29-71
Cisco ME 3800X and 3600X Switch Software Configuration Guide
OL-23400-01
Chapter 29 Configuring IP Unicast Routing
Configuring BFD
Configuring BFD
The Bidirectional Forwarding Detection (BFD) Protocol quickly detects forwarding-path failures for a
variety of media types, encapsulations, topologies, and routing protocols. It operates in a unicast,
point-to-point mode on top of any data protocol being forwarded between two systems to track IPv4
connectivity between directly connected neighbors. BFD packets are encapsulated in UDP packets with
a destination port number of 3784 or 3785.
In EIGRP, IS-IS, and OSPF deployments, the closest alternative to BFD is the use of modified
failure-detection mechanisms.
Although reducing the EIGRP, IS-IS, and OSPF timers can result in a
failure-detection rate of 1 to 2 seconds, BFD can provide failure detection in less than 1 second. BFD
can be less CPU-intensive than the reduced timers and, be
cause it is not tied to any particular routing
protocol, it can be used as a generic and consistent failure detection mechanism for multiple routing
protocols.
To create a BFD session, you must configure BFD on both systems (BFD peers). Enabling BFD at the
interface and routing protocol level on BFD peers creates a BFD session. BFD timers are negotiated and
the BFD peers send control packets to each other at the negotiated intervals. If the neighbor is not
directly connected, BFD neighbor registration is rejected.
Figure 29-6
shows a simple network with two routers running OSPF and BFD. When OSPF discovers a
neighbor (1), it sends a request to the BFD process to initiate a BFD neighbor session with the neighbor
OSPF router (2), establishing the BFD neighbor session (3).
Figure 29-6
Establishing a BFD Session
Figure 29-7
shows what happens when a failure occurs in the network (1). The BFD neighbor session
with the OSPF neighbor closes (2). BFD notifies the OSPF process that the BFD neighbor is no longer
reachable, and the OSPF process breaks the OSPF neighbor relationship (4). If an alternative path is
available, the routers start converging on it.
Figure 29-7
Breaking an OSPF Neighbor Relationship
172.16.10.2
172.16.10.1
172.17.0.1
172.18.0.1
BFD
BFD neighbors
3
OSPF
2
BFD
OSPF
2
127844
OSPF neighbors
1
Router A
Router B
172.16.10.2
172.16.10.1
172.17.0.1
172.18.0.1
BFD
BFD neighbors
2
1
X
X
X
OSPF
3
BFD
Router A
Router B
OSPF
3
127845
OSPF neighbors
4