Support Support Downloads Knowledge Base Juniper Support Portal Community

Knowledge Base

Search our Knowledge Base sites to find answers to your questions.

Ask All Knowledge Base Sites All Knowledge Base Sites JunosE Defect (KA)Knowledge BaseSecurity AdvisoriesTechnical BulletinsTechnotes Sign in to display secure content and recently viewed articles

LDP flaps after Junos 14.1 upgrade and interoperation with H3C router



Article ID: KB31088 KB Last Updated: 02 Aug 2019Version: 3.0

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.


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

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".


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 -> (none)
Aug  4 10:34:49.233954 ver 1, pkt len 2982, PDU len 2978, ID
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,
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 -> (none)
Aug  9 01:54:46.076117 ver 1, pkt len 38, PDU len 34, ID
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

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.

Related Links

Comment on this article > Affected Products Browse the Knowledge Base for more articles related to these product categories. Select a category to begin.

Getting Up and Running with Junos

Getting Up and Running with Junos Security Alerts and Vulnerabilities Product Alerts and Software Release Notices Problem Report (PR) Search Tool EOL Notices and Bulletins JTAC User Guide Customer Care User Guide Pathfinder SRX High Availability Configurator SRX VPN Configurator Training Courses and Videos End User Licence Agreement Global Search