Traffic engineering may be applied for some, or for all, traffic and may
be applied either in an ???online??? mode where each MPLS LER calculates the optimal path
MPLS 435
for an LSP using CSPF or in an ???offline??? mode where the service provider calculates optimal
paths for all required LSPs and then configures an explicit route for each LSP at
its ingress LER. A third mode where LSRs may request a path from a dedicated server
is being standardised and is known as the Path Computation Element (PCE).
Layer 2 VPNs A fourth application for MPLS is the establishment of Layer 2 VPNs either
by using sets of point-to-point connections (known as pseudowires) or by emulating
a switched Ethernet LAN (using a technology known as VPLS). These are discussed in
the next two sections of this chapter. Although service providers offering IP-VPN have
generally used LDP-signalled LSPs, providers offering Layer 2 services more commonly
use RSVP-TE in order to offer deterministic bandwidth and delay guarantees and rapid
service restoration in the event of failures.
Pseudowires
Historically, IP networks have been layered on top of connection-orientated Layer 1
or Layer 2 technologies??”for example, SDH/SONET virtual containers or ATM virtual
circuits??”which have then adapted the traffic onto the underlying transport infrastructure
(fibres or wavelengths).
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