LDP flaps after Junos 14.1 upgrade and interoperation with H3C router

Summary:

LDP (Label Distribution Protocol) is seen flapping after upgrading to Junos 14.1+. This occurs only when peering with H3C routers, not Cisco or Junos equipment.

This article explains how ELC (Entropy Label Capability) may be the cause. It can be disabled to resolve this issue.

Symptoms:

After upgrading the MX to Junos 14.1, LDP flaps with the H3C router. The H3C router reports that the LDP session is going up and down.

The logs on the MX report the LDP TCP session is reset by the peer device (H3C).

Aug  4 10:34:49.448715 Read from 172.16.1.18 failed: Connection reset by peer
Aug  4 10:34:49.448726 Error reading from 172.16.1.18: Connection reset by peer
Aug  4 10:34:49.448857 Session 172.16.1.18 GR state Operational -> Nonexistent
...
Aug  4 10:34:49.448867 Session 172.16.1.18 state Operational -> Closing
Aug  4 10:34:49.448875 LDP session 172.16.1.18 is down, reason: connection reset
Aug  4 10:34:49.448911 RPD_LDP_SESSIONDOWN: LDP session 172.16.1.18 is down, reason: connection reset

Cause:

Junos 14.1 and later enables by default the advertisement of ELC in LDP and RSVP. As a result, the first mapping message includes a ELC TLV (type length variable). This TLV length is "0". If the peer device does not support this feature, it may consider the TLV length as invalid and then reset the TCP session due to "bad message length".

Example:

Confirmed that the peering is using ELC:

labroot@lab1# run show ldp overview | match egress 
  Egress FEC capabilities enabled: entropy-label-capability <-- entropy label capability enabled

Notice the difference in LDP packets between 13.3 and 14.1.

Junos 13.3 traceoptions output:
The advertisement of ELC was not supported by Junos. There was no ELC (0X206) TLV in the label mapping message.

Aug  4 10:34:49.233946 LDP sent TCP PDU 172.16.1.115 -> 172.16.1.18 (none)
Aug  4 10:34:49.233954 ver 1, pkt len 2982, PDU len 2978, ID 172.16.1.115:0
Aug  4 10:34:49.233962   Msg LabelMap (0x400), len 28, ID 640022
Aug  4 10:34:49.233969     TLV FEC (0x100), U: 0, F: 0, len 8
Aug  4 10:34:49.233978       Prefix, family 1, 172.16.1.115/32
Aug  4 10:34:49.233985     TLV Label (0x200), U: 0, F: 0, len 4
Aug  4 10:34:49.233991       Label 3
Aug  4 10:34:49.234002     TLV ELC (0x206), U: 1, F: 1, len 0 <-- TLV Length is "0"

Junos 14.1 traceoptions output:

Aug  9 01:54:46.076097 LDP sent TCP PDU 50.0.0.13 -> 172.0.0.1 (none)
Aug  9 01:54:46.076117 ver 1, pkt len 38, PDU len 34, ID 50.0.0.13:0
Aug  9 01:54:46.076123   Msg LabelMap (0x400), len 24, ID 4
Aug  9 01:54:46.076128     TLV FEC (0x100), U: 0, F: 0, len 8
Aug  9 01:54:46.077172     TLV Label (0x200), U: 0, F: 0, len 4

Solution:

Disable ELP with the no-load-balance-label-capability knob:

user@router# set forwarding-options no-load-balance-label-capability

For more information, refer to the RFC: The Use of Entropy Labels in MPLS Forwarding

Modification History:

2019-07-31: Updated for clarity.
2017-03-22: Author corrected version number. Editor deleted reference to JTAC lab server.

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