KB.15.15

• If a previously configured unicast address is changed, a neighbor advertisement is sent on

the VLAN to notify other devices and for duplicate address detection.

• If DAD is disabled when an address is configured, the address is assumed to be unique and

is assigned to the interface.

Router access and default router selection

Traffic can be routed between destinations on different VLANs configured on the switch or to a

destination on an off-switch VLAN. This is done by placing the switch on the same VLAN interface

or subnet as an IPv6-capable router configured to route traffic to other IPv6 interfaces or to tunnel

IPv6 traffic across an IPv4 network.

Router advertisements

An IPv6 router periodically transmits RAs on the VLANs to which it belongs to notify other devices

of its presence. The switch uses these advertisements for purposes such as:

• Learning the MAC and link-local addresses of IPv6 routers on the VLAN. (For devices other

than routers, the switch must use ND to learn these addresses.)

• Building a list of default (reachable) routers, along with router lifetime and prefix lifetime data.

• Learning the prefixes and the valid and preferred lifetimes to use for stateless (autoconfigured)

global unicast addresses. (This is required for autoconfiguration of global unicast IPv6

addresses.)

• Learning the hop limit for traffic leaving the VLAN interface.

• Learning the MTU to apply to frames intended to be routed.

IPv6 router advertisement options for DNS configuration

Two new options in IPv6 Router Advertisements allow IPv6 routers to advertise a list of recursive

DNS Server (RDNSS) addresses and a DNS Search List (DNSSL) to IPv6 hosts. RA-based DNS

configuration enables the full configuration of basic networking information for hosts without

requiring DHCPv6. An IPv6 host can acquire the DNS configuration (that is, the DNS recursive

server addresses and DNS Search List) for the links to which the host is connected. The host learns

this DNS configuration from the same RA message that provides configuration information for the

link.

DNS options are included by default in every emitted RA unless the inclusion is suppressed via CLI

commands or the SNMP MIB. The suppress option can be configured either globally or for an IP

interface.

NOTE: This is supported in RFC 6106

Global configuration

This command suppresses the inclusion of RDNSS and SNSSL in outgoing Router Advertisements

across all interfaces on a switch.

Syntax

[no]nd suppress-ra-dns

Suppresses DNS options in router advertisements.

Router access and default router selection 33