You have the LDP signaled VPLS topology as shown in the exhibit. CE-B at Site-2 is multihomed to both PE-2 and PE-3. In this scenario, where would you configure loop prevention?
The correct answer is A. In LDP signaled VPLS, when you have a multihomed CE to two different PEs you can configure a neighbor and a backup neighbor in remotes PEs. In this case PE-1. If the VPLS is signaled with BGP then you have to configure the preference in PE-2 and PE-3. The spanning tree option mentioned in other comments needs the configuration of spanning tree in the CE and you also need to configure a l2control instance in PEs attached to the CE. So B is not correct.
What you are describing is redundancy
https://supportportal.juniper.net/s/article/MX-Example-Configuring-LDP-based-VPLS-using-primary-backup-for-redundancy?language=en_US
In our case it seems automatic
Remote PE routers in the VPLS network need to determine which of the multihomed PE routers should forward traffic to reach the CE device. To make this determination, remote PE routers use the VPLS path-selection process to select one of the multihomed PE routers based on its NLRI advertisement. Because remote PE routers pick only one of the NLRI advertisements, it establishes a pseudowire to only one of the multihomed PE routers,
https://www.juniper.net/documentation/us/en/software/junos/vpn-l2/topics/concept/vpn-vpls-multihoming.html
Remote PE routers in the VPLS network need to determine which of the multihomed PE routers should forward traffic to reach the CE device. To make this determination, remote PE routers use the VPLS path-selection process to select one of the multihomed PE routers based on its NLRI advertisement. Because remote PE routers pick only one of the NLRI advertisements, it establishes a pseudowire to only one of the multihomed PE routers, the PE router that originated the winning advertisement. This prevents multiple paths from being created between sites in the network, preventing the formation of Layer 2 loops. If the selected PE router fails, all PE routers in the network automatically switch to the backup PE router and establish new pseudowires to it.
https://www.juniper.net/documentation/us/en/software/junos/vpn-l2/topics/topic-map/vpls-bgp-multihoming.html
BEST PRACTICE: To prevent the formation of Layer 2 loops between the CE devices and the multihomed PE routers, we recommend that you employ the Spanning Tree Protocol (STP) on your CE devices. Layer 2 loops can form due to incorrect configuration. Temporary Layer 2 loops can also form during convergence after a change in the network topology.
https://www.juniper.net/documentation/us/en/software/junos/vpn-l2/topics/topic-map/vpls-bgp-multihoming.html
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