Cisco ASR 9000 Series Routing Configuration Manual page 275

Implementing BFD
• You can configure timers for up to 3600 seconds (1 hour) to allow for delays in receipt of BFD state
change notifications (SCNs) from peers before declaring a link bundle BFD session down. The
configurable timers apply to these situations:
• A BFD session sends a DOWN notification when one of these occurs:
• In these cases, a DOWN notification is not sent, but the internal infrastructure treats the event as if a
DOWN has occurred:
• The BFD session on a bundle member sends BFD state change notifications to the bundle manager.
Once BFD state change notifications for bundle member interfaces are received by the bundle manager,
the bundle manager determines whether or not the corresponding bundle interface is usable.
• A threshold for the minimum number of active member links on a bundle is used by the bundle manager
to determine whether the bundle remains active, or is down based on the state of its member links. When
BFD is started on a bundle that is already active, the BFD state of the bundle is declared when the BFD
state of all the existing active members is known.
Whenever a member's state changes, the bundle manager determines if the number of active members
is less than the minimum number of active links threshold. If so, then the bundle is placed, or remains,
in DOWN state. Once the number of active links reaches the minimum threshold then the bundle returns
to UP state.
◦BFD session startup (bfd address-family ipv4 timers start command)—Number of seconds to
allow after startup of a BFD member link session for the expected notification from the BFD peer
to be received to declare the session up. If the SCN is not received after that period of time, the
BFD session is declared down.
◦ Notification of removal of BFD configuration by a neighbor (bfd address-family ipv4 timers
nbr-unconfig command)—Number of seconds to allow after receipt of notification that BFD
configuration has been removed by a BFD neighbor so that any configuration inconsistency between
the BFD peers can be fixed. If the BFD configuration issue is not resolved before the specified
timer is reached, the BFD session is declared down.
◦ The BFD configuration is removed on the local member link.
The BFD system notifies the peer on the neighbor router that the configuration is removed. The
BFD session is removed from the bundle manager without affecting other bundle member interfaces
or the overall bundle state.
◦ A member link is removed from the bundle.
Removing a member link from a bundle causes the bundle member to be removed ungracefully.
The BFD session is deleted and BFD on the neighboring router marks the session DOWN rather
than NBR_CONFIG_DOWN.
◦ The BFD configuration is removed on a neighboring router and the neighbor unconfiguration timer
(if configured) expires.
The BFD system notifies the bundle manager that the BFD configuration has been removed on the
neighboring router and, if bfd timers nbr-unconfig is configured on the link, the timer is started.
If the BFD configuration is removed on the local router before the timer expires, then the timer is
stopped and the behavior is as expected for BFD configuration removal on the local router.
If the timer expires, then the behavior is the same as for a BFD session DOWN notification.
◦ The session startup timer expires before notification from the BFD peer is received.
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.3.x
BFD Over Member Links on Link Bundles
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