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BFD

Bidirectional Forwarding Detection (BFD) is a low‑overhead, short‑duration protocol that detects faults in the bidirectional path between two forwarding engines. This includes physical interfaces, sub‑interfaces, data links, and — as far as possible — the forwarding engines themselves, all with very low latency. It operates independently of media, data protocols, and routing protocols.

BFD is a hello protocol that checks bidirectional neighbor reachability. It provides sub‑second link failure detection and is not specific to any routing protocol, unlike protocol hello timers.

Control data is encapsulated in a UDP frame with destination port 3784; port 4784 is used for multihop paths. The source port range is 49152 – 65535.
Echo is encapsulated in a UDP frame with destination port 3785.

Standards and Technologies:

Features not yet supported​

  • Echo mode.
  • Enabling BFD for ip route gateways.
  • Authentication.

Configuration​

Allowing or forbidding BFD sessions can be done from the /routing/bfd/configuration menu. For example:

/routing bfd configuration
add interfaces=sfp12 forbid-bfd=yes
add interfaces=static

Configuration entries are order sensitive, which means that in the example above we are forbidding BFD sessions explicitly on the "sfp12" interface (using forbid-bfd) and allowing on the rest of the interfaces belonging to the "static" interface list.

To be able to filter multi-hop sessions, addresses or address-list properties can be used to match the destination, as well as the appropriate vrf, if a session is not running in the "main" VRF.

/ip firewall address-list
add address=10.155.255.183 list=bgp_allow_bfd
add address=10.155.255.217 list=bgp_allow_bfd

/routing bfd configuration
add addresses=111.111.0.0/16 vrf=vrf1
add address-list=bgp_allow_bfd

Anything not explicitly listed in the configuration is forbidden by default.

BFD with BGP​

To enable BFD for BGP sessions, set use-bfd on the required entries in the /routing/bgp/connection menu.

The BGP session output shows when the associated BFD session is down:

[admin@dr_02_BGP_MUM] /routing/bgp/session> print 
Flags: E - established 
 0 E ;;; BFD session down
     name="ovpn_test1-1" 
     remote.address=111.111.11.11@vrf1 .as=65530 .id=10.155.101.217 
     .capabilities=mp,rr,as4 .hold-time=infinity .messages=40717 
     .bytes=3436281 .eor="" 
     local.address=111.111.11.12@vrf1 .as=555 .id=111.111.11.12 
     .capabilities=mp,rr,gr,as4 .messages=1 .bytes=19 .eor="" 
     output.procid=20 
     input.procid=20 .filter=bgp-in ebgp 
     hold-time=infinity use-bfd=yes uptime=3s210ms 
     last-started=2023-05-19 09:54:04 prefix-count=3853

BFD with OSPF​

To enable BFD for OSPF neighbors, set use-bfd on the required entries in the /routing/ospf/interface-template menu.

Session Status​

View the status of current sessions in the /routing/bfd/session menu:

[admin@dr_02_BGP_MUM] /routing/bfd/session> print 
Flags: U - up, I - inactive 
 0 I ;;; BFD forbidden for destination address
     multihop=yes remote-address=10.155.101.183 local-address="" desired-tx-interval=0ms required-min-rx=0ms 
     multiplier=0 

 1   multihop=no remote-address=111.111.11.11%ovpn-out1@vrf1 local-address=111.111.11.12@vrf1 state=down 
     state-changes=0 desired-tx-interval=200ms required-min-rx=200ms remote-min-rx=1us multiplier=5 
     packets-rx=0 packets-tx=7674

BFD picks the higher of the local tx interval and the remote minimum rx interval as the desired transmit interval. If the session is not established, the desired minimum tx interval defaults to 1 second.