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Cisco Bug: CSCvs02859 - NCS5500 forwards mpls label over unlabelled tunnel-te when loadbalancing with labelled tunnel

Last Modified

Jun 23, 2020

Products (3)

  • Cisco ASR 9000 Series Aggregation Services Routers
  • Cisco Network Convergence System 5508
  • Cisco IOS XR Software

Known Affected Releases

7.2.1.BASE

Description (partial)

Release-note

Symptom:
An NCS5500 incorrectly forwards MPLS frames over a tunnel-te interface while this tunnel-te interface is supposed to be "unlabelled" as it's not configured under LDP
Drops due to unknown mpls label at the tail-end router of a tunnel-te. For instannce, on an NCS5500, these would be detected by having the "RxTrapMplsUnknownLabel" counter increment in "show contr npu stats traps-all".  But there would be different counters on different platforms to indicate that mpls frames with unknown mpls label are getting dropped.

MID node TE with mix label and unlabelled path not working.

Conditions:
An NCS5000 is the MPLS TE headend of 2 TE tunnels and some IGP prefixes are loadbalanced between these 2 tunnel-te interfaces:
- One of these 2 tunnel-te must have LDP enabled (the tunnel-te interface is configured under "mpls ldp")
- The other tunnel-te interface must not be configured under LDP.

The mpls forwarding entry for that IGP prefix would look like:

P/0/RP0/CPU0:router#sh mpls forwarding labels 32024
Wed Oct 23 17:15:56.258 CEST
Local  Outgoing    Prefix             Outgoing     Next Hop        Bytes       
Label  Label       or ID              Interface                    Switched    
------ ----------- ------------------ ------------ --------------- ------------
32024  32011       192.168.2.0/24     tt13         192.168.1.11    0           
       Unlabelled  192.168.2.0/24     tt28         192.168.1.12    0           
RP/0/RP0/CPU0:router#

We can see that there are 2 paths, both paths over a tunnel-te interface, one with an outgoing label and one "unlabelled" path.
The same problem would problem occur if there's more than 2 paths but with some labelled and unlabelled paths.

TE mid node with ecmp path with mix label and unlabelled.
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