3Com MSR 50 Series, Configuration Manual

Page: 663 / 2443

IPv6 Overview 663
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 198 shows the working
procedure of the PMTU discovery.
Figure 198 Working procedure of the PMTU discovery
The working procedure of the PMTU discovery is as follows:
1
The source host uses its MTU to fragment packets and then sends them to the
destination host.
2
If the MTU supported by the forwarding interface is less than the packet size, the
forwarding device will discard the packet and return an ICMPv6 error packet
containing the interface MTU to the source host.
3
After receiving the ICMPv6 error packet, the source host uses the returned MTU to
fragment the packet again and then sends it.
4
Step 2 to step 3 are repeated until the destination host receives the packet. In this
way, the minimum MTU of all links in the path from the source host to the
destination host is determined.
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