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Tutorial
MPLS
Network Reliance and Recovery
Rick
Gallaher is course
director for CCI,
President of Telecommunications Technical Services Inc., and author of Rick
Gallaher's MPLS Training Guide
December 17, 2001
This
series of tutorials has defined MPLS into two operations: data flow
and signaling. The previous tutorials have addressed these subjects
with special attention given to signaling protocols CR-LDP and
RSVP-TE. To complete this series, this article will cover the
failure recovery process.
Vocabulary
- Back-up
Path:
the path that traffic takes if there is a failure on the primary
path.
- Fast
ReRoute
(FRR): a protection plan in which a failure can be detected without
a need for error notification or failure signaling (Cisco).
- Link
Protection:
a backup method that replaces the entire link or path of a
failure.
- Make
Before Break:
a procedure in which the back-up path is switched in before the
failed path is switched out. For a small period of time, both
the primary and back-up paths carry the traffic.
- Node
Protection:
a backup procedure in which a node is replaced in a failure.
- Pre-provisioned
Path:
a path in the switching database on which traffic engineering
has been performed in order to accommodate traffic in case of a
failure.
- Pre-qualified
Path:
a path that is tested prior to switchover that meets the quality
of service (QoS) standards of the primary path.
- Primary
Path:
the path through which the traffic would normally progress.
- Protected
Path:
a path for which there is an alternative back-up path.
- Rapid
ReRoute (RRR):
a protection plan in which a failure can be detected without a
need for error notification or failure signaling (Generic).
Introduction
Around
the country you will find highways under repair.
A good many of these highways have bypass roads or detours to
allow traffic to keep moving around the construction or problem
areas. Traffic
rerouting is a real challenge for highway departments, but they have
learned that establishing detour paths before construction begins is
the only way they can keep traffic moving (Figure 1).
Figure
1: Traffic Detour
The
commitment to keeping traffic moving has been a philosophy in voice
and telephone communications since its inception. In a telephony
network, not only are detour paths set-up before a circuit is
disconnected (make before break), but the back-up or detour paths
must have at least the same quality as the links that are to be
taken down for repair. These paths are said to be pre-qualified
(tested) and pre-provisioned (already in place).
Historically
in IP networking, packets would find their own detours around
problem areas; there were no pre-provisioned bypass roads. The
packets were in no particular hurry to get to the destination.
However, with the convergence of voice onto data networks, the
packets need these bypass roads to be pre-provisioned so that they
do not have to slow down for the construction or road failures.
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