Comprehensive and Detailed 150 to 250 words of Explanation From: In a routed environment like the one shown in the exhibit, traffic forwarding involves a constant interaction between Layer 3 (Network) and Layer 2 (Data Link) addressing. When User A generates a packet destined for User B, the source and destination IP addresses remain static throughout the entire journey across the network (assuming no Network Address Translation is performed). However, the Layer 2 Ethernet headers must be rewritten at every hop because MAC addresses have only local significance on a physical segment.

As Router R1 receives the packet from User A, it performs a lookup in its Forwarding Information Base (FIB) and identifies that the path to User B requires forwarding the packet to Router R2. R1 decapsulates the incoming frame, stripping away the original Ethernet header that contained User A ' s source MAC and R1 ' s own destination MAC. To forward the packet to the next hop, R1 creates a new Ethernet header. The source MAC address becomes the MAC address of R1’s egress interface, and the destination MAC address is replaced with the MAC address of Router R2 . R1 cannot use User B ' s MAC address at this point because User B is not on a directly connected segment. This hop-by-hop MAC address replacement is essential for the Packet Forwarding Engine to successfully deliver the frame to the next Layer 3 device in the path. Reference: Networking Fundamentals, Packet Forwarding, Layer 2 and Layer 3 Addressing.