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Troubleshooting IP Default Gateway issues


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Article ID: 5621

Protocols/Features
IP
PING
Tracert

Goals
Configure default gateway

Symptoms
Can't ping
"Destination Host Unreachable"
"Request timed out"
Incorrect default gateway

Cause
A ping response of "Destination Host Unreachable" is a solid indicator that there's a Default Gateway misconfiguration on the initiating PC.

A ping response of "Request timed out" could suggest a Default Gateway misconfiguration, but lower-layer (L1/L2) issues could also be in play.

Solution
Your choices for setting the Default Gateway are three in number:

    No gateway IP - Conversations with other subnets will not take place. The host won't even write the packets to the wire. Failure symptom: "Destination Host Unreachable"
    Gateway IP is pointed to the local router's local interface - Conversations with other subnets will be addressed to the router, as an intermediary device (Dest MAC = router; Dest IP = intended destination host). Failure symptom: "Request timed out"
    Gateway IP is 'self-addressed' - All destinations will be treated as if local, so packet delivery relies upon either local presence of the destination host, or Layer 2 devices (bridges), or a router running Proxy ARP. Failure symptom: "Request timed out"
If the issue involves a router between two hosts, any host that can successfully ping the local router interface but cannot ping the remote router interface or the remote host, while the router can successfully ping both hosts, has a Default Gateway misconfiguration. A 'tracert <host>' command on one or both PC hosts can also assist with this type of investigation, detailing the router hops traversed.

Also available in GTAC Knowledge: "Troubleshooting IP Default Gateway issues".

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