Lab: static and default routes across three routers

Aim and what you need

The aim is to understand exactly how a router chooses where to send a packet, by building every route yourself. Static routing is still common for small sites, stub branches and default routes to an internet provider, and it is the foundation for understanding dynamic protocols such as OSPF.

• Packet Tracer or your own lab: three routers (1941, 2911 or 4321), two switches and two PCs.
• Copper cables between routers (Gigabit Ethernet keeps it simple).

Topology and addressing

A /30 gives exactly two usable addresses, ideal for point-to-point links. Check your maths with the VLSM calculator.

Steps

1. Address all router interfaces from the table and bring them up with no shutdown. Give PCs the .1 of their LAN as gateway.
2. Check each router can ping its directly connected neighbour before adding any routes.
3. Add a default route on R1 and R3, and two specific routes on R2.
4. Add a floating static route on R1 that points at a backup path (here a second link, or simply a test next hop) with a higher administrative distance (AD).

! R1
ip route 0.0.0.0 0.0.0.0 10.0.12.2
! R2
ip route 192.168.1.0 255.255.255.0 10.0.12.1
ip route 192.168.3.0 255.255.255.0 10.0.23.2
! R3
ip route 0.0.0.0 0.0.0.0 10.0.23.1

For the floating static route, add a second link between R1 G0/2 and R3 G0/2 (use 2911 routers, which have three Gigabit ports) using 10.0.13.0/30 (R1 .1, R3 .2). Then on R1 and R3 add a backup route with AD 10, so it only appears if the main route disappears.

! R1
ip route 192.168.3.0 255.255.255.0 10.0.12.2
ip route 192.168.3.0 255.255.255.0 10.0.13.2 10
! R3
ip route 192.168.1.0 255.255.255.0 10.0.23.1
ip route 192.168.1.0 255.255.255.0 10.0.13.1 10

Verify

The routing table on each router should show connected networks (C), local addresses (L) and your static routes (S, with S* for the default). Success is PC-A reaching PC-C, with a traceroute that passes R1, R2 and R3.

show ip route
show ip route static
show ip interface brief
ping 192.168.3.10 source g0/0
traceroute 192.168.3.10

Now test the floating route: shut down R1 G0/1. On R1, show ip route static should now list the 10.0.13.2 route, and the trace from PC-A should go straight to R3. Bring G0/1 back up and confirm the original route returns.

tracert 192.168.3.10

Troubleshooting and clean-up

• Ping fails one way only: every router needs a route back. Most static routing faults are a missing return route.
• Route missing from the table: a static route with a next hop is only installed if the router can reach that next hop. Check the exit interface is up.
• Wrong mask: ip route 192.168.3.0 255.255.0.0 silently matches far more than you meant.
• Floating route never appears: in Packet Tracer an interface only goes down if the cable or port really fails, so use shutdown on the interface to test.

To clean up, remove routes with no ip route ... or save a copy of the file before moving on. The same topology is reused in the OSPF lab, so keep the addressed but unrouted version too.