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:
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 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 |
Everything else that is not explicitly listed in the configuration by default is forbidden.
To enable the use of BFD for BGP sessions, enable use-bfd for required entries in /routing bgp connection menu.
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 |
To enable the use of BFD for OSPF neighbors, enable use-bfd for required entries in /routing ospf interface-template menu.
The status of the currently available sessions can be observed from /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 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.