The rx form enables strict Unicast Reverse Path Forwarding on an interface. For every received packet, the router checks the forwarding information base for the source address and verifies that the best return path uses the same interface on which the packet arrived. If that ingress-interface check fails, the packet is rejected, helping mitigate forged or spoofed source addresses. Therefore, B is correct. The any form enables loose mode, which accepts the source when any route to that address exists, regardless of the ingress interface. both and tx are not valid modes for the Cisco IOS XE command shown. Strict mode is suitable where routing is symmetric; loose mode is normally preferred where legitimate asymmetric routing could cause strict-mode checks to discard valid traffic. Cisco IOS XE Unicast RPF configuration

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