How to determine what BFD timers to configure

[Stuart Patton]

Hi,

I have 2 sites which each have a pair of Cat9500-32C switches in virtual stackwise as core switches and are connected together with 1x 10G L2 and 2x 1G L2 fibre (essentially dark fibre) circuits.  We have configured the links between the sites as L3 and have EIGRP adjacencies between the two cores.  Prefixes use the 10G and appear as feasible successors via the 1G links as expected.  Due to the circuit providers not forwarding link loss events, we had a gold partner implement BFD, and this is using 100,100,3 as the values and of course, the BFD interfaces are added to the EIGRP config.

Yesterday, we saw a number of BFD link failure events across multiple links.  I'm confident that the actual circuits did not fail, so we believe this is a result of the BFD timers being too aggressive.  The C9500 are ticking over and never go above 6% cpu utilisation.

According to the IOS XE guide, the Cat 9500 high-performance models should be using a minimum of 750 to prevent a BFD failure in the event of a redundancy switchover.  However I also note that BFD traffic falls into class-default, so I am guessing maybe there was a microburst and the BFD packets were QoS dropped?  We are using out-of-the box QoS config on the Cat 9500s and no softmax multipliers.

Can anyone point me towards any documentation or experience around calculating what BFD timers we should use? 

Thanks,

Stuart

[balaji.bandi]

Until we see the config and Logs was generate it is hard to assume what went wrong. we do have Cat 9500 same kind of setup never seen this issue personally myself.

Some time Connected to NTU - the NTU Link always stay up never go down - if you looking Physical connection concern.

But there may be short distrunption in the path, until you have any other IP SLA Track or another system in place to record that details.

as per CAt 9500 concerns cisco recomend  below values : (this may vary depends on deploym,ent) - but this help you to start with.

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst9500/software/release/16-11/configuration_guide/rtng/b_1611_rtng_9500_cg/configuring_bidirectional_forwarding_detection.html#id_122161

[Stuart Patton]

Hi BB, yes, that was the document I saw.  In fact, they are L3 interfaces so in my original post it should read 250 not 750ms.

In the case of the 10G link, this is what I'm showing:

C9500-32C#sh run int hu1/0/13

interface HundredGigE1/0/13
 no switchport
 ip address 192.168.254.248 255.255.255.254
 no ip redirects
 no ip unreachables
 bfd interval 100 min_rx 100 multiplier 3

C9500-32C#sh bfd neighbors interface hu1/0/13 details

IPv4 Sessions
NeighAddr                              LD/RD         RH/RS     State     Int
192.168.254.249                         7/12         Up        Up        Hu1/0/13
Session state is UP and using echo function with 100 ms interval.
Session Host: Software
OurAddr: 192.168.254.248
Handle: 2
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(94611)
Rx Count: 94666, Rx Interval (ms) min/max/avg: 1/2044/876 last: 159 ms ago
Tx Count: 94646, Tx Interval (ms) min/max/avg: 1/1005/878 last: 698 ms ago
Elapsed time watermarks: 0 0 (last: 0)
Registered protocols: CEF EIGRP
Uptime: 19:49:21
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.: 12               - Your Discr.: 7
             Min tx interval: 1000000    - Min rx interval: 1000000
             Min Echo interval: 100000

[Snip multiple occurrences of the below throughout the day]
May 26 15:18:17.311: %DUAL-5-NBRCHANGE: EIGRP-IPv4 500: Neighbor 192.168.254.249 (HundredGigE1/0/13) is up: new adjacency
May 26 15:18:17.311: %BFD-6-BFD_SESS_CREATED: BFD-SYSLOG: bfd_session_created, neigh 192.168.254.249 proc:EIGRP, idb:HundredGigE1/0/13 handle:2 act
May 26 15:25:30.377: %DUAL-5-NBRCHANGE: EIGRP-IPv4 500: Neighbor 192.168.254.249 (HundredGigE1/0/13) is down: BFD peer down notified
May 26 15:25:34.978: %DUAL-5-NBRCHANGE: EIGRP-IPv4 500: Neighbor 192.168.254.249 (HundredGigE1/0/13) is up: new adjacency
May 26 15:25:34.979: %BFD-6-BFD_SESS_CREATED: BFD-SYSLOG: bfd_session_created, neigh 192.168.254.249 proc:EIGRP, idb:HundredGigE1/0/13 handle:2 act