4A0-105 practice question 6 of 25

What is the result of initiating an oam mac-ping to an unknown unicast MAC address in a VPLS instance?

Answer options

A: The oam mac-ping will fail. The MAC address must be known in the local FDB to be successful.

B: The request will be flooded and the only response will come from the PE node that has learned the specified MAC address on a local SAP. Correct answer

C: The request will be flooded and the CE device with the corresponding MAC will reply.

D: All nodes will reply because the oam mac-ping is flooded.

Correct Answer:

B. The request will be flooded and the only response will come from the PE node that has learned the specified MAC address on a local SAP.

Explanation

The oam mac-ping is a VPLS OAM tool used to verify MAC address reachability across the service. When initiated for a MAC address that is not present in the originating PE's Forwarding Database (FDB), the request is treated as an unknown unicast frame. Consequently, the PE floods the mac-ping request to all other PEs participating in the VPLS instance. Upon receiving the flooded request, each PE checks its own FDB. Only the PE that has learned the target MAC address on one of its local Service Access Points (SAPs) will generate and send a reply. Other PEs that receive the flooded request but do not have the MAC in their FDB will discard it.

Why Incorrect

A. The mac-ping does not necessarily fail. Flooding is the mechanism used to discover a MAC address that is unknown to the originating PE.
C. The mac-ping is a PE-to-PE OAM tool. Customer Edge (CE) devices are not designed to process or respond to these specific OAM frames.
D. Although the request is flooded to all PEs, only the single PE that has the target MAC address learned on a local SAP will reply.

References

1. Nokia 7750 SR OS Services Guide, Release 21.10.R1, "VPLS OAM", Section: "MAC Ping and MAC Trace".

The guide states: "When a MAC ping is initiated for a MAC address that is not in the FDB of the originating PE, the MAC ping is flooded to all PEs in the VPLS service. When a PE in the VPLS service receives the MAC ping and the MAC address is present in its FDB, the PE responds to the MAC ping." This directly supports that the request is flooded and the PE with the learned MAC responds.

2. Nokia 7450 ESS, 7750 SR, 7950 XRS, and VSR OAM and Diagnostics Guide, Release 21.10.R1, "oam", Command Reference: "mac-ping".

The command description specifies: "If the destination MAC is not known by the local node, the MAC ping request is flooded throughout the VPLS domain. A reply is generated by the node that has the destination MAC in its FDB." This confirms the behavior described in the correct answer.

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