BFD (Bidirectional Forwarding Detection) in ACI

What is BFD?

• Bidirectional Forwarding Detection (BFD) is a network protocol that swiftly identifies failures in the forwarding path between two devices, such as routers or switches.

• It enables rapid detection of faults, often within milliseconds (sub-second), enhancing network reliability by reducing downtime.

  
  

When to Use BFD:

1. Indirect Connections:

   • In scenarios where routers are connected through a Layer 2 device or cloud and cannot directly detect each other’s failures, BFD offers quick failure detection, bypassing the longer timeouts of traditional protocols.​

2. Unreliable Media:

   • For connections over media lacking reliable failure detection mechanisms, like shared Ethernet, BFD provides swift detection, ensuring timely responses to issues.​

3. Multiple Protocols:

   • When multiple protocols operate between two routers, each with its own failure detection timers, BFD standardizes detection times, leading to consistent and predictable network behavior.​

     
     

BFD in Cisco ACI:

• Monitoring Spine-to-Leaf Connections:

  • BFD rapidly identifies failures in critical ACI fabric links between spine and leaf switches, maintaining network stability.​

• Enhancing Routing Protocols:

  • By integrating with protocols like OSPF, BGP, and static routes, BFD accelerates network convergence during failures, minimizing downtime.​

• Ensuring Application Availability:

  • BFD helps prevent application downtime by enabling swift rerouting of traffic in response to network issues.​

Configuration Steps for Fabric BFD

1. Enable Global BFD:

   • Navigate to: Fabric > Fabric Policies > Policies > Interface > L3 Interface > default.​

   • Enable the BFD ISIS Policy configuration.​

   • Verify neighbors using the command:​

     
     

     Spine/Leaf# show bfd neighbors vrf overlay-1

     
     

2. Modify Global BFD Timer:

   • Navigate to: Fabric > Access Policies > Policies > Switch > BFD > BFD IPv4.​

   • Create or edit a BFD policy with desired timers.​

   • Check neighbor details using:​

     
     

     Spine/Leaf# show bfd ipv4 neighbors details

     
     

   Enable Interface-level BFD:

   
   

   • Important: Disable the global BFD setting first.​

   • Create a new L3 Interface Policy (e.g., "NP") and enable the BFD ISIS Policy configuration within it.​

     
     

   • Assign this new L3 Interface Policy to the appropriate Policy Groups:​

     • Leaf Interface Policy Group: Create a new policy group (e.g., "LNPG") and attach the "NP" L3 interface policy.

     • Spine Interface Policy Group: Create a new policy group (e.g., "SNPG") and attach the "NP" L3 interface policy.

       
       

   • Associate these Policy Groups with the correct Interface Profiles:​

     • Leaf Interface Profile: Create a new profile and attach the "LNPG" policy group.

     • Spine Interface Profile: Create a new profile and attach the "SNPG" policy group.

   • Ensure that the relevant switch profiles utilize "LNPG" and "SNPG" to activate Fabric BFD on those interfaces.​

     
     

Guidelines and Limitations:

• Supported Features:

  • Starting from APIC Release 3.1(1), BFD supports IS-IS on fabric interfaces between leaf and spine switches. Additionally, BFD is supported on spine switches for OSPF and static routes.​

  • BFD is compatible with modular spine switches equipped with -EX and -FX line cards (or newer versions), as well as the Nexus 9364C non-modular spine switch (or newer versions).​

  • From APIC Release 5.0(1), BFD multihop is supported on leaf switches, and ACI supports C-bit-aware BFD, determining whether BFD is dependent or independent of the control plane.​

    
    

• Limitations:

  
  

  • BFD between vPC peers is not supported.​

  • BFD over iBGP is not supported for loopback address peers.​

  • BFD on Layer 3 Outs (L3Out) is supported only on routed interfaces, subinterfaces, and SVIs; it is not supported on loopback interfaces.​

  • BFD for BGP prefix peers (dynamic neighbors) is not supported.​

  • Enabling BFD subinterface optimization on one subinterface activates it for all subinterfaces on the same physical interface.

Enabling BFD on L3Out:

• To enable BFD on an L3Out:​

  • Check the BFD option within the Logical Interface Profile under the respective routing protocol (BGP, OSPF, or EIGRP).​

  • By default, BFD parameters are derived from the global default BFD policy located at: Fabric > Access Policies > Policies > Switch > BFD > BFD IPv4/v6 > default.​

  • For custom BFD settings, create a non-default BFD policy and apply it to specific switches via the Switch Policy Group and Switch Profile under Fabric > Access Policies > Switches.​

  • To override switch-level global BFD parameters at the interface level, create a BFD Interface Profile under the Logical Interface Profile. This interface-level BFD policy is located under: Tenant > Policies > Protocol > BFD.