Summary

Bidirectional Forwarding Detection (BFD) is a low-overhead and short-duration protocol intended to detect faults in the bidirectional path between two forwarding engines, including physical interfaces, sub-interfaces, data link(s), and to the extent possible the forwarding engines themselves, with potentially very low latency. It operates independently of media, data protocols, and routing protocols.

BFD is basically a hello protocol for checking bidirectional neighbor reachability. It provides sub-second link failure detection support. BFD is not routing protocol specific, unlike protocol hello timers or such.

BFD Control packets are transmitted in UDP packets with destination port 3784, BFD also uses port 4784 for multihop paths. The source port is in the range 49152 through 65535. And BFD Echo packets are encapsulated in UDP packets with destination port 3785.

Standards and Technologies:

  • RFC 5880 Bidirectional Forwarding Detection (BFD)
  • RFC 5881 BFD for IPv4 and IPv6
  • RFC 5882 Generic Application of BFD
  • RFC 5883Bidirectional Forwarding Detection (BFD) for Multihop Paths


Features not yet supported

  • echo mode
  • enabling BFD for ip route gateways
  • authentication


配置

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

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

配置entries are order sensitive, which means that in the example above we are forbidding BFD sessions explicitly on the "sfp12" interface and allowing on the rest of the interfaces belonging to the "static" interface list.


To be able to filter multi-hop sessions,addressesoraddress-listproperties 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


Everything else that is not explicitly listed in the configuration by default is forbidden.

BFD with BGP

使使用BFD for BGP sessions, enableuse-bfdfor required entries in/routing bgp connectionmenu.

A useful feature is that the BGP session will show that the BFD session for that particular BGP 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

使使用BFD for OSPF neighbors, enableuse-bfdfor required entries in/routing ospf interface-templatemenu.

Session Status

The status of the currently available sessions can be observed from/routing bfd sessionmenu:

[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 is picking the highest value between the local tx interval and remote minimum rx interval as desired transmit interval. If the session is not established then desired minimum tx interval is set to 1 second.


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