MPLS-Enabled Applications: Emerging Developments and New Technologies

3.8. ADDITIONAL CONSTRAINTS FOR THE COMPUTATION OF THE PROTECTION PATH

Until now, when discussing computation of the protection path we have focused on a single constraint: avoid using the protected resource in the protection path. However, is this enough to ensure adequate protection? The answer may be 'no', on a case-by-case basis, as we will see in this section.

3.8.1. Fate sharing

Using different links or nodes in the protected and backup paths seems like a good way to ensure resiliency in the case of a failure. However, underlying resources used in the two paths may be affected by a single event. For example, if the optical fiber for two links follows the same physical path, a single event (such as a bulldozer cutting through the fiber) affects them both. In this case, the two links are said to be in the same Shared Risk Link Group (SRLG) or fate-sharing group. Therefore, the protection path must avoid all links that are in the same SRLG with links in the main path (the protected path). This is applicable for both end-to-end path protection (discussed at the beginning of this chapter) and local protection. For example, for the path protection scenario in Figure 3.1, if links S-R1 and S-R4 belonged to the same SRLG, the secondary path would have been computed along the (less optimal) path S-R2-R3-D.

From the point of view of the CSPF computation, SRLG membership is just another constraint and it is conceptually very similar to link colors (discussed in the Traffic Engineering ...