Brocade Multi-Service IronWare Switching Configuration Guide (Supporting R05.6.00) - Manual (Page 944)

914 Multi-Service IronWare Switching Configuration Guide
53-1003036-02
BiDirectional Forwarding Detection (BFD)
25
Multi-Service IronWare software provides support for Bidirectional Forwarding Detection (BFD).
BFD provides rapid detection of the failure of a forwarding path by checking that the next-hop
device is alive. Without BFD enabled, it can take from 3 to 30 seconds to detect that a neighboring
device is not operational causing packet loss due to incorrect routing information at a level
unacceptable for real-time applications such as VOIP and video over IP. Using BFD, you can detect a
forwarding path failure in 300 milliseconds or less depending on your configuration.
A BFD session is automatically established when a neighbor is discovered for a protocol provided
that BFD is enabled on the interface on which the neighbor is discovered and BFD is also enabled
for the protocol (by interface or globally). Once a session is set-up, each device transmits control
messages at a high rate of speed that is negotiated by the devices during the session setup. To
provide a detection time of 150 milliseconds, it is necessary to process 20 messages per second of
about 70 to 100 bytes each per each session. A similar number of messages also need to be
transmitted out per each session. Once a session is set-up, that same message is continuously
transmitted at the negotiated rate and a check is made that the expected control message is
received at the agreed frequency from the neighbor. If the agreed upon messages are not received
from the neighbor within a short period of time, the neighbor is considered to be down.
For the NetIron CES and NetIron CER device, there are 20 Bidirectional Sessions per LP and 40
Bidirectional sessions system-wide.
NOTE
BFD session establishment on an interface does not start until 90 seconds after the interface
comes up. The reason for this delay is to ensure that the link is not effected by unstable link
conditions which could cause BFD to flap. This delay time is not user configurable.
The BFD Control Message is an UDP message with destination port 3784.
NOTE
BFD version 0 is not supported in this implementation and BFD version 1 is not compatible with BFD
version 0.
NOTE
BFD supports multi-slot LAGs in cases where all BFD packet are transmitted only on a single path
which does not change unless the LAG active membership changes. BFD is not be supported on
multi-slot LAGs where per-packet switching is used such that the path taken by the BFD packets will
vary per packet.
NOTE
When BFD is configured with stringent values of 100/300 msec, BFD may flap when learning a large
number of routes.
Number of BFD sessions supported
The devices have a set limit of 250 BFD sessions per system with a maximum number of 40
sessions per Interface Module. This number is inclusive of the fact that IS-IS and OSPF sessions on
an Interface Module will include both Tx and Rx sessions. Consequently, the 40 sessions per
Interface Module actually corresponds to 80 sessions where each OSPF and IS-IS session
consumes 2 sessions (1 Tx and 1 Rx).