During the "traceroute" phase of the test to determine the network path, the full path to the target host could not be traced.
- Determine whether there is a routing problem by reviewing secondary messages (below) and looking for one or more of these messages in the test results: "Routing loop in network path detected", "IP address for this hop appears more than once in network path", "ICMP TTL Expired messages received".
- If not a routing problem, ensure that ICMP Echo, ICMP Echo Reply and ICMP TTL Expired messages are enabled along the entire test path.
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.
In this case, either a device is not responding with the "time exceeded" message, or there is a routing loop along the network path.
Possible secondary messages
- "TTL Expiry messages appear to be blocked"
- "Target host is unreachable"
- "May be a routing problem"