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Software Configuration Guide—Release IOS XE 3.3.0SG and IOS 15.1(1)SG
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Chapter 37 Configuring Bidirection Forwarding Detection
Information About Bidirectional Forwarding Detection
Figure 37-2 Tearing Down an OSPF Neighbor Relationship
A routing protocol needs to register with BFD for every neighbor it acquires. Once a neighbor is
registered, BFD initiates a session with the neighbor if a session does not already exist.
OSPF registers with BFD when:
• A neighbor finite state machine (FSM) transitions to full state.
• Both OSPF BFD and BFD are enabled.
On broadcast interfaces, OSPF establishes a BFD session only with the designated router (DR) and
backup designated router (BDR), but not between any two switches (routers) in DROTHER state.
BFD Detection of Failures
Once a BFD session has been established and timer negations are complete, BFD peers send BFD control
packets that act in the same manner as an IGP hello protocol to detect liveliness, except at a more
accelerated rate. The following information should be noted:
• BFD is a forwarding path failure detection protocol. BFD detects a failure, but the routing protocol
must take action to bypass a failed peer.
• Typically, BFD can be used at any protocol layer. However, the Cisco implementation of BFD for
Cisco IOS Release 15.1(1)SG supports only Layer 3 clients, in particular, the BGP, EIGRP, and
OSPF routing protocols, and static routing.
• Cisco devices will use one BFD session for multiple client protocols in the Cisco implementation of
BFD for Cisco IOS Release 15.1(1)SG. For example, if a network is running OSPF and EIGRP
across the same link to the same peer, only one BFD session will be established, and BFD will share
session information with both routing protocols. However, IPv4 and IPv6 clients cannot share a BFD
session.
BFD Version Interoperability
Cisco IOS Release 15.1(1)SG supports BFD Version 1 as well as BFD Version 0. All BFD sessions come
up as Version 1 by default and will be interoperable with Version 0. The system automatically performs
BFD version detection, and BFD sessions between neighbors will run in the highest common BFD
version between neighbors. For example, if one BFD neighbor is running BFD Version 0 and the other
BFD neighbor is running Version 1, the session will run BFD Version 0. The output from the show bfd
neighbors [details] command will verify which BFD version a BFD neighbor is running.
See the “Example: Configuring BFD in an EIGRP Network with Echo Mode Enabled by Default” section
on page 37-17 for an example of BFD version detection.
172.16.10.2 172.16.10.1
172.17.0.1172.18.0.1
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BFD neighbors
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X
X
X
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Router A Router B
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