40-5
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Chapter 40 Configuring Bidirection Forwarding Detection
Information About Bidirectional Forwarding Detection
• Cisco devices will use one BFD session for multiple client protocols in the Cisco implementation of
BFD. For example, if a network is running OSPF and EIGRP across the same link to the same peer,
only one BFD session will be established, and BFD will share session information with both routing
protocols. However, IPv4 and IPv6 clients cannot share a BFD session.
BFD Version Interoperability
Starting with Cisco IOS Release 15.1(1)SG, the Catalyst 4500 series switch supports BFD Version 1 as
well as BFD Version 0. All BFD sessions come up as Version 1 by default and will be interoperable with
Version 0. The system automatically performs BFD version detection, and BFD sessions between
neighbors will run in the highest common BFD version between neighbors. For example, if one BFD
neighbor is running BFD Version 0 and the other BFD neighbor is running Version 1, the session will
run BFD Version 0. The output from the show bfd neighbors [details] command will verify which BFD
version a BFD neighbor is running.
See the “Example: Configuring BFD in an EIGRP Network with Echo Mode Enabled by Default” section
on page 40-17 for an example of BFD version detection.
BFD Session Limits
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 on the
Catalyst 4500 series switch.
The bfd interval command must be configured on an 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.
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