets of edge routers (e.g., one for
public Internet, one for IP-VPNs, one for Layer 2 VPNs, and one for Voice over IP services)
may be independently interconnected over a common MPLS core.
Traffic Engineering Many service providers have deployed MPLS traffic engineering
using RSVP-TE to enable them to optimise the efficiency of their networks and to specify
explicit routes for traffic in cases where certain constraints must be met (for example,
routing voice traffic over the shortest possible path and best-effort traffic over the cheapest
possible path).
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