BFD – Bidirectional Forwarding Detection

BFD is a routing protocol independent fast failure detection method(subsecond detection is possible). It is basically a faster way to signal to your dynamic routing protocols({Cisco – OSPF, IS-IS, EIGRP, and BGP}{Mikrotik – OSPF and BGP}) that there has been a failure of a neighbor.

The Cisco link above has a very good description of how the whole process works, so I’m simply going to paraphrase here.

Configurations

Interval
This is the desired rate at which this router will send BFD packets to a neighbor.

min-rx(Mikrotik) or min_rx(Cisco)
This is the desire rate at which this router will receive BFD packets from a neighbor.

Note: The rate negotiated for send/receive doesn’t have to be the same. Whichever rate is slower will be used for each direction.

Multiplier
The number of consecutively missed BFD packets before BFD informs the associated protocol that there was a failure.

Cisco Configuration

Setting baseline BFD parameters via interface configuration mode:

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bfd interval milliseconds min_rx milliseconds multiplier multiplier-value

If you are going to be connecting this Cisco router to a Mikrotik via BFD you must disable BFD echo mode on the Cisco:

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no bfd echo

Now you must inform individual protocols that they are to use BFD:
To enable on all active EIGRP interfaces:

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router eigrp 111
bfd all-interfaces

To enable on individual EIGRP interfaces:

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router eigrp 123
bfd interface Gig1/0

Mikrotik Configuration

Setting baseline BFD parameters for interfaces(Mikrotik defaults to already having 2MS timers with a multiplier of 5 configured for all interfaces):

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/routing bfd interface
add disabled=no interface=ether1 interval=0.2s min-rx=0.2s multiplier=5

Now inform the routing protocol:

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/routing ospf interface add interface=ether1 use-bfd=yes

or

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/routing bgp peer add remote-address=1.1.1.1 remote-as=1111 use-bfd=yes

Troubleshooting/Verification

Cisco
Neighbor info including neighbor address, interface, protocol, negotiated timers, uptime, up/down, and number of state changes.

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show bfd neighbors [details]

Debugging BFD:

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debug bfd packet [neighbor address]

Debug state transitions:

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debug bfd event

Mikrotik
This prints neighbor information including neighbor address, interface, protocol, negotiated timers, uptime, up/down, and number of state changes.

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/routing bfd neighbor print detail

Logging BFD related messages:

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/system logging
add topics=bfd

So BFD is an interesting addition to routing that may make a large difference to you in certain situations. Having said that, I don’t actually run this in my infrastructure. Since I’m in an ISP environment I generally like OSPF’s speed which is at a moderate pace.

There are some instances where I can see the real value of this. Most of the BGP sessions I establish occur over some layer two medium. If there is a failure between the peering router and me, my router doesn’t know about it until it’s default timers run out. In BGP the default hello is 60 seconds and the default dead is 3 iterations which means it takes 3 minutes for my router to realize there is an issue and to tear down the session. When establishing the BGP session I usually attempt to keep as much default as possible, but this would allow you to put a little zip in the process.