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:

3) Add an uplink interface to the group:

4) Repeat Step 3 for each uplink interface you add to the group.

• Add a downlink interface to the group:

5) Repeat Step 4 for each downlink interface you add to the group.

Limitations:

• JunOS 11.1 Release - Currently, a maximum of 8 groups are supported. The maximum number of interfaces under link-to-monitor/link-to-disable under each of these groups is also 8. Starting with JunOS release 12.1, both of  these numbers will increase to 32.
• Aggregated Ethernet (ae) interfaces are currently not supported to be link-to-disable.
• UFD is currently only available on fixed platforms, namely EX2200, EX3200, EX4200 and EX4500. Starting with JunOS release 12.1, modular platforms EX8208 and EX8216 will also be supported.