Routing Concepts and How the Routing Table Works

That table is the trust hierarchy of routing. When multiple sources provide a route to the same destination, the router picks the source with the lowest administrative distance. It's not about bandwidth, not about hop count — it's about which source the router trusts more to be correct.

#What a Router Does, Step by Step#link

1. Receive the frame on an interface — check FCS, strip the frame header
2. Read the destination IP from the packet header
3. Look up the destination IP in the routing table — find the longest prefix match
4. Identify the next-hop IP and the egress interface
5. Resolve the next-hop's MAC — ARP if unknown
6. Rewrite the frame header — new source MAC (this router), new destination MAC (next hop)
7. Decrement TTL — drop if it hits zero
8. Recalculate the IP header checksum
9. Forward the frame out the egress interface

That's the whole job. Nine steps, tens of nanoseconds per packet on a modern ASIC. Every routing decision comes down to the lookup in step 3 — everything else is mechanical.

#The Routing Table Lookup#link

#Connected and Local Routes#link

When you assign `ip address 10.10.10.1 255.255.255.0` to Gi0/1, the router installs two routes: a Connected route for `10.10.10.0/24` and a Local route for `10.10.10.1/32`. The Local route is what allows the router to receive traffic destined for its own interface IP — without it, the router would drop packets addressed to itself.

#Check Your Understanding#link