KB.15.15

Default IPv6 Gateway

Instead of using static or DHCPv6 configuration, a default IPv6 gateway for an interface (VLAN

or tunnel) is determined from the default router list of reachable or probably reachable routers the

switch detects from periodic multicast router advertisements (RAs) received on the interface. For a

given interface, there can be multiple default gateways, with different nodes on the link using

different gateways. If the switch does not detect any IPv6 routers that are reachable from a given

interface, it assumes (for that interface) that it can reach only the other devices connected to the

interface. See “Router access and default router selection” (page 33).

NOTE: In IPv6 for the switches covered in this guide, the default route cannot be statically

configured. Also, DHCPv6 does not include default route configuration.)

Enabling DHCPv6

Enabling the DHCPv6 option on a VLAN allows the switch to obtain a global unicast address. It

also allows the switch to obtain additional information such as an NTP server address or a DNS

server address that can be used by the switch.

Syntax:

[no] ipv6 address dhcp full [ rapid-commit ]

Configures DHCPv6 on a VLAN, which initiates transmission of DHCPv6 requests

for service. If IPv6 is not already enabled on the VLAN by the ipv6 enable

command, this option enables IPv6 and causes the switch to autoconfigure a

link-local unicast address with an EUI-64 interface identifier.

[ rapid-commit ]

Expedites DHCP configuration by using a two-message exchange with the server

(solicit-reply) instead of the default four-message exchange

(solicit-advertise-request-reply).

NOTE: A DHCPv6 server does not assign link-local addresses, and enabling

DHCPv6 on a VLAN does not affect a pre-existing link-local address.

A DHCPv6-assigned address can be configured on a VLAN when the following is

true:

• The assigned address is not on the same subnet as a previously configured

autoconfig address.

• The maximum IPv6 address limit on the VLAN or the switch has not been

reached.

If the switch is an IPv6 host, ipv6 address dhcp full must be configured on

the DHCPv6 client to obtain relevant information from the DHCPv6 server. M-bit

and O-bit settings in RAs from a router are not used by the switch in host mode. If

the switch is operating as an IPv6 router, it includes M-bit and O-bit values in the

RAs it transmits. See chapter “IPv6 Router Advertisements” (page 186) for routing

switch operation.

After verification of uniqueness by DAD, an IPv6 address assigned to the VLAN by

a DHCPv6 server is set to the preferred and valid lifetimes specified in an RA

received on the VLAN for the prefix used in the assigned address and is configured

as a preferred address.

Default: Disabled

The no form of the command removes the DHCPv6 option from the configuration

and, if no other IPv6-enabling command is configured on the VLAN, disables IPv6

on the VLAN. See “About disabling IPv6 on a VLAN” (page 30).

Enabling DHCPv6 15