4 Questions
Explain the process a router follows when receiving a packet from a host and searching for a matching destination network entry in the routing table.
The router decapsulates the packet, searches the routing table for a matching destination network entry, uses the static route if available, applies the default static route if configured, drops the packet and sends an ICMP message back if no match is found, and encapsulates the packet and forwards it out of the appropriate interface if a routing table entry is matched.
What does a router do if the destination IP address does not match a specific route to the destination network?
The router will use the default static route (if configured) to identify the next hop IP address or exit interface.
What action does a router take if the destination IP address does not match a route table entry?
The router will drop the packet and send an ICMP message back to the source.
Describe the process a router follows when the packet reaches the destination network and the destination IP address matches a directly connected Ethernet interface.
The router searches the ARP table for the Layer 2 MAC address of the destination IP address, sends an ARP request out of the Ethernet interface if no ARP entry exists, receives an ARP reply from the destination host containing its MAC address, and then encapsulates the packet in a new frame.
Study Notes
Router Packet Processing
- A router receives a packet from a host and checks the destination IP address against the routing table entries.
- The router searches for a matching destination network entry in the routing table.
Handling Unknown Destinations
- If the destination IP address does not match a specific route to the destination network, the router checks for a default route (or gateway of last resort).
- If a default route is configured, the router forwards the packet to the default route.
- If no default route is configured, the router discards the packet and sends an ICMP Destination Unreachable message to the source IP address.
Packet Delivery
- When the packet reaches the destination network, the router checks if the destination IP address matches a directly connected Ethernet interface.
- If a match is found, the router forwards the packet to the directly connected Ethernet interface for delivery to the destination host.
Test your knowledge about the process of packet forwarding and routing in computer networks. Learn how routers handle incoming packets and make decisions based on routing tables.
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