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multi-hop BFD not working when only one-hop present

Latest reply: Jun 28, 2016 00:35:06 2477 2 0 0 0

Attempting to use multi-hop BFD with iBGP, but when the other router happens to only be one hop away, BFD will not establish.

Due to the topology the other router may or may not be only one hop away at any given time, but it appears that the session is rejected as multi-hop if it has a TTL of 255. RFCs 5881 and 5883 only say that single-hop has to be TTL of 255, but does not prohibit multi-hop from also being 255.

I have configured BFD like this:

  multi-hop destination-port 4784
  peer-ip x.x.21.1 32 ttl single-hop 255
  peer-ip x.x.23.1 32 ttl single-hop 255
  peer-ip x.x.23.1 32 ttl multi-hop 254
  peer-ip x.x.21.1 32 ttl multi-hop 254

bgp xxxxx
  peer reflectors bfd enable

and receive the following error:
Jun 27 2016 14:25:32.173.2-06:00 DST rtr.yung BFD/7/Packet:
16:04701: Receive BFD Packet :
MD(8556) YD(8205) Diag(0) State(2) P(0) F(0) TX(1000) RX(1000) DM(7)
Echo RX(0) Length(24) Ver(1) C(0) A(0) D(0) R(0)
TTL(255) DstPort(4784)
deb bfd all
Jun 27 2016 14:25:32.173.3-06:00 DST rtr.yung BFD/7/Packet:
16:04701: Receive BFD Packet. Discr:8205, SrcAddr:x.x.23.1, DestAddr:x.x.21.8, TOS:C0
deb bfd all
Jun 27 2016 14:25:32.173.4-06:00 DST rtr.yung BFD/7/Error:
16:04701: BFD ERR : Para(IP:0X____1701, TTL:255, Type:2) Check Err
                By(TTLIP:0X____1701, Mask:32, Type:2, TTL:254)
deb bfd all
Jun 27 2016 14:25:32.173.5-06:00 DST rtr.yung BFD/7/Error:
16:04731: BFD PKT : Local(Discr:8205) TTL Check ERR (Type:4)

It will not let me specify a value of 255 for multi-hop, and I cannot control the TTL the other side is using either. Is there a way to disable TTL checking for multi-hop BFD?

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Created Jun 27, 2016 21:56:40

I have discovered that if I do not specify a single-hop TTL at all, it will let me specify a multi-hop TTL of 255.

Earlier I was trying to cure a problem where sessions 2-hops away would not come up due to the received TTL matching the the single-hop value, so I changed it to the RFC specified value of 255, which moved my problem from two hops away to one hop. Specifying only a multi-hop TTL of 255, seems to cure both issues.

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Created Jun 28, 2016 00:35:06

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