Configuring Bfd - Cisco 3845 - Security Bundle Router Software Manual

Chapter 29 Configuring IP Unicast Routing

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, because 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
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
172.18.0.1
Figure 29-7
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
172.18.0.1
OL-23400-01
shows a simple network with two routers running OSPF and BFD. When OSPF discovers a
Establishing a BFD Session
OSPF
2
BFD
172.16.10.2
Router A
shows what happens when a failure occurs in the network (1). The BFD neighbor session
Breaking an OSPF Neighbor Relationship
OSPF
3
BFD
172.16.10.2
Router A
1
OSPF neighbors
BFD neighbors
3
172.16.10.1
4
OSPF neighbors
X
BFD neighbors
X
2
1
X
172.16.10.1
Cisco ME 3800X and 3600X Switch Software Configuration Guide
Configuring BFD
OSPF
2
BFD
Router B
172.17.0.1
OSPF
3
BFD
Router B
172.17.0.1
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