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4-27
IPv6 Addressing Configuration
Router Access and Default Router Selection
Router Access and Default Router
Selection
Routing traffic between destinations on different VLANs configured on the
switch or to a destination on an off-switch VLAN 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 router advertisements (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 neighbor discovery
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 (Maximum Transmission Unit) to apply to frames
intended to be routed
Router Solicitations
When an IPv6 interface becomes operational on the switch, a router solicita-
tion is automatically sent to trigger a router advertisement (RA) from any IPv6
routers reachable on the VLAN. (Router solicitations are sent to the All-
Routers multicast address; ff02::2. Refer to “Multicast Application to IPv6
Addressing” on page 3-21.) If an RA is not received within one second of
sending the initial router solicitation, the switch sends up to three additional
solicitations at intervals of four seconds. If an RA is received, the sending
router is added to the switch's default router list and the switch stops sending
router solicitations. If an RA is not received, then IPv6 traffic on that VLAN
cannot be routed, and the only usable unicast IPv6 address on the VLAN is the
link-local address.