Bidirectional Forwarding Detection (BFD)

Bidirectional Forwarding Detection (BFD) is a protocol defined in RFC 5880 that helps your routing protocol detect link failures much faster.
The topology below has three routers, all running OSPF with default timers.

Without BFD

If the link between R1 and R2 goes down, OSPF waits up to 40 seconds before declaring the neighbor dead.
That is the dead timer.

Figure 1 – The dead timer makes R1 wait 40 seconds

During those 40 seconds, OSPF simply hasn't noticed the neighbor is gone.
This is not efficient.

R1 could reroute through R3 and converge right away.

Figure 2 – The path through R3 stays unused

But the dead timer forces it to wait, so your traffic keeps going into the dead link.

With BFD

BFD detects a link failure in milliseconds instead of seconds.

With BFD running, the failure is caught almost instantly.
OSPF converges and reroutes through R3 right away.

Figure 3 – OSPF reroutes as soon as BFD reports the failure

BFD detects failures independent of the routing protocol and the media type.
The same session works for any routing protocol, over any media.

BFD does not run once for your whole topology.

One Session per Link Pair

BFD runs as a separate session between each pair of directly connected routers.

Figure 4 – One BFD session per link pair

R1 and R2 have their own session, R1 and R3 have theirs, and R2 and R3 have theirs.
Each session is independent and only concerns the two routers on that link.

BFD Terminology

Every BFD session runs on three values:

• Desired Min TX Interval: how fast your router would like to send packets

• Required Min RX Interval: the fastest rate it can actually receive at

• Detect Mult: how many consecutive missed packets it takes to declare the session down, typically 3

Figure 5 – The slower interval always wins

Your router's Desired Min TX is always compared against the neighbor's Required Min RX.
When R1 offers to send every 50ms but R2 can only receive every 150ms, the session settles at 150ms.

The slower value always wins.

At a 150ms interval with a Detect Mult of 3, that's a 450ms detection time.
Still far faster than the 40-second dead timer you started with.