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Chapter 18 SSO-BFD
Information about SSO-BFD
across a switchover, the Cisco 10000 series router needs the addition of SSO support for the BFD
protocol. With this addition, a planned or an unplanned switchover does not result in the peer router
declaring a failure in the forwarding path.
For configuring the SSO-BFD feature, see the How to Configure Bidirectional Forwarding Detection
section in the
Bidirectional Forwarding Detection
guide at the following link:
http://www.cisco.com/en/US/docs/ios/iproute_bfd/configuration/guide/irb_bfd.html#wp1054190
The SSO-BFD feature is dependent on the following three modules for its execution.
•
Enhanced Timers
•
BFD HA Process
•
Early Packet Send
Enhanced Timers
After the switchover on the active RP, the BFD protocol needs a periodic signal to continue sending
packets to its peer router in the absence of its pseudo preemptive process. The enhanced timers
infrastructure sends the BFD protocol packets until the RP is completely active. This infrastructure
allows the clients to register to it, and specify the interval at which the clients are to be called. The
enhanced timers infrastructure runs until the control plane on the Cisco 10000 series router comes up
which can take upto 17 seconds. After the control plane is operational, the BFD protocol deregisters
from the enhanced timers infrastructure and spawns its own pseudo preemptive process.
The enhanced timers infrastructure is a mechanism for the BFD protocol to perform the add or register,
and callback or notification functions that are invoked at regular intervals. The callback or notification
function starts when the data path is established and packets can be transmitted out of an interface.
BFD HA Process
The BFD High Availability (HA) process is a platform-independent effort for the SSO-BFD feature to
enhance the BFD protocol to be stateful. The BFD HA process maintains sessions on the standby RP, if
those sessions are in the up state. After the switchover, the process on the newly active RP begin to
transmit the BFD packets. The BFD clients need to create or reclaim those sessions after the standby RP
becomes active. The BFD HA process starts a timer for reclaiming sessions after switchover, and any
session that is not reclaimed before the timer expires is deleted.
Early Packet Send
The early packet send routine is specific to the Cisco 10000 series router, and takes care of the BFD
protocol. It sends keep-alive packets using the data plane, before the control plane is up. Therefore, when
the standby RP is signaled of a switchover and becomes active, the active RP performs platform-specific
tasks and brings up the enhanced timers infrastructure. At regular time intervals, this infrastructure
invokes the BFD protocol that sends keep-alive packets. At this stage, the Interface Descriptor Block
(IDB) state on newly active RP is not up; so, it is not possible to send packets out through the Cisco
Express Forwarding (CEF) path that is normally used by the BFD protocol. As a result, the platform
provides a specific send path to the BFD protocol until the CEF path is able to send packets out. When
the CEF path is able to send packets, the BFD protocol deregisters from the enhanced timers
infrastructure, spawns its own pseudo preemptive process, and uses the CEF path for sending and
receiving packets.
