H3C Technologies H3C S7500E Series Switches - Manual (Page 128)

13-9
z In addition to an address prefix, the prefix information option also contains the preferred lifetime
and valid lifetime of the address prefix. After receiving a periodic RA message, the node updates
the preferred lifetime and valid lifetime of the address prefix accordingly.
z An automatically generated address is applicable within the valid lifetime and is removed when the
valid lifetime times out.
Redirection
When a host is started, its routing table may contain only the default route to the gateway. When
certain conditions are satisfied, the gateway sends an ICMPv6 redirect message to the source host so
that the host can select a better next hop to forward packets (similar to the ICMP redirection function in
IPv4).
The gateway sends an IPv6 ICMP redirect message when the following conditions are satisfied:
z The receiving interface is the forwarding interface.
z The selected route itself is not created or modified by an IPv6 ICMP redirect message.
z The selected route is not the default route.
z The forwarded IPv6 packet does not contain any routing extension header.
IPv6 PMTU Discovery
The links that a packet passes from the source to the destination may have different MTUs. In IPv6,
when the packet size exceeds the path MTU ( the minimum MTU of all links), the packet will be
fragmented at the source end so as to reduce the processing pressure of forwarding devices and
utilize network resources properly.
The path MTU (PMTU) discovery mechanism is to find the minimum MTU of all links in the path from
the source to the destination. Figure 13-5 shows the working procedure of PMTU discovery.
Figure 13-5 Working procedure of PMTU discovery
The working procedure of the PMTU discovery is as follows:
1) The source host uses its MTU to send packets to the destination host.