H3C Technologies H3C S10500 Series Switches - Manual (Page 105)

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To do… Use the command… Remarks
Enable sending of ICMP redirect packets ip redirects enable
Required
Disabled by default.
Enable sending of ICMP timeout packets ip ttl-expires enable
Required
Disabled by default.
Enable sending of ICMP destination
unreachable packets
ip unreachables enable
Required
Disabled by default.
NOTE:
When sending ICMP timeout packets is disabled, the device will not send “TTL timeout” ICMP error
packets. However, “reassembly timeout” error packets will be sent normally.
Enabling support for ICMP extensions
Introduction
ICMP messages are of a fixed format and cannot carry extension information. With support for ICMP
extensions enabled, a device appends an extension information field to the ICMP messages as needed.
The device can append only MPLS label information to ICMP messages.
ICMP extensions for MPLS
In MPLS networks, when a packet's TTL expires, MPLS strips the MPLS header, encapsulates the
remaining datagram into an ICMP time exceeded message, and sends the message to the egress router
of the MPLS tunnel. Then the egress router sends the message back to the ingress router of the tunnel. The
ICMP message, however, does not contain the label information that is very important to the ingress
router. With support for ICMP extensions enabled, the device appends the MPLS label to the ICMP time
exceeded message before sending it back to the ingress router of the tunnel.
ICMP extensions are usually used for an enhanced traceroute implementation in MPLS networks, in
which MPLS label information of each hop the original datagram arrives at is printed.
Handling ICMP messages
ICMP messages can be classified into the following types:
• Common ICMP messages: Without any extension information.
• Extended ICMP messages with a length field: Carry extension information and a length field. The
length field indicates the length of the original datagram that is encapsulated within the ICMP
header and excludes the ICMP extension length. Such an ICMP message complies with RFC 4884.
• Extended ICMP messages without a length field: Carry extension information but does not contain
a length field. Such an ICMP message does not comply with RFC 4884.
Based on how these messages are handled, the device can work in one of these modes: common mode,
compliant mode, and non-compliant mode. Table 6 shows how ICMP messages are handled in different
working modes.