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