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The MPLS FAQ - Page 1 of 3


1. Administrivia  

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 at http://www.ietf.org/html.charters/mpls-charter.html.  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:

  • The IETF's MPLS Working Group mailing list, which can be joined https://www1.ietf.org/mailman/listinfo/mpls.  This list is for discussion of MPLS standards development.  Note that several of the other IETF working groups also host mailing lists for discussion of MPLS standards for specific applications.  To see a brief description of these groups, visit http://www.mplsrc.com/standards.shtml 

  • The MPLS-OPS mailing list, which can be joined by visiting http://www.mplsrc.com/mplsops.shtml.  This list is for the discussion of issues related to the design, deployment and management of MPLS-based networks

  • LINUXMPLS - A Yahoo-based group and mailing list for the discussion of MPLS implementations for LINUX can be accessed at:
    http://groups.yahoo.com/group/linuxmpls

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 MPLS?

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 at http://www.watersprings.org/links/mlr/.  

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, they include:

  • 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 applications
  • 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?
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

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 site (http://www.ietf.org), 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.

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.

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