How a Router Chooses a Route: Forwarding Tutorial

It’s common knowledge that once a packet arrives at a router interface, the router forwards it toward its destination. But how does the router actually decide which route is the best one to use? This tutorial walks through exactly how a router makes that forwarding decision.

To make that decision, the router first has to build a table holding all of its best routes. Then, when a packet actually arrives, all it has to do is check that table for the best match. That table is the routing table.

So what does a routing table actually contain? Here’s a look via the show ip route command, which displays the local router’s routing table:

R1# show ip route
--output omitted--
D    192.168.20.0/26 [90/24513456] via 10.10.10.1
R    192.168.20.0/24 [120/5] via 10.10.10.2
O    192.168.0.0/19 [110/219414] via 10.10.10.13
D    192.168.20.0/27 [90/4123710] via 10.10.10.12
D    192.168.20.0/25 [90/14464211] via 10.10.10.11
S*   0.0.0.0/0 [1/0] via 10.10.10.14

A few things worth noting about this routing table:

1. There are several entries for 192.168.20.0 with different prefix lengths (subnet masks) — /26, /24, /19, and so on. That’s completely normal: routes with different prefix lengths are treated as different routes, even when the network address looks the same.
2. These 192.168.20.0 routes were learned through different routing protocols, shown by the letter at the start of each line — “D” for EIGRP, “O” for OSPF, “R” for RIP.
3. The next hop for each entry is the IP address that follows “via” — here, 10.10.10.1, 10.10.10.2, 10.10.10.13, and so on.