[c-nsp] Cisco IGP / BFD Dampening (Suppressing Unstable WAN Links)

[James Bensley]

Hi,

If you use carrier-delay with BFD then the interface won't be
signalled as up to the IGP until BFD has been established. Or am I
miss-remembering this?

I belive that if you use something like "carrier-delay down 0 up 5000"
on an interface with BFD, if the interface goes down hard down or a
layer 2 hop in the middle goes down (which BFD catches) then the
interface is signalled as down to the IGP without delay (also required
global config command "ip routing protocol purge interface" I
believe). When the interface comes hard up again (or BFD confirms
end-to-end connectivity in the case of a layer 2 hop issue) then
carrier delay will prevent the IGP from being signalled strait away
(in this example for 5 seconds).


Something like the following has worked for us (can't remember exactly
off the top of my head and you should lab test to match the config to
your needs etc.):

conf t
 ip routing protocol purge interface
 int x/x
  carrier-delay down 0 up 5000
  dampening
  bfd interval 50 min_rx 50 multiplier 3


We have had scenarios of third party links that flapped multiple times
and were caught by the "damping" command which we see in syslog a
message like "InterfaceX/X Routing state to DOWN, interface is
suppressed"

Cheers,
James.