Finisar Surveyor, User Manual

Page: 275 / 454

10-71
Expert Features
Network Layer
10
ICMP Fragmentation Needed [D/F set]
Counter
ICMP Fragmentation Needed [D/F set] events are counted in the ICMP All Errors
and the ICMP Destination Unreachable counters. A count of all destination
unreachable ICMP symptoms and a count of all ICMP errors displays in the
Overview
counters of Expert View. A threshold can be set in Expert Alarms for all
destination unreachable ICMP errors or for all ICMP errors.
Expert Symptom
ICMP Fragmentation Needed [D/F] set events are automatically logged as expert
symptoms. The
Symptom Summary
field provides information about the IP
addresses involved. For example:
MTU of next Hop=2 to reach [206.250.228.69]. Cannot be
reached by [206.250.228.11] as D/F Set.
SA=[206.250.228.11] DA=[206.250.228.69]
Diagnostic Details
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Problem Description:
An ICMP Destination (Fragmentation needed, but, D/F set) Unreachable
message has been sent.
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Probable Cause(s):
1. If a router has a routing table problem, it may send this message.
2. If the packet needs to be fragmented and the “don’t fragment” flag is set the
host/router will send this message.
3. The source may have an incorrectly configured subnet mask.
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Recommended Action(s):
1. Check the routing tables of the router that this message was generated from.
2. Check the netmask configuration of the source.
3. Ignore this message if the D/F is meant to be set (no action required).