BFD multihop with Junos

[Arzhel Younsi]

Bingo! As soon as I the system TTL to 255, the session went up. Thanks a lot!

Now that we know where to look, we started to dig a bit in the code to not have to change the TTL system wide but only for Bird.

It seems like there a TODO to make the TTL value customizable: https://github.com/BIRD/bird/blob/master/proto/bfd/packets.c#L453
And in some (so far unknown) cases, it sets the TTL to 255 https://github.com/BIRD/bird/blob/master/proto/bfd/packets.c#L456

-- 
Arzhel

On Sun, Mar 10, 2019, at 19:10, Ondrej Zajicek wrote:
> On Thu, Mar 07, 2019 at 07:13:58PM -0500, Arzhel Younsi wrote:
> > Thanks for your reply Ondrej,
> > 
> > I changed the port range as suggested, confirmed that BFD packets were leaving from a correct port, but the BFD session still stays down.
> 
> 208.80.153.77            Down                     0.000     1.000        3
>  Client BGP, TX interval 0.300, RX interval 0.300
>  Local diagnostic None, remote diagnostic None
>  Remote state AdminDown, version 1
>  Replicated
>  Session type: Multi hop BFD
>  Min async interval 0.300, min slow interval 1.000
>  Adaptive async TX interval 2.000, RX interval 2.000
>  Local min TX interval 2.000, minimum RX interval 0.300, multiplier 3
>  Remote min TX interval 0.000, min RX interval 0.000, multiplier 0
>  Local discriminator 3556, remote discriminator 0
>  Echo mode disabled/inactive, no-absorb, no-refresh
>  Multi-hop min-recv-TTL 254, route table 0, local-address 208.80.153.192
> 
> Perhaps there is an issue with 'min-recv-TTL 254'. For single-hop BFD
> sessions, the RFC 5880 requires TTL security mechanism and therefore
> BIRD specifies outgoing TTL 255. For multi-hop BFD there is no such
> requirement and therefore BIRD uses OS default TTL, which is AFAIK 64
> on Linux.
> 
> You can check that with tcpdump and perhaps disable the check on Juniper
> or set /proc/sys/net/ipv4/ip_default_ttl on Linux.
> 
> -- 
> Elen sila lumenn' omentielvo
> 
> Ondrej 'Santiago' Zajicek (email: santiago at crfreenet.org)
> OpenPGP encrypted e-mails preferred (KeyID 0x11DEADC3, wwwkeys.pgp.net)
> "To err is human -- to blame it on a computer is even more so."
>