Re: BFD strict in combination with OSPFv3

dvanoudheusden · ‎05-29-2019

I use OSPFv3 between the two routers, where we use 2 address-families, 1 for IPv4 and one for IPv6. As the operation of strict-mode requires a bfd session to be up, before the OSPF adjacency will form, I'm facing the problem that because (I suppose) OSPFv3 uses the IPv6 link-local address for the hello singnalling, that no IPv4 BFD neighbor is forming and because of strict mode, the IPv4 AF OSPF adjacency will not form.

I've also tested to only enable BFD strict-mode for the IPv6 address-family, if I do that and the BFD neighbor is broken, BFD will tear down only the IPv6 OSPF adjacency, leaving the IPv4 adjacency up.

I’ve attached a drawing, that explains our configuration and setup. I've searched multiple configuration guides for different software releases, however we’ve not yet found an answer. We also use BGP in multi address-family mode (IPv4/IPv6) and BFD strict-mode does work in combination with BGP.

Kind regards,

Dylan

Giuseppe Larosa · ‎05-29-2019

Hello ,

this is really new topic.

I have found the following draft (it is not already an approved RFC) about OSPF and BFD strict mode that provides guidelines for backward compatibility.

https://datatracker.ietf.org/doc/draft-ketant-lsr-ospf-bfd-strict-mode/?include_text=1

However, in older RFCs something different is stated, that BFD for IPv4 and IPv6 on the same interface should use two separate sessions each of them using a different L3 protocol

see

>>

 If BFD is to be used in conjunction with both IPv4 and IPv6 on a
   particular path, a separate BFD session MUST be established for each
   protocol (and thus encapsulated by that protocol) over that link.

on

https://datatracker.ietf.org/doc/rfc5881/?include_text=1

section 2 Applications and Limitations.

OSPFv3 works based on IPv6 link local addresses.

How should be implemented BFD for IPv4 unicast address family in OSPFv3?

According to RFC 5881 an IPv4 BFD session should be used for it, but your tests show that enabling BFD strict mode for IPv4 unicast family prevents neighbors to form adjacencies. In the case of OSPFv3 as described in the draft the OSPF session can be formed only for those neighbors having an active BFD session.

See in the first link in the draft

>>

Init (with BFD strict-mode)

      In this state, an Hello packet has recently been seen from the
      neighbor.  However, bidirectional communication has not yet been
      established with the neighbor (i.e., the router itself did not
      appear in the neighbor's Hello packet).  A BFD session
      establishment to the neighbor is requested, if not already done
      (e.g. in the event of transition from 2-way state).  All neighbors
      in higher than Init state and those in Init state with BFD session
      up are listed in the Hello packets sent from the associated
      interface.

I would suggest you to open a Cisco TAC describing this issue. It looks like implementation needs to be tuned.

Hope to help

Giuseppe

dvanoudheusden · ‎05-31-2019

Hi Giuseppe,

I did find the same draft as you did, I've contacted the authors of the draft to see and try if they can help. If I know more I'll update the post!

Regards,

Dylan