Allied Telesis AT-S63 User Manual

Chapter 27: Internet Protocol Version 4 Packet Routing

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Section VII: Routing

ICMP allows routers to send error and control messages to other routers 
or hosts. It provides the communication between IP software on one 
system and IP software on another. The switch implements the non-
obsolete ICMP functions listed in Table 24.

Table 24. ICMP Messages Implemented on the AT-9400 Switch

Echo reply (0)

This is used to implement the 
“ping” command common to most 
UNIX and TCP implementations. 
The switch sends out an “Echo 
reply” packet in response to an 
“Echo request.”

Destination unreachable (3)

This message is sent out when the 
switch drops a packet because it 
did not have a route to the 
destination.

Source Quench (4)

The switch will send a “Source 
Quench” if it must drop a packet 
due to limited internal resources. 
This could be because the source 
was sending data too fast to be 
forwarded.

Redirect (5)

The switch will issue a “redirect” 
packet to inform a local host that 
its target is located on the same 
LAN (no routing is required) or 
when it detects a host using a non-
optimal route (usually because a 
link has failed or changed its 
status).

Ech request (8)

This is related to (1) and results in 
an “echo reply” packet being sent. 
The switch can also generate an 
“echo request” packet as a result 
of the PING command.