Detecting and Resolving Network Loops with Traffic Analysis

When a network loop occurs, access becomes slow, packets are lost, and services may fail. Detecting such loops in large networks is difficult, but this case study shows how network analysis can uncover them.

Problem Description

The company’s network is completely internal, with an edge firewall connecting to the corporate backbone and a core switch linking to firewalls of subordinate units.

At around 8–10 am, network and application access became abnormal, with slow speeds and even server outages. Ping tests to a DMZ server showed heavy packet loss, but initial investigations could not pinpoint the cause. The organization used the Kela network trace analysis system to replay traffic and locate the fault.

Analysis Process

Traffic statistics for the incident period showed a peak of 682.35Mbps, about 70% bandwidth utilization, with occasional spikes higher than that, indicating potential packet loss.

Business Data Analysis

Analysis of application traffic revealed that an unknown UDP flow accounted for over 99% of total traffic.

Further inspection showed massive one‑way communication on UDP port 2425.

Discovering the Network Loop

Packet capture of two hosts showed identical IP IDs, indicating the same packet was being retransmitted.

TTL values decreased by 2 per hop, proving the packet passed through two layer‑3 devices before returning to the core switch interface, forming a loop.

A static route on the firewall pointing to X.X.0.0/16 caused packets destined for that subnet to be sent back to the core switch, which then default‑routed them back to the firewall, creating the routing loop.

Conclusion and Recommendations

Conclusion

The unknown UDP 2425 traffic consumed 99% of bandwidth, and the root cause was a routing loop between the subordinate unit’s firewall and the core switch.

After instructing the subordinate unit to block UDP 2425 traffic on its firewall, the issue disappeared and normal traffic resumed.

Recommendations

Prevent similar loops by using black‑hole routes: configure aggregate routes on upstream routers, default routes on downstream routers, and add a static null route for the unused aggregated subnet on the downstream device (e.g., X.X.0.0/16 → Null 0 on Cisco). This discards packets for undefined subnets without affecting valid routes.

Value

With the Kela network trace analysis system, operators can quickly detect network loops by observing changes in IP TTL and IP ID, enabling precise routing adjustments and avoiding unnecessary bandwidth consumption.