The Resilient Ethernet Protocol (REP) Fast feature allows faster link failure detection and convergence on the switch copper Gigabit Ethernet (GE) ports.

This document describes only REP Fast.

REP was originally designed for Fast Ethernet (FE 10/100) ports. Link down detection time on FE ports is 10 milliseconds (ms) and convergence time is about 50 ms. On Fiber GE ports, link down time is 10 ms, but on GE copper interfaces, the IEEE 802.3 specification mandates the link drop detection and recovery times to be 750 ms for a master and 350 ms for a slave. As a result, link loss and recovery can be detected a lot more quickly on GE fiber interfaces than on corresponding copper interfaces. This in turn means that the convergence time for REP is a lot higher when using GE copper interfaces.

To improve link down detection time, a real-time operating system (RTOS)/beacon mechanism is implemented to trigger faster link failure detection (within 5-10 ms) when a REP interface is configured for REP Fast mode. RTOS has two timers for each REP interface. The first timer is triggered every 3 ms to transmit the beacon frame to the neighbor node. After successful transmission and reception of the frame, both the timers are reset. If the packet is not received after the transmission, then the second timer is triggered to check the reception within 10 ms. If the packet is not received, upon the timer expiry, a link down packet is sent to the switch.

If the neighbor acknowledges and is configured for REP Fast mode, convergence occurs within 50 ms. If a neighbor switch does not support RTOS, normal REP mode must be used for link up/down detection. In this case, you need to disable fastmode on both ends of the link.