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Cisco Bug: CSCvr97992 - NCS5500 pushes incorrect mpls label stack with ECMP and "hw- module fib mpls ldp lsr-optimized"

Last Modified

Jun 18, 2020

Products (3)

  • Cisco Network Convergence System 5500 Series
  • Cisco Network Convergence System 5508
  • Cisco IOS XR Software

Known Affected Releases

6.5.3.LC

Description (partial)

Symptom:
MPLS traffic loss.

Conditions:
This will happen when an NCS5500 has the command "hw-module fib mpls ldp lsr-optimized" configured and it has 2 ECMP paths for an incoming label:

RP/0/RP0/CPU0:router#sh mpls forwarding labels 64008
Tue Nov  5 03:29:52.718 IST
Local  Outgoing    Prefix             Outgoing     Next Hop        Bytes       
Label  Label       or ID              Interface                    Switched    
------ ----------- ------------------ ------------ --------------- ------------
64008  24031       10.222.253.62/32   BE7          10.222.71.17    0           
       64029       10.222.253.62/32   BE8          10.222.77.10    0           
RP/0/RP0/CPU0:router#

In the example above, when an mpls frame is received with top label 64008 (the local label), the NCS5500 will forward the packet to the next hop with an incorrect label stack which means that traffic will get dropped at some point as some next hop won't understand the incoming mpls label.
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