Bfd Session Limits - Cisco Catalyst 4500 Series Configuration Manual

Chapter 37 Configuring Bidirection Forwarding Detection

BFD Session Limits

Starting with Cisco IOS Release 15.1(1)SG, the minimum number of BFD sessions that can be created
varies with the "hello" interval. With "hello" intervals of 50ms, 128 sessions are permitted. More
sessions are permitted at larger hello intervals.
BFD Support for Nonbroadcast Media Interfaces
Starting with Cisco IOS Release 15.1(1)SG, the BFD feature is supported on VLAN interfaces.
The bfd interval command must be configured on the interface to initiate BFD monitoring.
BFD Support for Nonstop Forwarding with Stateful Switchover
Typically, when a networking device restarts, all routing peers of that device detect that the device went
down and then came back up. This transition results in a routing flap, which could spread across multiple
routing domains. Routing flaps caused by routing restarts create routing instabilities, which are
detrimental to the overall network performance. Nonstop forwarding (NSF) helps to suppress routing
flaps in devices that are enabled with stateful switchover (SSO), thereby reducing network instability.
NSF allows for the forwarding of data packets to continue along known routes while the routing protocol
information is being restored after a switchover. With NSF, peer networking devices do not experience
routing flaps. Data traffic is forwarded while the standby RP assumes control from the failed active RP
during a switchover. The ability of line cards and forwarding processors to remain up through a
switchover and to remain current with the Forwarding Information Base (FIB) on the active RP is key to
NSF operation.
In devices that support dual RPs, SSO establishes one of the RPs as the active processor; the other RP is
designated as the standby processor, and then synchronizes information between them. A switchover
from the active to the standby processor occurs when the active RP fails, when it is removed from the
networking device, or when it is manually taken down for maintenance.
BFD Support for Stateful Switchover
The BFD protocol provides short-duration detection of failures in the path between adjacent forwarding
engines. In network deployments that use dual RP switches (to provide redundancy), the switches have
a graceful restart mechanism that protects the forwarding state during a switchover between the active
RP and the standby RP.
Stateful BFD on the Standby RP
To ensure a successful switchover to the standby RP, the BFD protocol uses checkpoint messages to send
session information from the active RP Cisco IOS instance to the standby RP Cisco IOS instance. The
session information includes local and remote discriminators, adjacent router timer information, BFD
setup information, and session-specific information such as the type of session and the session version.
In addition, the BFD protocol sends session creation and deletion checkpoint messages to create or
delete a session on the standby RP.
The BFD sessions on the standby RP do not receive or send packets and do not process expired timers.
These sessions wait for a switchover to occur and then send packets for any active sessions so that
sessions do not time out on adjacent switches.
OL-25340-01
Information About Bidirectional Forwarding Detection
Software Configuration Guide—Release IOS XE 3.3.0SG and IOS 15.1(1)SG
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