18.5 IPv6 important reminder for use

(1) Changing MTU length of IPv6 trunks

The minimum packet length for IPv6 is defined as 1280 bytes (RFC 2460). Therefore, if the MTU size is less than 1280 bytes, IPv6 communication is not possible. Specify at least 1280 bytes for the MTU size for interfaces that perform IPv6 communication.

(2) Configuring Multiple Global Addresses for an Interface

When you assign multiple global addresses to an interface, if terminals that are connected on the same link as the interface use different global addresses to communicate with one another, IPv6 forwarding might occur via the Switch.

If forwarding does occur, the hardware forwards packets to the software so that the software can determine whether an ICMPv6 Redirect message can be sent. Because this process significantly increases the load on the CPU of the Switch, keep the following in mind:

• Use the same prefix for all the terminals that are connected on the same link by using IPv6 address auto-configuration that has been enabled by Router Advertisement messages or other methods.

• If you need to use different prefixes for the terminals that are connected on the same link for security reasons, we recommend that you avoid high CPU load by using the appropriate configuration command to stop the hardware from making a determination regarding whether ICMPv6 Redirect messages can be sent.

(3) Duplicate IPv6 addressing

IPv6 supports Duplicate Address Detection (DAD) as defined in RFC 2462. If DAD detects a duplicate IPv6 address, that address cannot be used for communication. If you execute the show ipv6 interface command or show ip-dual interface command and duplicated is indicated next to an IPv6 address in the list, that IPv6 address is already being used by another device.For each case, perform the following procedure:

• If the IPv6 address assigned to the other device is incorrect

  First correct the IPv6 address of the other device, and then delete and set again the IPv6 address of the Switch. Alternatively, restart the Switch.

• If the IPv6 address assigned to the Switch is incorrect

  Delete the duplicate IPv6 address assigned to the Switch in the configuration, and assign a correct IPv6 address.

• If duplicate IPv6 addresses are automatically generated

  A loop occurred on a VLAN interface or a terminal is illegally using the IPv6 address of the Switch. Eliminate the problem and execute the no ipv6 enable command.Next, execute the ipv6 enable command.

(4) Notes on Static NDP

When you specify the IPv6 address that is assigned to an interface in the Switch as a static NDP entry, the operation of the Switch becomes unstable and communication might become impossible. To prevent this problem, when you configure an interface, the Switch checks whether the IPv6 address assigned to the interface is specified as a static NDP entry.However, the Switch does not check the following IPv6 addresses for duplication:

• Link-local addresses (automatically and manually generated)

• Global addresses that are automatically generated when interface IDs are omitted

Therefore, do not specify these IPv6 addresses assigned to interfaces as static NDP entries. If you have mistakenly specified such a static NDP entry, delete it and restart the VLAN for the applicable interface.

(5) Layer 3 forwarding of packets with IPv6 extension option

• If you perform Layer 3 forwarding for the packets with the Hop-by-Hop Options header, the software performs forwarding.

• If the receiving side uses QoS control, the software performs Layer 3 forwarding for the TCP packets with the Routing header or Destination Options header.