[c-nsp] BFD not working on ASR920

Gert Doering gert at greenie.muc.de
Sun Apr 3 06:01:03 EDT 2022


this is no question (except "why oh why?") but for the sake of google
and to help the next one that runs into this...

We have a number of ASR920-12 on 16.06.05a - because we cannot be bothered
to go to "the new licensing scheme" - and we have customer lines on them,
"because SFP handoff".

All the interfaces follow a standard scheme

interface GigabitEthernet0/0/2
 description <customer>/lid=nnn (primary)
 ip address 195.xx.xx.12
 ip flow ingress
 negotiation auto
 cdp enable
 ipv6 address 2001:608:xxx:xxx::1234/64
 bfd interval 200 min_rx 200 multiplier 10

(really nothing special here)

and then there's BGP config to the customer's router, with BFD

  neighbor 195.xx.xx.13 fall-over bfd
  neighbor 2001:608:xxx:xxx::1235 fall-over bfd

... this used to work fine on our 6500s, but BFD just did not want to
come up on the ASR920s, no matter what I tested and debugged.

Today I found the culprit - the configs were copied over from the 6500,
including an "ip flow ingress" line - we do not have netflow active on
the ASR920 (and from my understanding of "how it works", this is not
something we actually want) so I considered this more of a no-op / config
wart than a problem.

Today I went cleaning up... and lo and behold

ar21(config)#interface GigabitEthernet0/0/2
ar21(config-if)#no ip flow ing
Apr  3 11:41:11: %SYS-5-CONFIG_I: Configured from console by gert on vty0 (2001:608:0:736::18)
Apr  3 11:41:18: %BFDFSM-6-BFD_SESS_UP: BFD-SYSLOG: BFD session ld:37 handle:9 is going UP

... all the "down and we cannot find a reason" BFD sessions came up the
moment I removed the "ip flow ingress" from the interface config.

long story short: 

 "ip flow ingress" kills BFD on ASR920, both for IPv4 and for IPv6.

(and no, I'm not going to bother spending another month of my life 
explaining this to TAC - documenting this on c-nsp is more helpful to
other victims than trying to get things fixed on this platform)


"If was one thing all people took for granted, was conviction that if you 
 feed honest figures into a computer, honest figures come out. Never doubted 
 it myself till I met a computer with a sense of humor."
                             Robert A. Heinlein, The Moon is a Harsh Mistress

Gert Doering - Munich, Germany                             gert at greenie.muc.de
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