Information Technology Reference
In-Depth Information
Firewalls and routers can be configured to not allow devices to be pinged but to still permit other types
of IP traffic. For this reason, a ping failure between two devices should not be misconstrued as a lack of
IP connectivity between those devices. Table 7-2 shows a list of some of the codes returned by the Cisco
ping utility, along with their meanings and possible cause.
Table7-2
Cisco ping Return Codes
Code
Meaning
Possible Cause(s)
!
Each exclamation point indicates receipt
of an ICMP echo reply.
The ping completed successfully.
.
Each period indicates that the network
server timed out while waiting for a reply.
This message can indicate many
problems:
ping was blocked by an
access list or firewall.
A router along the path did
not have a route to the
destination and did not send
an ICMP destination
unreachable message.
A physical connectivity
problem occurred
somewhere along the path.
U
An ICMP unreachable message was
received.
A router along the path did not
have a route to the destination
address.
C
An ICMP source quench message was
received.
A device along the
path—possibly the
destination—may be receiving to
much traffic; check input queues.
&
An ICMP time exceeded message was
received.
A routing loop may have
occurred.
traceroute
The traceroute utility sends out either ICMP echo request (Windows) or UDP (most implementations)
messages with gradually increasing IP TTL values to probe the path by which a packet traverses the
network (see Example 7-3). The first packet with the TTL set to 1 will be discarded by the first hop, and
the first hop will send back an ICMP TTL exceeded message sourced from its IP address facing the
source of the packet. When the machine running the traceroute receives the ICMP TTL exceeded
message, it can determine the hop via the source IP address. This continues until the destination is
reached. The destination will return either an ICMP echo reply (Windows) or a ICMP port unreachable,
indicating that the destination had been reached. Cisco's implementation of traceroute sends out three
packets at each TTL value, allowing traceroute to report routers that have multiple equal-cost paths to
the destination.
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