H3C Technologies H3C S12500 Series Switches - Manual (Page 140)

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NOTE:
•
You can configure this feature only on the VLAN interfaces of an Ethernet interface card.
• When sending ICMP timeout packets is disabled, the switch will not send "TTL timeout" ICMP error
packets. However, "reassembly timeout" error packets will be sent correctly.
Enabling support for ICMP extensions
Generally, ICMP messages are of a fixed format and cannot carry extension information. With support
for ICMP extensions enabled, a switch appends an extension information field to the ICMP messages as
needed. The switch can append only MPLS label information to ICMP messages.
ICMP extensions for MPLS
In MPLS networks, when a packet's TTL expires, MPLS stripes 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 switch 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 about 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—Carries 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—Carries 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 switch can operate in one of these modes: common
mode, compliant mode, and non-compliant mode. Table 6 shows how ICMP messages are handled in
different operating modes.
Table 6 Handling ICMP messages
Device mode ICMP messages sent ICMP messages received Remarks
Common mode Common ICMP messages Common ICMP messages
Extension information in
extended ICMP messages
will not be processed.
Compliant mode
Common ICMP messages
Extended ICMP messages
with a length field Common ICMP messages
Extended ICMP messages
with a length field
Extended ICMP messages
without a length field are
handled as common ICMP
messages.