Routing loop in network path detected

During the "traceroute" phase, a repeating pattern of IP addresses was detected in the network path, indicating an error condition.

Recommended action

  1. Review diagnostics at intermediate hops to determine which devices are in the loop. Look for "IP address for this hop appears more than once in network path".
  2. If you have access to the affected devices, review their routing tables.

Detailed explanation

The "traceroute" phase of Delivery monitoring works by sending packets with low time-to-live (TTL) values in the IP headers. The TTL value specifies how many hops the packet is allowed before it is returned. When a packet can't reach its destination because the TTL value is too low, the last hop returns the packet and identifies itself with an ICMP "time exceeded" message. By sending a series of packets and incrementing the TTL value with each successive packet, the location of intermediate hops can be found.

A routing loop exists when one or more network forwarding devices have been misconfigured to route packets through each other in an infinite loop. Packets entering the loop will cycle among the involved devices until their TTL field is decremented to zero, at which time they are discarded. Routing loops completely break network connectivity.

Possible secondary messages

  • "Target host is unreachable"
  • "ICMP TTL Expired messages are likely caused by this condition"
  • "Packet loss primarily attributed to TTL expiry"
  • "Review mid-path diagnostics to identify affected hops"

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