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Traffic Engineering, RSVP-TE Path

Manuplation and Constraints

Traffic Engineering is one of the most important routing lessons. With traffic engineering, as a network engineer you are directing your network traffic according to some constraints. By directing the traffic, you provide efficient usage of resources and become proactive for any congestion and packet drops in your network.

Traffic Engineering is used in MPLS with RSVP-TE. RSVP-TE brings many benefit to traffic engineering. With RSVP-TE you can do bandwidth reservation, you can define the LSP paths and you can manuplate path selection by giving additional constraints to the links.

RSVP-TE can define LSP Paths in two types in general. These types are “Strict” and “Loose” Paths. What are these “Strict” and “Loose” Paths?

Strict and Loose Paths

In a network, you can define a path as full strict, fully loose or mixture of strict and loose.

RSVP-TE can use a fixed defined LSP path. This is called “Strict” path. And if we configure a hop as “Strict hop”, this means that this is the immetiate next-hop that have layer 3 connection.

With this Strict path, the LSP is fixed and if any node on this LSP is down, the LSP is also down. Not an efficient way in many case.

Beside this fixed defined Strict paths, there is also “Loose” paths. If we configure a hop as “Loose hop”, this is any downstream node from the begginning router to the destination. In other words, you will not determine all the nodes between the beginning and the destination. Here, you are using CSPF and, instead of you, CSPF is calculating the path dynamically, according to the contraints that you had defined.What are these traffic engineering constraints ?

Traffic Engineering Advanced Path Calculation with Constraints

In a network, for traffic engineering path calculation, you can define some contraints. CSPF (Constraint Shortest Path First) calculates the best path according to these constraints.So, what are these constraints?

• Bandwidth Reservation,
• Link Coloring (Administrative Groups),
• Hop Limiting,
• Using TE-Metric,
• Using SRLG (Shared Risk Link Group)

Bandwidth Reservation

The first constraints that you can define to manuplate network traffic is using bandwidth constraints. With bandwidth constraint, as an administrator, network engineers define unreserved bandwidth values to the links. After manual setting, special bandwidth requirements can be selected to route the traffic.

Think about the below topology. As you can see the unreserved bandwidth values are defined.


traffic engineering bandwidth contraints

In the below topology, you can see the logical network after eliminating some links because of their bandwidth.


traffic manuplation bandwidth contraints

Administrative Groups (Link Coloring)

Administrative Groups or with the other name link coloring are used to combine similar characteristic links. With this combine, different traffic types can easily differentiated and send through different links.

There are many types of traffics in a network. Best effort data traffics, delay sensitive voice traffics and etc. Different traffic types need different behaviour. Because for example it is not recommended for a delay sensitive traffic like voice, to use a link that has too much or variable lenght delay.

With Administrative Groups (Link Coloring), we can group the links with different colors. And during traffic engineering, we can send different types of traffic through these different color links.

Below, there are some logical examples for Administrative Groups (Link Coloring).


administrative groups link coloring

In above topology, there are links that are coloured as orange and blue as you can see. Let’s assume that the orange links are delay sensitive links, blue links are not. And we need to force the voice traffic goes through these orange links. We will do this by assigning orange and blue code (Administrative Group) to these specific links.

In traffic engineering, if we use Administrative Groups as a constraint, as you can see below, only the desired colors will be in our logical topology that the CSPF calculation will do. Remember, using CSPF (Constraint Spanning Path First) is not too different that using SPF (Spanning Path First) algorithm. In CSPF, the only difference is, before the calculation, the mentioned constraints are met and after that the normal SPF calculation is done on this logical topology.


administrative groups link coloring orange

CSPF calculate the best path on the above logical diagram. And lastly, the best path is determined like below.


administrative groups link coloring after cspf

Hop Limit Constraint

Hop Limit can be imposed to an LSP path that provides a restriction about hop count. With this value, the path is selected accourding to the Hop Limit value.

Below, there is a topology for hop limit example.In this example from A to D, there are three paths with different hop count. Out hop limit will be 3 here.


hop limit constraint ipcisco

After hop limit contraint, on the remainning links CSPF calculate the best path. For our example, there is only one path, so the selected best path is that, the path with 3 hop from A to D.


hop limit constraint max 3 hop ipcisco

Hop Limit is generally used to avoid too much delay for delay sensitive traffics. By restricting the number of hops, minimum hop counted link is selected and this results minimum network delay.

Traffic Engineering Metric (TE-Metric)

Traffic Engineering Metric (TE-Metric) is used beside the normal metric (IGP metric). IGP metrics show the bandwidth of the links. This is useful in many times. But for Traffic Engineering, there is one more metric called Traffic Engineering Metric (TE-Metric).

By default, IGP metric and TE-metric is equal. For traffic engineering, some links can be configured with different TE-Metrics. And at the source router, network engineers can manuplate the LSP with forcing a strict TE-Metric.

Below, there is a network topology that the paths are only determined with normal IGP metrics. As you know, the lowest metric is desired and the traffic is going along the above path.


traffic engineering metric without

If we set TE-metric like below, then we can manuplate the path selection. Althought the above path has lower IGP cost, we will give higher TE-cost to above links and provide the selection of below path.


traffic engineering metric with

As you can see, we force the path selection with TE-metric and determine the selected path with traffic engineering TE-metric constraint.

Shared Risk Link Groups (SRLG)

Shared Risk Link Groups (SRLG) is another grouping mechanism like Administrative Groups (Link Coloring). In Shared Risk Link Groups (SRLG), we are creating the SRLGs and then assigning links to the SRLGs. Here, we are also controlling the sorce router during backup path building.

The main aim of Shared Risk Link Groups (SRLG) is avoiding double failure point. By double failure points I mean that, a primary path and a secondary path that use some common links with the primary path. Think about it. You have a failure on primary path and you need to use your secondary path. But because of the fact that in your backup link you have some common links with your primary path, you can not use your secondary path too.

Shared Risk Link Groups (SRLG) is used to aviod this difficult situation. With Shared Risk Link Groups (SRLG), we are grouping the links that has common links and then avoid selecting the same SRLG during secondary path selection.


traffic engineering srlg ipcisco

Shared Risk Link Groups (SRLG) in MPLS traffic engineering refer to situations in which links in a network share a common fiber (or a common physical attribute). These links have a shared risk, and that is when one link fails, other links in the group might fail too.

OSPF and Intermediate System-to-Intermediate System (IS-IS) flood the SRLG value information (including other TE link attributes such as bandwidth availability and affinity) using a sub-type length value (sub-TLV), so that all routers in the network have the SRLG information for each link.

To activate the SRLG feature, configure the SRLG value of each link that has a shared risk with another link. A maximum of 30 SRLGs per interface is allowed. You can configure this feature on multiple interfaces including the bundle interface.

In the following articles, we will see, how to configure traffic engineering constraints on Alcatel-Lucent (Nokia) 7750 SR.

Keep On, IPcisco.com ;)




About the Author
Gokhan Kosem is a telecommunation and network engineer. His ambition to IP networks and end-to-end system installation made him to prepare this web-site. By sharing his experiences about various networking protocols beside different system installation experiences and Cisco, Juniper, Alcatel-Lucent devices configurations, he is aimed to be helpful for his collegues in all over the world. He is currently lives in Istanbul, Turkey.

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