Node protection extends the capabilities of link protection. Link protection helps to ensure that traffic going over a specific interface to a neighboring router can continue to reach this router if that interface fails. Node protection ensures that traffic from an LSP traversing a neighboring router can continue to reach its destination even if the neighboring router fails.
When you enable node protection for an LSP, you must also enable link protection. Once enabled, node protection and link protection establish the following types of bypass LSPs:
In Figure 22, both node protection and link protection are enabled on Interface B on Router 1. Both node protection and link protection are also enabled on LSP A, an LSP that traverses the link transiting Router 1, Router 2, and Router 3. If the link between Router 1 and Router 2 fails, traffic from LSP A is quickly switched to the next-hop bypass LSP generated by link protection. If Router 2 suffers a hardware or software failure, traffic from LSP A is switched to the next-next-hop bypass LSP generated by node protection.
Figure 22: Node Protection Creating a Next-Next-Hop Bypass LSP
The time needed by node protection to switch traffic to a next-next-hop bypass LSP can be significantly longer than the time needed by link protection to switch traffic to a next-hop bypass LSP. Link protection relies on a hardware mechanism to detect a link failure, allowing it to quickly switch traffic to a next-hop bypass LSP.
Node failures are often due to software problems on the node router. Node protection relies on the receipt of hello messages from a neighboring router to determine whether it is still functioning. The time it takes node protection to divert traffic partly depends on how often the node router sends hello messages and how long it takes the node-protected router to react to having not received a hello message. However, once the failure is detected, traffic can be quickly diverted to the next-next-hop bypass LSP.
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