3Com Switch 8800 Advanced Software V5 Configuration Guide

IPv6 Overview 219
Stateless address autoconfiguration means that a host automatically configures an
IPv6 address according to the information obtained through router/prefix
discovery.
The router/prefix discovery is implemented through RS and RA messages. The
router/prefix discovery procedure is as follows:
1 After started, a host sends an RS message to request the router for the address
prefix and other configuration information for the purpose of autoconfiguration.
2 The router returns an RA message containing information such as address prefix
and flag bits. (The router also regularly sends an RA message.)
3 The host automatically configures an IPv6 address and other information for its
interface according to the address prefix and other configuration parameters in
the RA message.
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 will send an IPv6 ICMP redirect message when the following
conditions are satisfied:
■ The receiving interface is the forwarding interface.
■ The selected route itself is not created or modified by an IPv6 ICMP redirect
message.
■ The selected route is not the default route.
■ The forwarded IPv6 packet does not contain any routing 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 link MTU, the packet
will be fragmented at the source end so as to reduce the processing pressure of
the forwarding device and utilize network resources rationally.
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 67 shows the working
procedure of the PMTU discovery.
Figure 67 Working procedure of the PMTU discovery
Source
MTU = 1500 MTU = 1500 MTU = 1350 MTU = 1400
Packet with MTU = 1500
ICMP error: packet too big;
use MTU = 1350
Packet with MTU = 1350
Packet received