Uplink Failure Detection (UFD) is a feature available in JunOS starting in release 11.1. It allows the propagation of uplink failures to dual-homed downstream servers, to which the failures are invisible, so that they can stop using their interface connected to the switch where the failure occurred. This article explains the functionality of the feature, how it is configured, as well as the limitations associated with it.
In most redundant networks, end devices (i.e. servers) are connected to two access layer switches using two different interfaces, which often share the same IP address. The two interfaces are used by the server in an active/standby fashion, meaning only the active interface sends traffic out at any given time. The failover of the active interface to the standby interface is only triggered by a failure of the link directly connected to the server interface. This protects the server against the failure of the links that are directly connected to it, however the failure of a link upstream that is not directly to the end device might also cause the traffic from the server to be blackholed until the network converges around the failure. UFD is a feature that prevents the servers from dropping traffic in case of such failures.
In the diagram above, the upstream link whose failure to be propagated downstream is the link between EX Series Switch 1 and the upstream Switch ; and the link to be disabled to prevent traffic drop is the link between NIC1 on the server and EX Series Switch 1 . In this setup, UFD defines the former link as the link-to-monitor and the latter as the link-to-disable . Here's how UFD is configured: 1) Navigate to the protocols stanza. 2) Specify a name for an uplink-failure-detection group: