The MPLS FAQ - Page 1 of 3
a. Who wrote the FAQ?
The FAQ is maintained by Irwin Lazar (ilazar at mplsrc.com).
Contributors are listed at the end of this FAQ.
b. How do I contribute to the FAQ?
Anyone may contribute to the FAQ, either by submitting a question for inclusion,
revising a current answer, or answering a question that currently doesn't have
an answer. Please send all FAQ submissions to faq at mplsrc.com.
c. Where can I find the FAQ?
The FAQ is hosted by the MPLS Resource Center and is available at http://www.mplsrc.com/.
d. What is the purpose of this FAQ?
The MPLS FAQ is meant to provide a central place where one can find answers to
frequently asked questions about MPLS. The questions included within this
FAQ are focused primarily on issues related to design, deployment and management
of MPLS-based networks. Questions related to the development of MPLS and related
standards should be addressed to respective IETF working group mailing lists.
2. MPLS Resources
a. What group is responsible for creating MPLS standards?
The IETF's MPLS Working Group is charged with establishing core MPLS standards.
More information about the activities of the WG can be found on their home page
Other IETF working groups are charged with developing standards
covering areas such as Generalized MPLS, MPLS network management, Layer 2
encapsulation, L2 & L3 VPN services, and MPLS
Traffic Engineering. See http://www.mplsrc.com/standards.shtml
for a complete list of MPLS-related IETF working groups.
In addition, industry groups such as the Optical
Internetworking Forum (OIF), The Optical
Ethernet Forum, and the MFA Forum (MPLS/Frame/ATM) are working on other MPLS standards not
related to the areas of focus of the IETF.
b. Where can I find additional information on MPLS?
For a large collection of articles, papers, and additional resources, see
the MPLS Resource Center at http://www.mplsrc.com/.
An extensive collection of RFCs and Internet Drafts on MPLS
can be found at Noritoshi Demizu's Multilayer Routing Page at http://www.watersprings.org/links/mlr/.
c. What is the MFA Forum?
The MFA is the union of the MPLS Forum, Frame Relay Forum, and ATM Forum. The MFA is an industry consortium dedicated to accelerating the
adoption of Multiprotocol Label Switching (MPLS) and its associated
technologies. More information about the alliance can be found at http://www.mfaforum.org/.
d. What MPLS related mailing lists are there and what are
they used for?
There following is a list of current MPLS-related mailing lists:
e. Are there any books on MPLS?
There are currently dozens of books that cover MPLS and related technologies.
A list can be found on the MPLS Resource
Center's "Books" page at http://www.mplsrc.com/books.shtml
f. Are there any shareware or freeware implementations of
There are several shareware and freeware implementations of
MPLS. For a complete list, see http://www.mplsrc.com/vendor.shtml
3. MPLS History
a. What is MPLS?
MPLS stands for "Multiprotocol Label Switching". In an
MPLS network, incoming packets are assigned a "label" by a "label
edge router (LER)". Packets are forwarded along a "label switch
path (LSP)" where each "label switch router (LSR)" makes
forwarding decisions based solely on the contents of the label. At each
hop, the LSR strips off the existing label and applies a new label which tells
the next hop how to forward the packet.
Label Switch Paths (LSPs) are established by network operators
for a variety of purposes, such as to guarantee a certain level of performance,
to route around network congestion, or to create IP tunnels for network-based
virtual private networks. In many ways, LSPs are no different than
circuit-switched paths in ATM or Frame Relay networks, except that they are not
dependent on a particular Layer 2 technology.
An LSP can be established that crosses multiple Layer 2
transports such as ATM, Frame Relay or Ethernet. Thus, one of the true
promises of MPLS is the ability to create end-to-end circuits, with specific
performance characteristics, across any type of transport medium, eliminating
the need for overlay networks or Layer 2 only control mechanisms.
To truly understand "What is MPLS", RFC
3031 - Multiprotocol Label Switching Architecture, is required reading.
b. How did MPLS evolve?
MPLS evolved from numerous prior technologies including Cisco's "Tag
Switching", IBM's "ARIS", and Toshiba's "Cell-Switched
Router". More information on each of these technologies can be found
The IETF's MPLS Working Group was formed in 1997.
c. What problems does MPLS solve?
The initial goal of label based switching was to bring the speed of Layer 2
switching to Layer 3. Label based switching methods allow routers to make
forwarding decisions based on the contents of a simple label, rather than by
performing a complex route lookup based on destination IP address. This
initial justification for technologies such as MPLS is no longer perceived as
the main benefit, since Layer 3 switches (ASIC-based routers) are able to
perform route lookups at sufficient speeds to support most interface types.
However, MPLS brings many other benefits to IP-based networks,
- Traffic Engineering - the ability to set the path traffic
will take through the network, and the ability to set performance
characteristics for a class of traffic
- VPNs - using MPLS, service providers can create IP tunnels
throughout their network, without the need for encryption or end-user
- Layer 2 Transport - New standards being defined by the
IETF's PWE3 and PPVPN working groups allow service providers to carry Layer
2 services including Ethernet, Frame Relay and ATM over an IP/MPLS core
- Elimination of Multiple Layers - Typically most carrier
networks employ an overlay model where SONET/SDH is deployed at Layer 1, ATM is used at Layer 2 and IP is used
at Layer 3. Using MPLS, carriers can migrate many of the functions of
the SONET/SDH and ATM control plane to Layer 3, thereby simplifying network management and
network complexity. Eventually, carrier networks may be able to
migrate away from SONET/SDH and ATM all-together, which means elimination of ATM's
inherent "cell-tax" in carrying IP traffic.
d. What is the status of the MPLS standard?
There's no such thing as a single MPLS "standard".
Instead there a set of RFCs and IDs that together allow the building of an
MPLS system. For example, a typical IP router spec. sheet will list
about 20 RFCs to which this router will comply. If you go to the IETF web
then click on "I-D Keyword Search", enter "MPLS" as your
search term, and crank up the number of items to be returned, (or visit http://www.mplsrc.com/standards.shtml)
you'll find over 100 drafts currently stored. These drafts have a lifetime
of 6 months.
Most MPLS standards are currently in the "Internet
Draft" phase, though several have now moved into the RFC-STD phase.
See "MPLS Standards" for a complete
listing of current ID's and RFC's. For more information on the
current status of various Internet Drafts, see the IETF's MPLS Working Group
home page at http://www.ietf.org/html.charters/mpls-charter.html
Some of these drafts have been adopted by the IETF WG for
MPLS. The filename for these drafts is prefixed by "draft-ietf-".
Some of these drafts are now on the IETF Standards Track. This is
indicated in the first few lines of the document with the term "Category:
Standards Track". You can read up on this process in RFC 2600.