Ethan Banks On productivity. CCIE-LAB List – Best of 2/11/2008 – L2TP Topology Loops + Display K-Values + Proctors Won’t Break Your Lab + Loopback Interface Command + service unsupported-transceiver


Some of the comments that interested me from CCIE-LAB mailing list over the last 2 or 3 weeks:

  • While running dot1q tunnels with L2TP running through them, you might see errors like the following:
    • ETHCNTR-3-LOOP_BACK_DETECTED: Loop-back detected on
    • PM-4-ERR_DISABLE: l2ptguard error detected

    This may indicate a topology loop in your L2TP infrastructure. Deal with it by applying the “l2tunnel-protocol stp” command on the appropriate interfaces.

  • show ip protocols | i EIGRP metric” will display the K values set on your IOS device running EIGRP.
  • Erik Vangrunderbeek, a lab proctor in Brussels, stated “Proctors do NOT interfere (modify config, delete config, etc.) in any way during the exam. We encourage people who claim differently, to address this to the proctor during the time of your lab exam AND to officially address this by opening a case on”
  • The “loopback” interface command allows you to loop a lot of different interface types, including ethernet. I only ever thought of looping an interface in the context of serial interfaces, usually when troubleshooting a WAN circuit with a carrier. From the Doc CD regarding using the loopback command on an ethernet interface: “On the MCI and MEC Ethernet cards, the interface receives back every packet it sends when the loopback command is enabled. Loopback operation has the additional effect of disconnecting network server functionality from the network.”
  • Check out undocumented command “service unsupported-transceiver” that allows you to use non-Cisco SFPs or GBICs in a switch. I’m not sure what switches or IOS revisions might have this command hidden away, but it’s there on a 3750G running C3750-ADVIPSERVICESK9-M, Version 12.2(37)SE1.

    CAT1(config)#service unsupported-transceiver
    Warning: When Cisco determines that a fault or defect can be traced to the use of third-party transceivers installed by a customer or reseller, then, at Cisco’s discretion, Cisco may withhold support under warranty or a Cisco support program. In the course of providing support for a Cisco networking product Cisco may require that the end user install Cisco transceivers if Cisco determines that removing third-party parts will assist Cisco in diagnosing the cause of a support issue.


By Ethan Banks
Ethan Banks On productivity.

You probably know Ethan Banks because he writes & podcasts about IT. For example, he co-authored "Computer Networks Problems & Solutions" with Russ White.

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