BFD multihop with Junos

[Ondrej Zajicek]

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."