OSPF BFD

OSPF BFD configuration is as below.

ABC#sh running-config interface g0/1/0
Building configuration...

Current configuration : 571 bytes
!
interface GigabitEthernet0/1/0
 bandwidth 60000
 ip address 172.16.21.58 255.255.255.252
 ip ospf bfd
 bfd interval 400 min_rx 400 multiplier 4

!
!


As per above OSPF BGD configuration,BFD will make OSPF down after 1600milisecond (400*4).Can anyone confirm my understanding is i am wrong?

In the below output how i can confirm that 1600milisecond is ospf bfd dead interval ?


ABC#sh bfd neighbors ipv4 172.16.21.57 details

NeighAddr                         LD/RD    RH/RS     State     Int
172.16.21.57                       7/311   Up        Up        Gi0/1/0
Session state is UP and using echo function with 400 ms interval.
OurAddr: 172.16.21.58
Local Diag: 0, Demand mode: 0, Poll bit: 0
MinTxInt: 1000000, MinRxInt: 1000000, Multiplier: 4
Received MinRxInt: 1000000, Received Multiplier: 4
Holddown (hits): 0(0), Hello (hits): 1000(459450)
Rx Count: 458897, Rx Interval (ms) min/max/avg: 2/1057/880 last: 1 ms ago
Tx Count: 459451, Tx Interval (ms) min/max/avg: 754/1056/876 last: 567 ms ago
Elapsed time watermarks: 0 0 (last: 0)
Registered protocols: CEF OSPF
Uptime: 4d15h
Last packet: Version: 1                  - Diagnostic: 0
             State bit: Up               - Demand bit: 0
             Poll bit: 0                 - Final bit: 0
             Multiplier: 4               - Length: 24
             My Discr.: 311              - Your Discr.: 7
             Min tx interval: 1000000    - Min rx interval: 1000000
             Min Echo interval: 400000

Comments

  • Your understanding is correct-if BFD Echo packets are not received for 1600ms (400ms interval x 4 multiplier), BFD will cause OSPF to bring down adjacency.

    Cheers,

    AB.

  • In the below output how i can confirm that 1600milisecond is ospf bfd dead interval ?


    ABC#sh bfd neighbors ipv4 172.16.21.57 details

    NeighAddr                         LD/RD    RH/RS     State     Int
    172.16.21.57                       7/311   Up        Up        Gi0/1/0
    Session state is UP and using echo function with 400 ms interval.
    OurAddr: 172.16.21.58
    Local Diag: 0, Demand mode: 0, Poll bit: 0
    MinTxInt: 1000000, MinRxInt: 1000000, Multiplier: 4
    Received MinRxInt: 1000000, Received Multiplier: 4
    Holddown (hits): 0(0), Hello (hits): 1000(459450)
    Rx Count: 458897, Rx Interval (ms) min/max/avg: 2/1057/880 last: 1 ms ago
    Tx Count: 459451, Tx Interval (ms) min/max/avg: 754/1056/876 last: 567 ms ago
    Elapsed time watermarks: 0 0 (last: 0)
    Registered protocols: CEF OSPF
    Uptime: 4d15h
    Last packet: Version: 1                  - Diagnostic: 0
                 State bit: Up               - Demand bit: 0
                 Poll bit: 0                 - Final bit: 0
                 Multiplier: 4               - Length: 24
                 My Discr.: 311              - Your Discr.: 7
                 Min tx interval: 1000000    - Min rx interval: 1000000
                 Min Echo interval: 400000

  • ABC#sh bfd neighbors ipv4 172.16.21.57 details

    NeighAddr                         LD/RD    RH/RS     State     Int
    172.16.21.57                       7/311   Up        Up        Gi0/1/0
    Session state is UP and using echo function with 400 ms interval.
    OurAddr: 172.16.21.58
    Local Diag: 0, Demand mode: 0, Poll bit: 0
    MinTxInt: 1000000, MinRxInt: 1000000, Multiplier: 4
    Received MinRxInt: 1000000, Received Multiplier: 4
    Holddown (hits): 0(0), Hello (hits): 1000(459450)
    Rx Count: 458897, Rx Interval (ms) min/max/avg: 2/1057/880 last: 1 ms ago
    Tx Count: 459451, Tx Interval (ms) min/max/avg: 754/1056/876 last: 567 ms ago
    Elapsed time watermarks: 0 0 (last: 0)
    Registered protocols: CEF OSPF
    Uptime: 4d15h
    Last packet: Version: 1                  - Diagnostic: 0
                 State bit: Up               - Demand bit: 0
                 Poll bit: 0                 - Final bit: 0
                 Multiplier: 4               - Length: 24
                 My Discr.: 311              - Your Discr.: 7
                 Min tx interval: 1000000    - Min rx interval: 1000000
                 Min Echo interval: 400000

  • It's not strickptly speaking the ospf dead time. It's the time in which BFD will signal ospf the link is no longer available.  The ospf hello and dead timers are irrelevant. You could set you ospf timers to really high values to limit the amount of processing and rely on BFD with no problems.

     

    There's  a decent article here, unfortunately the follow up never seemed to come along.  http://www.networkworld.com/community/blog/bidirectional-forwarding-detection-bfd-–-little-about-timers-0

  • You wont see any specific timers for BFDs refecting 1600 however if you send any traffic over this link and there is any alternate path, BFD will take max of 1600ms to converge and BFD will ask OSPF to reroute the traffic so that 40sec usual dead tiime of OSPF can be avoided  and you will get max of 1600ms loss of traffic.

    Regards

    Mohit

     

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