The Bidirectional Forwarding Detection (BFD) provides fast forwarding-path failure detection times for media types, encapsulations, topologies, and routing protocols. You can use BFD to detect forwarding path failures at a uniform rate, rather than at variable rates for different protocol hello mechanisms. BFD makes network profiling and planning easier and reconvergence time consistent and predictable.

BFD makes all routing and control plane applications less CPU intensive by offloading Layer 3 hello message adjacency detection to a single process. The BFD process runs uniformly for all applications and detects failures at subsecond intervals.

A BFD global configuration command is available for fabricpath interfaces. The bfd keyword in the config-farbicpath-isis command mode, enables BFD on all the FabricPath interfaces. The interface specific commands override the global values.

For FabricPath BFD, fabricpath-isis is the client. FabricPath-ISIS is a part of feature-set fabricpath.

BFD has the following prerequisites:

• You must enable the BFD feature, see Enabling the BFD Feature.

• For any client protocols that you want to enable BFD on, enable BFD in that client protocol, see Configuring BFD Support for Routing Protocols.

• Disable ICMPv4 and ICMPv6 redirect messages on BFD Layer 3 interfaces for Cisco Nexus 5548 and Cisco Nexus 5596 platforms.

• Disable the IP packet verification check for identical IP source and destination addresses in the default VRF.

• See other detailed prerequisites that are listed with the configuration tasks.

BFD has the following configuration guidelines and limitations for the Cisco Nexus 5500 Series switch:

• BFD supports BFD version 1.

• BFD supports IPv4.

• BFD supports single-hop BFD.

• BFD supports the Border Gateway Protocol (BGP).

• BFD for BGP supports single-hop External Border Gateway Protocol (EBGP) and internal Border Gateway Protocol (iBGP) peers.

• BFD supports the Enhanced Interior Gateway Routing Protocol (EIGRP).

• BFD supports the Open Shortest Path First (OSPF) routing protocol.

• BFD supports keyed SHA-1 authentication.

• BFD supports the following Layer 3 interfaces: physical interfaces, port channels, subinterfaces, and VLAN interfaces.

• BFD does not support Anycast HSRP.

• BFD does not support BFD per link.

• BFD depends on Layer 3 adjacency information to discover topology changes, including Layer 2 topology changes. A BFD session on a VLAN interface (SVI) may not be up after the convergence of the Layer 2 topology if there is no Layer 3 adjacency information available.

• Port-channel configuration limitations are as follows:

  • Fabricpath BFD sessions are not supported on a port channel logical interface on any type of a line card.

  • For Layer 3 port channels used by BFD, you must enable Link Aggregation Control Protocol (LACP) on the port channel.

  • For Layer 2 port channels used by SVI sessions, you must enable LACP on the port channel.

• BFD is supported on SVI interfaces that are formed over virtual port channels (vPCs), vPC peer-links and FabricPath links.

• SVI limitations are as follows:

  • An ASIC reset causes traffic disruption for other ports and could possibly cause SVI sessions on other ports to flap.

  • When you change the topology (for example, when you add or delete a link into a VLAN or delete a member from a Layer 2 port channel), the SVI session could be affected. The SVI session might go down first and then come up after the topology discovery is finished.

Tip

If you do not want the SVI sessions to flap and you need to change the topology, you can disable the BFD feature before making the changes and reenable BFD after the changes have been made. You can also configure the BFD timer to be a large value (for example, 5 seconds), and change it back to a fast timer after the topology change is complete.

• When you configure the BFD Echo function on the distributed Layer 3 port channels, reloading a member module flaps the BFD session hosted on that module, which results in a packet loss.

You can configure BFD at the global level and at the interface or subinterface level (for physical interfaces and port channels). The interface or subinterface configuration overrides global configuration. On supported interfaces, the subinterface-level configuration overrides the interface or port-channel configuration unless you enable subinterface optimization on the port-channel interface. See Optimizing BFD on Subinterfaces for more information.

For physical ports that are members of a port channel, the member port inherits the master port-channel BFD configuration. The member port subinterfaces can override the master port-channel BFD configuration, unless subinterface optimization is enabled.

You must enable the BFD feature before you can configure BFD on an interface and protocol within a device VRF.

You can configure the BFD session parameters for all BFD sessions on the device. The BFD session parameters are negotiated between the BFD peers in a three-way handshake.

See “Configuring BFD on an Interface” to override these global session parameters on an interface.

You can configure the BFD session parameters for all BFD sessions on an interface. The BFD session parameters are negotiated between the BFD peers in a three-way handshake.

This configuration overrides the global session parameters for the configured interface.

You can configure the BFD session parameters for all BFD sessions on a port channel. The BFD session parameters are negotiated between the BFD peers in a three-way handshake.

This configuration overrides the global session parameters for the configured port channel. The member ports of the port channel inherit the port channel BFD session parameters, unless you configure subinterface-level BFD parameters on a member port. In that case, the member port subinterface uses the subinterface BFD configuration if subinterface optimization is not enabled. See “Optimizing BFD on Subinterfaces” for more information.

You can optimize BFD on subinterfaces. BFD creates sessions for all configured subinterfaces. BFD sets the subinterface with the lowest configured VLAN ID as the master subinterface, and that subinterface uses the BFD session parameters of the parent interface. The remaining subinterfaces use the slow timer. If the optimized subinterface session detects an error, BFD marks all subinterfaces on that physical interface as down.

The Cisco Nexus 5500 provides BFD support for these routing protocols:

• Border Gateway Protocol (BGP)

• Enhanced Interior Gateway Routing Protocol (EIGRP)

• Open Shortest Path First Version 2 (OSPFv2)

• Hot Standby Router Protocol (HSRP)

• Virtual Router Redundancy Protocol (VRRP)

• Static routes

• Protocol-Independent Multicast (PIM)

• FabricPath on Intermediate System to Intermediate System (IS-IS)

This example shows how to configure BFD for OSPFv2 on Ethernet 2/1, using the default BFD session parameters:

switch# configure terminal
switch(config)# feature bfd
switch(config)# feature ospf
switch(config)# router ospf Test1
switch(config-router)# interface ethernet 2/1
switch(config-if)# ip ospf bfd
switch(config-if)# no shutdown

This example shows how to configure BFD for all EIGRP interfaces, using the default BFD session parameters:

switch# configure terminal
switch(config)# feature bfd
switch(config)# feature eigrp
switch(config)# bfd interval 250 min_rx 250 multiplier 4
switch(config)# router eigrp Test2
switch(config-router)# bfd