networking n stuff

Friday, April 10, 2020

Demystifying BFD intervals

If I was to make a list of most wide-used and still very poorly-documented protocols, I'd definitely put BFD in it. In this post I'll try to answer several questions I was looking answers for.

TLDR: by default when you enable BFD there's both Echo and Asynchronous BFD modes running simultaneously (so there's two different types of packets your devices exchange with each other). The one's responsible for failure detection is Echo Mode and it's the one you are configuring intervals and multipliers for. In outputs you'll also see intervals for Asynchronous Mode packets which are significantly higher but you don't have to worry because Echo Mode packets are used for failure detection.

Let's say you configured BFD as follows (I'm using CSR1000V with IOS XE 16.04.03 for this example):

interface GigabitEthernet2
 bfd interval 50 min_rx 50 multiplier 3
router ospf 1
 bfd all-interfaces

So we're all good, setting our timers for minimal possible values. But when you start to check things out, you see this:

Core3#sh bfd nei details 

IPv4 Sessions
NeighAddr                              LD/RD         RH/RS     State     Int                           4097/4097       Up        Up        Gi2
Session state is UP and using echo function with 50 ms interval.
Session Host: Software
Handle: 1
Local Diag: 0, Demand mode: 0, Poll bit: 0
MinTxInt: 1000000, MinRxInt: 1000000, Multiplier: 3
Received MinRxInt: 1000000, Received Multiplier: 3
Holddown (hits): 0(0), Hello (hits): 1000(1590)
Rx Count: 1587, Rx Interval (ms) min/max/avg: 2/1003/875 last: 358 ms ago
Tx Count: 1594, Tx Interval (ms) min/max/avg: 2/1004/871 last: 315 ms ago
Elapsed time watermarks: 0 0 (last: 0)
Registered protocols: OSPF CEF 
Uptime: 00:23:09
Last packet: Version: 1                  - Diagnostic: 0
             State bit: Up               - Demand bit: 0
             Poll bit: 0                 - Final bit: 0
             C bit: 0                                   
             Multiplier: 3               - Length: 24
             My Discr.: 4097             - Your Discr.: 4097
             Min tx interval: 1000000    - Min rx interval: 1000000
             Min Echo interval: 50000  

What's this? Why do we have Rx/TxInt of 1 second? And what's echo? OK, let's figure out how exactly BFD works.