Default route loop: packets bounce until the TTL runs out

The scenario

PC (10.0.1.10) sits behind Router-A. Router-B connects to Server's network (10.0.3.0/24) and, over 10.0.4.0/30, to Core, the router in front of the company Intranet server (10.0.5.10). Router-A's default route points to Router-B (10.0.2.2), which is right. Router-B's default route points back to Router-A (10.0.2.1), which is the bug: whoever set it up assumed Router-A was the way out. PC can still reach Server, because Router-B is connected to Server's network. But for any other destination, such as the Intranet or 8.8.8.8, Router-A hands the packet to Router-B and Router-B hands it straight back. OSPF is switched off on every router, so these static routes are all there is.

What you will work out

Understand how circular default routes create a routing loop, how TTL ends it, and how default routes should form a chain toward the exit.

Why this matters

Routing loops appear when two teams each assume the other's router is the way out, or when a default route is copied to the wrong device. The symptoms are traceroutes that repeat the same addresses and links that carry traffic going nowhere. Following default routes hop by hop until they reach an exit, and giving the top of the chain specific routes for everything behind it, prevents them.

Running it

Default Route Loop: Infinite Bounce 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.