To effectively troubleshoot routing issues, it is helpful to review how routing works and the related information tables.

  1. Receive a packet to send to a destination IP address.

  2. Check the routing table and determine the IP address of the next hop.

  3. Determine which of your network interfaces can reach it.

  4. Get a MAC address of that next hop (via ARP).

  5. Build an L2 frame.

  6. Send the frame out the interface.

So to do routing, you need:

  • An interface table (with interface IP addresses and netmasks)

  • A routing table

  • An ARP table