Attempting to run LinkTrace message over EVPN with Up direction.

markk2316 · ‎12-04-2024

Hello,
I'm using an ASR9K and i've configured a bridge domain attached to an EVI that is connected to another device.
The configuration of the L2VPN Bridge Group is as follows.

RP/0/RSP0/CPU0:cisco-asr9k(config)#do show run l2vpn bridge group Mark_Kazakov
Wed Dec  4 16:01:02.068 EET
l2vpn
 bridge group Mark_Kazakov
  bridge-domain Mark
   interface HundredGigE0/0/0/1.1394
   !
   evi 100
   !
  !
 !
!

I'll save the shows for the service itself given that the CCM session is Up on the local MEP and the direction is noticed as Up.

RP/0/RSP0/CPU0:cisco-asr9k(config)#do show ethernet cfm peer meps domain Mark_Level_5  detail 
Wed Dec  4 16:08:57.236 EET
Domain Mark_Level_5 (level 5), Service Mark_BBB
Up MEP on HundredGigE0/0/0/1.1394 MEP-ID 1531
================================================================================
Peer MEP-ID 1390, MAC 8440.761b.e134
   CFM state: Ok, for 00:32:43
   Received CCM handling offloaded to software
   Port state: Up
   CCMs received: 2124
     Out-of-sequence:             0
     Remote Defect received:      0
     Wrong level:                 0
     Cross-connect (wrong MAID):  0
     Wrong interval:              0
     Loop (our MAC received):     0
     Config (our ID received):    0
   Last CCM received:
     Level: 5, Version: 0, Interval: 1s
     Sequence number: 0, MEP-ID: 1390
     MAID: NULL, String: Bravo_Foxtrot
     Port status: Up, Interface status: Up

Now the issue is that when i attempt to send a LinkTrace Message (LTM) to the remote MEP that id with MEP-ID 1390. I'm noticing via wireshark that the LTM itself isn't sent. However, trying to send an LoopBack Message (LBM) via the ping ethernet cfm results in a successful request being generated and thus a reply as well.

LTM request.

RP/0/RSP0/CPU0:cisco-asr9k(config)#do traceroute ethernet cfm domain Mark_Level_5 service Mark_BBB mep-id 1390 source interface hundredGigE 0/0/0/1.1394
Wed Dec  4 16:11:31.654 EET
Type escape sequence to return to prompt.

Traceroutes in domain Mark_Level_5 (level 5), service Mark_BBB
Source: MEP-ID 1531, interface HundredGigE0/0/0/1.1394
================================================================================
Traceroute at 2024-12-04 16:11:31 to 8440.761b.e134,
TTL 64, Trans ID 1952015588:

  Running (7s) ...

Hop Hostname/Last            Ingress MAC/name       Egress MAC/Name        Relay
--- ------------------------ ---------------------- ---------------------- -----
  1 cisco-asr9k              40b5.c13b.029c [Ok]                           FDB  
      0000-40b5.c13b.029c    Hu0/0/0/1.1394                                     
Replies dropped: 0

No reply received, Wireshark shows no LTM sent.

Attempting to send LBM.

RP/0/RSP0/CPU0:cisco-asr9k(config)#do ping ethernet cfm domain Mark_Level_5 service Mark_BBB mep-id 1390 source interface  hundredGigE 0/0/0/1.1394 
Wed Dec  4 16:11:57.406 EET
Type escape sequence to abort.
Sending 5 CFM Loopbacks, timeout is 2 seconds -
Domain Mark_Level_5 (level 5), Service Mark_BBB
Source: MEP ID 1531, interface HundredGigE0/0/0/1.1394
Target: 8440.761b.e134 (MEP ID 1390):
  Running (5s) ...
Success rate is 100.0 percent (5/5), round-trip min/avg/max = 1/1/2 ms
Out-of-sequence: 0.0 percent (0/5)
Bad data: 0.0 percent (0/5)
Received packet rate: 1.3 pps

LBM successful, Wireshark shows the LBM sent from the Cisco.

I've also attempted to run Xconnect instead of Bridge-Domain and noticed that the LTM are generated in the case of Xconnect. This caused me to suspect that LTM might not be supported over Bridge-Domain running on an EVI instance ?

Is there any special config regarding the LinkTrace that should allow me to send the LTM over the EVI ?

Any help would be greatly appreciated,
Thank you in advance,
Mark Kazakov.

AshSe · ‎01-09-2025

Hello @markk2316

The issue you're encountering with the inability to send LinkTrace Messages (LTM) over a Bridge-Domain attached to an EVI instance on the ASR9K is likely related to the platform's support for Ethernet CFM (Connectivity Fault Management) operations in this specific configuration. Let's break this down and address your concerns:

Observations:

CFM LBM (Loopback Message) Works:
- The ping ethernet cfm command successfully sends LBMs and receives replies, indicating that the CFM configuration is functional and the MEPs are properly communicating.
CFM LTM (LinkTrace Message) Fails:
- The traceroute ethernet cfm command does not generate LTMs, and Wireshark confirms that no LTM packets are sent.
Xconnect Works for LTM:
- When using an Xconnect configuration instead of a Bridge-Domain with EVI, LTMs are successfully generated and sent.

Analysis:

The behavior you're observing suggests that LTMs may not be fully supported or operational in the context of a Bridge-Domain attached to an EVI instance. This is because the EVI (Ethernet Virtual Instance) is a specific technology used for Layer 2 extension over an MPLS or IP core, and it may impose certain limitations on CFM operations.

CFM operations like LTM rely on the ability to trace the path through the network, which requires the platform to generate and process specific CFM messages. In the case of an EVI, the encapsulation and forwarding mechanisms might not fully support the generation or handling of LTMs in a Bridge-Domain context.

Recommendations:

Verify Platform and Software Support:
- Check the Cisco ASR9K documentation and release notes for your specific software version to confirm whether LTMs are supported in a Bridge-Domain with EVI. Look for any caveats or limitations related to CFM and EVI.
Enable CFM on the EVI Interface:
- Ensure that CFM is explicitly enabled on the EVI interface. While this is typically not required for basic CFM operations, some configurations may require additional commands to enable full CFM functionality.
Test with a Different Configuration:
- If possible, test the same CFM configuration on a Bridge-Domain without EVI to confirm that LTMs work as expected. This will help isolate whether the issue is specific to EVI.
Use Xconnect if Necessary:
- If LTMs are critical for your use case and the limitation is confirmed to be with the Bridge-Domain + EVI configuration, consider using an Xconnect configuration instead, as you have already observed that LTMs work in this scenario.
Contact Cisco TAC:
- If the issue persists and you cannot find documentation confirming the limitation, consider opening a case with Cisco TAC. They can provide definitive answers regarding the platform's capabilities and any potential workarounds.

Additional Notes:

CFM and EVI Limitations:
- EVI is designed for Layer 2 extension and may not fully support all CFM features, especially those that require interaction with the underlying transport network (e.g., MPLS or IP). LTMs, which trace the path through the network, may be affected by this limitation.
Debugging:
- Use the following debug commands to gather more information about CFM operations:
```
debug ethernet cfm packets
debug ethernet cfm errors
debug ethernet cfm events
```
  These commands can help identify why LTMs are not being generated.
Software Version:
- Ensure that your ASR9K is running a software version that supports the features you are using. If you're running an older version, consider upgrading to a more recent release.

Conclusion:

The inability to send LTMs in a Bridge-Domain with EVI is likely due to a platform or configuration limitation. Verify the support for LTMs in this specific configuration, and consider using Xconnect if LTMs are critical for your deployment. If needed, escalate the issue to Cisco TAC for further assistance.

Hope This Helps!!!

AshSe

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