Routing Loop Detector

About Routing Loops

A routing loop occurs when packets continuously cycle between routers instead of reaching their destination. This tool analyzes traceroute results to detect:

• IP Repetition: Same IP address appearing multiple times in the path
• Alternating Patterns: Sequences like A -> B -> A -> B indicating packets bouncing between routers

Common Causes

• Misconfigured routing tables or incorrect static routes
• BGP/OSPF/IGP convergence issues or route flapping
• Missing null/blackhole route for an announced prefix - if a router advertises a prefix but lacks a specific route for part of it, packets may loop back to the default gateway
• Asymmetric routing combined with stateful firewalls

Testing Tips

For accurate results when testing your own IP space, try tracing to an unassigned IP address within your prefix. This helps identify missing blackhole routes - if the prefix is advertised but no host exists at that IP, packets should be dropped, not looped. A loop to an unassigned IP often indicates a missing null route for the covering prefix.

Example of a Routing Loop

Below is an example traceroute showing a routing loop. Notice how packets alternate between two routers (hops 5-8) and then one router keeps appearing repeatedly (hops 10-30):

traceroute to 123.123.123.99, 30 hops max, 60 byte packets
 1  ***.***.***.***  0.070 ms
 2  ***.***.***.***  0.578 ms
 3  123.123.124.1    0.735 ms
 4  123.123.124.2    0.690 ms
 5  123.123.125.64   1.525 ms  <-- Loop starts here
 6  123.123.126.8    1.639 ms
 7  123.123.125.64   1.144 ms  <-- Back to hop 5's IP
 8  123.123.126.8    1.886 ms  <-- Alternating pattern
 9  * * *
10  123.123.126.8    1.844 ms  <-- Same IP repeating
11  * * *
12  123.123.126.8    1.845 ms
...
28  123.123.126.8    3.746 ms
29  * * *
30  123.123.126.8    5.310 ms  <-- Never reaches destination

This trace never reaches its destination because packets are stuck bouncing between routers. The TTL eventually expires and the trace ends at hop 30.