There are many aspects of overall high availability network strategy, including improvements in routing convergence times. This article focuses on one important aspect, protection switching below the Internet Protocol (IP) layer. The IP networks respond to a link failure by finding an alternate traffic path after the failure is discovered. This results in a convergence time during which traffic may be dropped at the failed link until an alternate path is found. While there has been steady progress in decreasing IP routing convergence times, these approaches are limited by the need to communicate among multiple routers. However, IP traffic can be protected using techniques below layer 3, which are not subject to this constraint. This article discusses mechanisms where a link failure can be protected locally, synchronous optical networks (SONET)/synchronous digital hierarchy like, at sub-50-ms protection times. Descriptions of linear SONET automatic protection switching for routers, Resilient Packet Ring protection, IP interface bundling, and multiprotocol label switching fast reroute are covered. Protection triggers and coexistence of various protection methods are also discussed