BFD Operation

BFD (Bidirectional Forwarding Detection) mechanism uses packets to inform both ends that the protocol is configured. With these packets, BFD provide basic connectivity and the parameter negotiation.

The main role of BFD is detecting the link failures by sending BFD packets in regular intervals. It sends the packet and wait. If there is no reply until a certain of time, it determines that the link is down. This is the same as Hello protocols.

Let’s give an example to understand BFD Mechanims better.Below, there is a topology, that BFD Sessions are also established in each ends. Think about that a traffic is coming from Router 1 to Router 2.

What if the connection between these two routers fails.

Both end Router is informed by BFD mecahnism about this link failure. They say that “Link failed! You need to do a new route calculation”. And after this message, router calculates a new route to the same destination.

As you can see above, the traffic is go through the new calculated route, through Router 3.

BFD has different operation modes. These are :

• Async Mode
• Demand Mode
• Echo Mode

In Asyncronous Mode, two end node send BFD Conrol Packets eachother periodically. If they do not get some of the Control Packets, they decide that there is failure. In Async Mode, BFD control packets flow in each direction.

In Demand Mode, two end nodes send BFD Control Packets only for a short time to dedect if are there any failure or not. In Demand Mode, BFD control packets flow in each direction.

In Echo Mode, BFD Echo Packets also used between two end beside BFD Control packets. Echo packets is used to test forwarding plane not host stack.

BFD Applications

AS we mentioned above, BFD (Bidirectional Forwarding Detection) is used to detect link failure. But, there can be many type of link failure. BFD is generally used for:

• Routing Protocol link availability
• Ethernet Link availability
• MPLS and GRE Tunnel link availability
• Edge Network availablity

Protocols Used With BFD

BFD (Bidirectional Forwarding Detection) is used with many protocols for link failure detection. These protocols are:

• Static routes
• IGPs (OSPF, IS-IS)
• BGP (eBGP, iBGP)
• LDP
• RSVP

Disadvantages of BFD

Beside the advantages of BFD, it has also some disadvantages. First of all it can have more resource demands. This changes from platform to platform.

Secondly, BFD is not aware of Layer 2 link bundelling. When you configure it on one link, it operetas for single link, rather than the other bunde member.

Other BFD Articles
Bidirectional Forwarding Detection – BFD Overview
Bidirectional Forwarding Detection – BFD Operation