Backup paths are widely used to protect IP links from failures. Existing solutions such as the commonly used independent and Shared Risk Link Group models do not accurately reflect the correlation between IP link failures, and thus may not choose reliable backup paths. We propose a cross-layer approach for IP link protection. We develop a correlated failure probability (CFP) model to quantify the impact of an IP link failure on the reliability of backup paths. With the CFP model, we propose two algorithms for selecting backup paths. The first algorithm focuses on choosing the backup paths with minimum failure probability. The second algorithm further considers the bandwidth constraint and aims at minimizing the traffic disruption caused by failures. It also ensures that the rerouted traffic load on each IP link does not exceed the usable bandwidth to avoid interfering with the normal traffic. Simulations based on real ISP networks show that our approach can choose backup paths that are more reliable and achieve better protection.