Redundant Paths: Two Routers, One Destination

The scenario

PC-A (10.0.1.10) and Server (10.0.2.10, a web server on TCP port 80) are on two networks. Two routers are plugged into both: Router-Primary (10.0.1.1 and 10.0.2.1) and Router-Backup (10.0.1.2 and 10.0.2.2). PC-A's default gateway is 10.0.1.1 and Server's is 10.0.2.1, so all traffic goes through Router-Primary and Router-Backup sits idle. You will unplug Router-Primary from PC-A's network and see that nothing moves over by itself: PC-A keeps sending to a gateway that no longer answers, and Server keeps sending replies to the router that lost its way back. OSPF is switched off on both routers; they do not need it, because each is connected to both networks.

What you will work out

Understand how default gateways choose the path in each direction, and why a second router is not failover on its own.

Why this matters

Two routers do not make a network redundant unless the hosts can use the second one. Hosts send to one configured gateway, and replies follow the other side's gateway, which is why a half-finished failover shows up as requests arriving with no answers. First-hop redundancy protocols (VRRP, HSRP, GLBP) or routing on the hosts themselves are how production networks close that gap, and asymmetric paths are why stateful firewalls need both directions to pass through them.

Running it

Redundant Paths: Two Routers, One Destination opens on a canvas, already wired and addressed. Trace a packet through it hop by hop, or run the same topology live on real Linux network namespaces. How the two modes differ.