BFD parameters conflict

[nadeem-akhtar]

We plan to have BFD configured but with following conflicting parameters due to different support groups and standards:

CE Router: bfd interval 300 min_rx 300 multiplier 3

PE Router: bfd interval 500 min_rx 500 multiplier 5

Per my understanding, the heartbeat interval will get negotiated to the higher value (which will be 500 msec) during the handshake.

What will happen to the multiplier? Will it also get negotiated to the higher value of 5? Or will multiplier become an issue if configured differently as above?

Thank you,

Nadeem

[Bilal Nawaz]

Hello Nadeem,

Good question! the interval as you say negotiates at the higher level, 500msec. Based on some previous tests I did with Juniper devices, and just did test on Cisco too, i've come to this conclusion.

The multiplier stays the same, during BFD each of them CE and PE tell each other what their multiplier values are, and abide by that, so they expect each others multiplier. Just to explain more i have CE and PE exactly how you have there.

The calculated BFD liveness detection interval can be different on each peer. The multiplier value is roughly equivalent to the number of packets that can be missed before the BFD session is declared to be down.

Hope it helps.

CE

CE#show bfd neighbors details          

IPv4 Sessions

NeighAddr                              LD/RD         RH/RS     State     Int

10.10.10.1                              1/1          Up        Up        Et0/0

Session state is UP and using echo function with 500 ms interval.

Session Host: Software

OurAddr: 10.10.10.2     

Handle: 1

Local Diag: 0, Demand mode: 0, Poll bit: 0

MinTxInt: 1000000, MinRxInt: 1000000, Multiplier: 3

Received MinRxInt: 1000000, Received Multiplier: 5

Holddown (hits): 0(0), Hello (hits): 1000(440)

Rx Count: 445, Rx Interval (ms) min/max/avg: 1/1006/872 last: 759 ms ago

Tx Count: 444, Tx Interval (ms) min/max/avg: 1/1005/875 last: 114 ms ago

Elapsed time watermarks: 0 0 (last: 0)

Registered protocols: OSPF 

Uptime: 00:06:14

Last packet: Version: 1                  - Diagnostic: 0

             State bit: Up               - Demand bit: 0

             Poll bit: 0                 - Final bit: 0

             C bit: 0                                   

             Multiplier: 5               - Length: 24

             My Discr.: 1                - Your Discr.: 1

             Min tx interval: 1000000    - Min rx interval: 1000000

             Min Echo interval: 500000 

PE

PE#show bfd neighbors details 

IPv4 Sessions

NeighAddr                              LD/RD         RH/RS     State     Int

10.10.10.2                              1/1          Up        Up        Et0/0

Session state is UP and using echo function with 500 ms interval.

Session Host: Software

OurAddr: 10.10.10.1     

Handle: 1

Local Diag: 0, Demand mode: 0, Poll bit: 0

MinTxInt: 1000000, MinRxInt: 1000000, Multiplier: 5

Received MinRxInt: 1000000, Received Multiplier: 3

Holddown (hits): 0(0), Hello (hits): 1000(342)

Rx Count: 347, Rx Interval (ms) min/max/avg: 1/1005/874 last: 50 ms ago

Tx Count: 349, Tx Interval (ms) min/max/avg: 1/1006/869 last: 359 ms ago

Elapsed time watermarks: 0 0 (last: 0)

Registered protocols: OSPF 

Uptime: 00:04:49

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.: 1                - Your Discr.: 1

             Min tx interval: 1000000    - Min rx interval: 1000000

             Min Echo interval: 300000