The MC-EP mechanisms are built to minimize the possibility of loops. It is possible that human error could create loops through the VPLS service. One way to prevent loops is to enable the MAC move feature in the gateway PEs (PE3, PE3', PE1, and PE2).
An MC-EP passive mode can also be used on the second PE pair, PE1 and PE2, as a second layer of protection to prevent any loops from occurring if the operator introduces operational errors on the MC-EP PE3, PE3 pair. An example is shown in Figure: MC-EP in passive mode.
When in passive mode, the MC-EP peers stay dormant as long as one active pseudowire is signaled from the remote end. If more than one pseudowire belonging to the passive MC-EP becomes active, the PE1 and PE2 pair applies the MC-EP selection algorithm to select the best choice and blocks all others. No signaling is sent to the remote pair to avoid flip-flop behavior. A trap is generated each time MC-EP in passive mode activates. Every occurrence of this kind of trap should be analyzed by the operator as it is an indication of possible misconfiguration on the remote (active) MC-EP peering.
For the MC-EP passive mode to work, the pseudowire status signaling for active/standby pseudowires should be enabled. This requires the following CLI configurations:
For the remote MC-EP PE3, PE3 pair:
config>service>vpls>endpoint# no suppress-standby-signaling
When MC-EP passive mode is enabled on the PE1 and PE2 pair, the following command is always enabled internally, regardless of the actual configuration:
config>service>vpls>endpoint no ignore-standby-signaling