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The BFD Session Cannot Go Up When NE40 IConnected to a Non-Huawei Device receiving BFD packets with TTL value 255

Publication Date:  2013-01-08 Views:  65 Downloads:  0
Issue Description

Networking:

NE40E—MGW from Z supplier

When the Layer 3 interface of an NE40 is connected to an MGW from Z supplier, the link works properly. However, when a BFD session is set up between the Layer 2 LPU of the NE40 and the MGW, the BFD session cannot go Up. Because the NE40 uses the Layer LPU, an outbound interface cannot be specified.

Handling Process

1.         The on-site Huawei technical support personnel check the configuration on the NE40 and find no fault. Multi-hop BFD is configured on the NE40. An example configuration is shown as follows.

bfd ssw_signal-1 bind peer-ip 10.0.163.162 vpn-instance cqunicom_signal source-ip 10.0.163.161 auto

min-tx-interval 30

min-rx-interval 30

commit

#

bfd ssw_signal-2 bind peer-ip 10.0.163.170 vpn-instance cqunicom_signal source-ip 10.0.163.169 auto

min-tx-interval 30

min-rx-interval 30

commit

2.         Then they enable the debugging function of BFD on the NE40 (If the interval at which BFD packets are sent is very short, the time for debugging is also very short). They find the following error messages on the NE40.

19:08287: BFD SCM : Get Out Forwarding Information For Session(Discr:0)

   Peer : 10.0.163.170 VrfIndex : 0

   Got OutIf: NULL Nexthop: 0.0.0.0

*1.1458372156 CQCQ-NFHY-N40-AR1 BFD/7/Error:

00:03639: [BFD]: BFD_ADP_ProcRefreshTeFlagTimer: Get IfIndex failed, PeerIp=0x000000

*1.1458372156 CQCQ-NFHY-N40-AR1 BFD/7/Proc:

3.         They check whether the BFD packets are discarded at the bottom layer and find that the BFD packets that the MGW sends carry the TTL value of 255. The NE40 considers that the BFD packets with the TTL value 255 are sent for single-hop BFD session negotiation. As a result, the NE40 discards the BFD packets that the MGW sends, which causes the failure in session negotiation.

Root Cause

An NE40 supports different TTL values carried in BFD packets transmitted for single-hop BFD session negotiation and for multi-hop BFD session negotiation. When the NE40 is configured with multi-hop BFD, the TTL value in the BFD packets is not 255. When the NE40 is configured not to be bound to multi-hop BFD on the outbound interface, the TTL value in the BFD packets is not 255. After the NE40 receives the BFD packets from the MGW, the NE40 decide whether the received BFD packets are for single-hop BFD session negotiation or multi-hop BFD session negotiation based on the carried TTL value. If the TTL value in the received BFD packets is different from the locally configured TTL value, the NE40 silently discards the BFD packets.

When the NE40 receives the BFD packets with TTL value 255 from the MGW, the BFD packets are considered to be sent for single-hop BFD session negotiation, and therefore are discarded. That is why the BFD session negotiation fails.

Solution

Replace the NE40 with an NE40E that can support the configuration in this scenario.

END