Bfd Support For Stateful Switchover - Cisco Catalyst 4500 Series Software Configuration Manual

Information About Bidirectional Forwarding Detection

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.
When the BFD protocol on the standby RP is notified of a switchover it changes its state to active,
registers itself with Cisco Express Forwarding so that it can receive packets, and then sends packets for
any elements that have expired.
BFD also uses checkpoint messages to ensure that sessions created by clients on the active RP are
maintained during a switchover. When a switchover occurs, BFD starts an SSO reclaim timer. Clients
must reclaim their sessions within the duration specified by the reclaim timer or else the session is
deleted.
Timer values are different based on the number of BFD sessions and the platform.
Table 40-1
Table 40-1
Maximum Number of
BFD Sessions
100
BFD Support for Static Routing
Unlike dynamic routing protocols, such as OSPF and BGP, static routing has no method of peer
discovery. Therefore, when BFD is configured, the reachability of the gateway is completely dependent
on the state of the BFD session to the specified neighbor. Unless the BFD session is up, the gateway for
the static route is considered unreachable, and therefore the affected routes will not be installed in the
appropriate Routing Information Base (RIB).
For a BFD session to establish successfully, BFD must be configured on the interface on the peer and
there must be a BFD client registered on the peer for the address of the BFD neighbor. When an interface
is used by dynamic routing protocols, the latter requirement is usually met by configuring the routing
protocol instances on each neighbor for BFD. When an interface is used exclusively for static routing,
this requirement must be met by configuring static routes on the peers.
BFD is supported on IPv4 and IPv6 static routes.
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
40-6
describes the timer value on Cisco 4500 series switches.
BFD Timer Values on a Cisco 4500 Series Switches
BFD Session
Type
Async/echo
Chapter 40 Configuring Bidirection Forwarding Detection
Minimum Timer
Value (ms) Clients
100 All
multiplier 3
Comments
A multiple of 5
is recommended
for SSO
switches.