JUNOS Enterprise Switching

Bidirectional Forwarding Detection (BFD)

BFD is a protocol that was created to detect link failures very quickly. Essentially, it is a very fast hello protocol that gets around the slower protocol timers in OSPF, RIP, Intermediate System to Intermediate System (IS-IS), and so on. In a routed environment, this is especially useful when two Layer 3 devices are connected to a Layer 2 device in the middle (Figure 11-5). The problem is that the Router1 link could go down on Tequila, but Router2 would not detect the failure until the protocol timed out as a result of its local link remaining up. If OSPF was running, this could take up to 40 seconds with default timers. BFD was created to speed up this detection.

Figure 11-5. Layer 2 interconnection issue

The exact operation of BFD is beyond the scope of this book, but let’s take a look at a basic description. BFD establishes a session between two endpoints over a particular link. If more than one link exists between two systems, multiple BFD sessions may be established to monitor each link. The session is established with a three-way handshake, and is torn down the same way.

BFD does not discover neighbors that are explicitly configured. It uses any underlying media that is available, and also piggybacks on protocols such as BGP, OSPF, or IS-IS. The idea is that BFD should be implemented in the PFE of the system for quicker detection and closer ...