BFD for OSPF
Definition
Bidirectional Forwarding Detection (BFD) is a mechanism to detect communication faults between forwarding engines.
To be specific, BFD detects connectivity of a data protocol on a path between two systems. The path can be a physical link, a logical link, or a tunnel.
In BFD for OSPF, a BFD session is associated with OSPF. The BFD session can quickly detect a link fault and then notify OSPF of the fault. This speeds up OSPF's response to the change of the network topology.
Purpose
A link fault or a topology change may cause devices to re-calculate routes. Therefore, the convergence of routing protocols must be as quick as possible to improve the network performance.
Link faults are unavoidable. Therefore, a feasible solution is required to detect faults fast and notify routing protocols of the faults immediately. With BFD being associated with OSPF, once a fault occurs on a link between neighbors, BFD can speed up the OSPF convergence.
Associated with BFD |
Link Fault Detection Mechanism |
Convergence Speed |
|---|---|---|
No |
An OSPF dead timer expires. The default value of the dead timer is 40s. |
At the second level |
Yes |
A BFD session goes Down. |
At the millisecond level |
Implementation
Figure 5-8 shows the implementation of BFD for OSPF.
OSPF neighbor relationships are established between three routers.
After the status of a neighbor relationship reaches Full, BFD is instructed to establish a BFD session.
The outbound interface from Router A to Router B is GE 2/0/0. When the link fails, BFD immediately detects the fault and then notifies Router A of the fault.
Router A processes the event that a neighbor relationship becomes Down and re-calculates routes. After calculation, the outbound interface becomes GE 1/0/0 on Router C. Traffic from Router A passes through Router C to get to Router B.