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CCNA 3.1.c: Frame Rewrite

Overview:

Frame rewrite is a crucial step in routing a packet through a router.  Frame rewrite is determining how to construct the Layer 2 frame header to allow the packet to be properly encapsulated toward its next hop and forward the packet out the correct interface.

 

Study Notes:

  • Here is the process:
    • Packet arrives at a router and the router makes a forwarding decision
    • The router then rewrites the Layer 2 frame, encapsulating the packet appropriately based on the exit interface and sends the packet toward the next hop
    • The header is not changed significantly
      • only the TTL and checksum are modified (IPv4)
      • only the hop count is decremented (IPv6)
    • Process switching is very inefficient in that:
      • The routing table lookup is slow and might need recursive iterations until the directly attached next hop and egress interface are identified
      • The next hop must then be translated in ARP and the frame header constructed
      • Only then can the packet be encapsulated and foarwarded
      • With each packet, the process repeats from the beginning

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