H3C Technologies H3C S12500-X Series Switches - Manual (Page 136)

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Enabling sending ICMPv6 destination unreachable messages
The device sends ICMPv6 destination unreachable messages as follows:
• If a packet does not match any route, the device sends a No Route to Destination ICMPv6 error
message to the source.
• If the device fails to forward the packet because of administrative prohibition (such as a firewall
filter or an ACL), the device sends the source a Destination Network Administratively Prohibited
ICMPv6 error message.
• If the device fails to deliver the packet because the destination is beyond the scope of the source
IPv6 address (for example, the source IPv6 address is a link-local address whereas the destination
IPv6 address is a global unicast address), the device sends the source a Beyond Scope of Source
Address ICMPv6 error message.
• If the device fails to resolve the link layer address for the destination IPv6 address, the device sends
the source an Address Unreachable ICMPv6 error message.
• If a UDP packet received is destined for the device but its UDP destination port number does not
match any process, the device sends the source a Port Unreachable ICMPv6 error message.
If a device is generating ICMPv6 destination unreachable messages incorrectly, disable the sending of
ICMPv6 destination unreachable messages to prevent attack risks.
To enable sending ICMPv6 destination unreachable messages:
Step Command Remarks
1. Enter system view. system-view N/A
2. Enable sending ICMPv6 destination
unreachable messages.
ipv6 unreachables enable
By default, this function is
disabled.
Enabling sending ICMPv6 time exceeded messages
The device sends ICMPv6 Time Exceeded messages as follows:
• If a received packet is not destined for the device and its hop limit is 1, the device sends an ICMPv6
Hop Limit Exceeded message to the source.
• Upon receiving the first fragment of an IPv6 datagram destined for the device, the device starts a
timer. If the timer expires before all the fragments arrive, the device sends an ICMPv6 Fragment
Reassembly Timeout message to the source.
If the device receives large numbers of malicious packets, its performance degrades greatly because it
must send back ICMP Time Exceeded messages. To prevent such attacks, disable sending ICMPv6 Time
Exceeded messages.
To enable sending ICMPv6 time exceeded messages:
Step Command Remarks
1. Enter system view. system-view N/A
2. Enable sending ICMPv6 time
exceeded messages.
ipv6 hoplimit-expires enable The default setting is disabled.