Folks, here let’s discuss how main subject and obviously interesting subject in Cisco Syllabus for the CCIE SP Track, i omit some description to ease the memorization of such topic so i hope you all have well understanding of MPLS and it’s function, keep in mind that the Notion of Traffic Engineering has existed since the first networks were invented.

It relates to the art of making efficient use of network resources given a network topology and a set of traffic flows, in other words, the fundamental objective of traffic engineering is to route the traffic so as to avoid network congestion and increase the network’s ability to absorb the maximum amount of traffic.

We all know that the IP routing relies on the fundamental concept of destination-based routing, let’s assume below scenario and see how the traffic flows:


In the above topology as soon as the two traffic flows originated by the Router 1 and Router 6 towards Router 5 reaches Router 2, they follow the same IGP shortest path because IP Packets are router by default based on their destination IP Address, In this example, the two flows follow the path R2R3R4R5. If the sum of traffic from these two flows exceeds the network capacity along this path, this inevitably leads to network congestion. This congestion can be avoided by routing some of the traffic along the other available path (R2R7R8R4, where some spare capacity is available).

In such a simple network it is easy to adjust the IGP Metrics so as to load-balance the traffic across the two available paths, for the complex networks IGP Metric manipulation is not an easy task for any Network Admin, in this case deployment of the MPLS Traffic Engineering, which provides a rich set of features with very high granularity to efficiently traffic-engineer an MPLS/IP Networks.


The fundamental idea of MPLS TE is to use a Traffic-Engineered Label-Switched Path (TE LSP or tunnel) to forward the traffic across the network by taking into account a set of constraints and the network topology and resources available with the objective of making efficient use of the network.

In short, the TE LSP attributes (constraints) determine the desired characteristics of the LSP (between its source and destination).

Attributes are:

  • Destination
  • Bandwidth
  • Affinities
  • Preemption
  • Protection by Fast ReRoute
  • Optimization Metric

Below i will debrief all the above attributes, however they will be only conceptual description which will ease the attributes concepts:


The source of the TE LSP is the headend router where the TE LSP is configured, whereas its destination must be explicitly configured. Note that the source of a TE LSP must also be explicitly specified in case the TE LSP path is computed by some offline tool.


Although there are many ways to determine the Bandwidth attribute requirements, but the most obvious way is to obtain a traffic matrix between the routers involved in a mesh of TE LSPs.  This can be achieved by the various tools like NETFLOW ( Network Management Tool ).

It is also worth mentioning that some more recent inferring techniques rely on the measurement of link utilization to compute the traffic matrix. However, such methods usually just provide estimates whose accuracy is variable and that vary greatly with the network topology.


A 32-bit flag field that must match the set of links a TE LSP traverses represents affinities. In a nutshell, this can be seen as a coloring scheme (with up to 32 colors). Each link of the network can also have up to 32 colors. It might be desirable in certain circumstances to ensure that a TE LSP exclusively traverses links of specified colors. For the sake of illustration, consider a network with a mix of terrestrial and satellite links. They mainly differ by their propagation delays (which are significantly higher for the satellite links). Hence, the network administrator may decide to color those links (by setting up a specific bit of the 32-bit affinity field). For a TE LSP that carries sensitive traffic for which a short propagation delay is desired, the constraint of avoiding links marked with this specific color can be enforced. Conversely, it is also possible to impose the constraint of selecting links that have a specific color. Any combination is possible, offering a high degree of flexibility.


The notion of preemption refers to the ability to define up to seven levels of priority. In the case of resource contention, this allows a higher-priority TE LSP to preempt (and, consequently, tear down) lower-priority TE LSP(s) if both cannot be accommodated due to lack of bandwidth resources on a link.

Protection by Fast Re-Route:

MPLS Traffic Engineering provides an efficient local protection scheme called fast reroute to quickly reroute TE LSPs to a presignaled backup tunnel within tens of milliseconds. Such a local protection scheme can be used for some TE LSPs requiring fast rerouting on network failure and is signaled as a TE LSP attribute. In other words, it is possible when setting up a TE LSP to explicitly require the protection by Fast Reroute for the TE LSP whenever Fast Reroute is available on the traversed router.

Optimized Metric:

The notion of shortest path is always related to a particular metric. Typically, in an IP network, each link has a metric, and the shortest path is the path such that the sum of the link metrics along the path is minimal.

MPLS TE also uses metrics to pick the shortest path for a tunnel that satisfies the constraints specified. MPLS TE has introduced its own metric. When MPLS TE is configured on a link, the router can flood two metrics for a particular link: the IGP and TE metrics (which may or may not be the same).

Above are the information which i gathered for now and i will come with another post regarding the hierarchy of these attributes and other advance features.


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