[c-nsp] BFD not working on ASR920

[Gert Doering]

Hi,

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.12 255.255.255.252
 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
ar21(config-if)#^Z
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)

gert

-- 
"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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