Foundry NetIron M2404C and M2404F Metro Access Switches
Configuring Advanced Routing Information (Rev. 03)
Bidirectional Forwarding Detection (BFD)
© 2008 Foundry Networks, Inc
Page 55 of 68
Bidirectional Forwarding Detection (BFD)
Overview
Bidirectional Forwarding Detection (BFD) feature validates the operation of the LSP or the IP
routing neighbor.
IP is used as infrastructure in many Service Provider networks, including those using IP
infrastructure for MPLS. The failure detection for these networks is typically accomplished via
hardware detection mechanisms, for example Ethernet link signal when link is directly connected
to the neighboring device. In some cases link failure can not be detected on Ethernet networks, for
example when an intermediate layer2 device exists between the routers. In this case or when the
signaling does not reach the upper protocol layers, the protocols must rely on their much slower
strategies to detect failures. The detection times in existing protocols are typically greater than one
second, and most of the times require tens of seconds for such detection.
BFD provides low-overhead, short-duration detection of failures in the path between adjacent
forwarding engines. BFD allows a single mechanism to be used for failure detection over any
media and at any protocol layer, with a wide range of detection times and overhead. The fast
detection of failures provides immediate reaction to failure in the event of a failed link or neighbor.
BFD implementation is required to provide OSPF BFD, providing compliance to the draft-ietf-bfd-
base-05 and draft-ietf-bfd-v4v6-1hop-05. The device implements BFD for fast detection of the
neighbor status change, without relying on link status.
Figure 5: BFD Protection Domain: OSPF Neighbors
Initial BFD Session Setup for OSPF
BFD receives a list of neighbors for each interface it is enabled. This is the retransmission interval
for packets when the system is not in the Up state.
Upon receiving a BFD control packet, the system takes the Required Min RX Interval and
compares it to its own Desired Min TX Interval. The device then takes the greater of the two
values and uses it as the transmission rate for its BFD packets. Thus, the slowest device (the device
with the highest minimum TX interval) determines the transmission rate.
Desired Min TX Interval is the minimum interval which the local system would like to use when
transmitting BFD Control packets.