Link Protection Overview

In many networks that are deployed today, it is common to see high-bandwidth links carrying traffic belonging to "important" flows and other flows that are not so important. If MPLS TE is deployed in such networks, "important flows" translates to "important LSPs." These LSPs might be carrying critical information or time-sensitive data that requires a real-time response. In such cases, it would be nice if all the "important LSPs" could be protected while ignoring the less-important LSPs. FRR allows you to protect some of your TE tunnels (just the ones you deem important) or all of your TE tunnels. With link protection, you can protect links that are carrying these important LSPs by using presignalled backup tunnels that bypass the protected link. Figure 7-5 depicts one such sample network.

Figure 7-5. Link Protection

Figure 7-5. Link Protection

Here, link 12008a12008c is considered the crucial link over which a primary tunnel is signalled. This link is called the protected link. In order to protect this link and the primary tunnel over it, a backup tunnel is signalled around the link so that the headend of the backup tunnel is the node that is immediately upstream of the protected link, and the tail of the backup tunnel is the node that is immediately downstream of the protected link. In Figure 7-5, the headend of the backup tunnel is 12008a (PLR), and the tail is 12008c (MP). As described in the preceding section, when the protected link fails, label stacking delivers the primary tunnel packets to 12008c so that 12008c sees the label it is expecting. Link protection relies on the fact that, although a protected link has gone down, the router at the other end of that protected link is still up. Link protection uses NHop backup tunnels; the backup tunnels terminate on the node on the other end of the protected link. As you can imagine, this allows you to protect against a link failure but not a node failure. If the node you terminate your backup tunnel on goes down, link protection can't help you.

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