Home > Articles > Cisco Network Technology > IP Communications/VoIP > Multiprotocol Label Switching Traffic Engineering Technology Overview

Multiprotocol Label Switching Traffic Engineering Technology Overview

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Sep 22, 2006.

Chapter Description

This chapter presents a review of Multiprotocol Label Switching Traffic Engineering (MPLS TE) technology. MPLS TE can play an important role in the implementation of network services with quality of service (QoS) guarantees.

From the Book

QoS for IP/MPLS Networks

QoS for IP/MPLS Networks


Basic Operation of MPLS TE

The operation of MPLS TE involves link information distribution, path computation, LSP signaling, and traffic selection. This section explains the most important concepts behind each of these four steps. LSRs implement the first two steps, link information distribution and path computation, when they need perform constraint-based routing. MPLS networks that do not use constraint-based routing (or use an offline tool for that purpose) perform only LSP signaling and traffic selection. MPLS TE does not define any new protocols even though it represents a significant change in how MPLS networks can route traffic. Instead, it uses extensions to existing IP protocols.

Link Information Distribution

MPLS TE uses extensions to existing IP link-state routing protocols to distribute topology information. An LSR requires detailed network information to perform constraint-based routing. It needs to know the current state of an extended list of link attributes to take a set of constraints into consideration during path computation for a TE LSP. Link-state protocols (IS-IS and OSPF) provide the flooding capabilities that are required to distribute these attributes. LSRs use this information to build a TE topology database. This database is separate from the regular topology database that LSRs build for hop-by-hop destination based routing.

MPLS TE introduces available bandwidth, an administrative group (flags), and a TE metric as new link attributes. Each link has eight amounts of available bandwidth corresponding to the eight priority levels that TE LSPs can have. The administrative group (flags) acts as a classification mechanism to define link inclusion and exclusion rules. The TE metric is a second link metric for path optimization (similar to the IGP link metric). In addition, LSRs distribute a TE ID that has a similar function to a router ID. The OSPF and IS-IS extensions mirror each other and have the same semantics. Table 2-1 shows the complete list of link attributes. RFC 3784 and RFC 3630 define the IS-IS and OSPF extensions for TE respectively.

Table 2-1. Extended Link Attributes Distributed for TE

Link Attribute


Interface address

IP address of the interface corresponding to the link

Neighbor address

IP address of the neighbor's interface corresponding to the link

Maximum link bandwidth

True link capacity (in the neighbor direction)

Reservable link bandwidth

Maximum bandwidth that can be reserved (in the neighbor direction)

Unreserved bandwidth

Available bandwidth at each of the (eight) preemption priority levels (in the neighbor direction)

TE metric

Link metric for TE (may differ from the IGP metric)

Administrative group

Administratively value (flags) associated with the link for inclusion/exclusion policies

MPLS TE can still perform constraint-based routing in the presence of multiple IGP areas or multiple autonomous systems. OSPF and IS-IS use the concept of areas or levels to limit the scope of information flooding. An LSR in a network with multiple areas only builds a partial topology database. The existence of these partial databases has some implications for path computation, as the next section describes. LSRs in an inter-autonomous system TE environment also need to deal with partial network topologies. Fortunately, inter-area TE and inter-autonomous system TE use similar approaches for constraint-based routing in the presence of partial topology information.

Path Computation

LSRs can perform path computation for a TE LSP using the TE topology database. A common approach for performing constraint-based routing on the LSRs is to use an extension of the shortest path first (SPF) algorithm. This extension to the original algorithm generally receives the name of constraint-based, shortest path first (CSPF). The modified algorithm executes the SPF algorithm on the topology that results from the removal of the links that do not meet the TE LSP constraints. The algorithm may use the IGP link metric or the link TE metric to determine the shortest path. CSPF does not guarantee a completely optimal mapping of traffic streams to network resources, but it is considered an adequate approximation. MPLS TE specifications do not require that LSRs perform path computation or attempt to standardize a path computation algorithm.

Figure 2-2 illustrates a simplified version of CSPF on a sample network. In this case, node E wants to compute the shortest path to node H with the following constraints: only links with at least 50 bandwidth units available and an administrative group value of 0xFF. Node E examines the TE topology database and disregards links with insufficient bandwidth or administrative group values other than 0xFF. The dotted lines in the topology represent links that CSPF disregards. Subsequently, node E executes the shortest path algorithm on the reduced topology using the link metric values. In this case, the shortest path is {E, F, B, C, H}. Using this result, node E can initiate the TE LSP signaling.


Figure 2-2 Path Computation Using the CSPF Algorithm

Path computation in multi-area or inter-autonomous system environments may involve several partial computations along the TE LSP. When the headend does not have a complete view of the network topology, it can specify the path as a list of predefined boundary LSR (Area Border Router [ABR] in the case of inter-area and Autonomous System Boundary Router [ASBR] in the case of inter--autonomous system). The headend can compute a path to the first boundary LSR (which must be in its topology database and initiate the signalling of the TE LSP signaling can be initiated). When the signaling reaches the boundary LSR, that LSR performs the path computation to the final destination if it is in its topology. If the destination is not in the topology, the signaling should indicate the next exit boundary LSR, and the path computation will take place to that boundary LSR. The process continues until the signaling reaches the destination. This process of completing path computation during TE LSP signaling is called loose routing.

Figure 2-3 shows path computation in a sample network with multiple IGP areas. All LSRs have a partial network topology. The network computes a path between nodes E and H crossing the three IGP areas in the network. Node E selected nodes F and G, which have as the boundary LSRs that the TE LSP will traverse. Node E computes the path to node F and initiates the TE LSP signaling. When node F receives the signaling message, it computes the next segment of the path toward node G. When the signaling arrives at node G, it completes the path computation toward node H in area 2. The next section explains how LSRs signal TE LSPs.


Figure 2-3 Multi-Area Path Computation

Signaling of TE LSPs

MPLS TE introduces extensions to RSVP to signal up LSPs. RSVP uses five new objects: LABEL_REQUEST, LABEL, EXPLICIT_ROUTE, RECORD_ROUTE, and SESSION_ATTRIBUTE. RSVP Path messages use a LABEL_REQUEST object to request a label binding at each hop. Resv messages use a LABEL object to perform label distribution. Network nodes perform downstream-on-demand label distribution using these two objects. The EXPLICIT_ROUTE object contains a hop list that defines the explicit routed path that the signaling will follow. The RECORD_ROUTE object collects hop and label information along the signaling path. The SESSION_ATTRIBUTE object lists the attribute requirements of the LSP (priority, protection, and so forth).

RFC 3209 defines these RSVP TE extensions. Table 2-2 summarizes the new RSVP objects and their function.

Table 2-2. New RSVP Objects to Support MPLS TE

RSVP Object

RSVP Message




Label request to downstream neighbor



MPLS label allocated by downstream neighbor



Hop list defining the course of the TE LSP


Path, Resv

Hop/label list recorded during TE LSP setup



Requested LSP attributes (priority, protection, affinities)

Figure 2-4 illustrates the setup of a TE LSP using RSVP. In this scenario, node E signals a TE LSP toward node H. RSVP Path messages flow downstream with a collection of objects, four of which are related to MPLS TE (EXPLICIT_ROUTE, LABEL_REQUEST, SESSION_ATTRIBUTE, and RECORD_ROUTE). Resv messages flow upstream and include two objects related to MPLS TE (LABEL and RECORD_ROUTE). Each node performs admission control and builds the LSP forwarding information base (LFIB) when processing the Resv messages. The structure of the LFIB and the MPLS forwarding algorithm remain the same regardless of the protocols that populated the information (for example, RSVP in the case of MPLS TE).


Figure 2-4 TE LSP Setup Using RSVP

Traffic Selection

MPLS TE separates TE LSP creation from the process of selecting the traffic that will use the TE LSP. A headend can signal a TE LSP, but traffic will start flowing through the LSP after the LSR implements a traffic-selection mechanism. Traffic can enter the TE LSP only at the headend. Therefore, the selection of the traffic is a local head-end decision that can use different approaches without the need for standardization. The selection criteria can be static or dynamic. It can also depend on the packet type (for instance, IP or Ethernet) or packet contents (for example, class of service). An MPLS network can make use of several traffic-selection mechanisms depending on the services it offers.

3. DiffServ-Aware Traffic Engineering | Next Section Previous Section

Cisco Press Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from Cisco Press and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about Cisco Press products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites; develop new products and services; conduct educational research; and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@ciscopress.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by Cisco Press. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.ciscopress.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020