KB.15.15

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, IPv6 traffic on that VLAN cannot

be routed, and the only usable unicast IPv6 address on the VLAN is the link-local address.

NOTE: If the switch does not receive an RA after sending the router solicitations, as described

above, no further router solicitations are sent on that VLAN unless a new IPv6 setting is configured,

IPv6 on the VLAN is disabled and then re-enabled, or the VLAN itself is disconnected and then

reconnected.

Default IPv6 router

If IPv6 is enabled on a VLAN where there is at least one accessible IPv6 router, the switch selects

a default IPv6 router.

• If the switch receives RAs from a single IPv6 router on the same VLAN or subnet, the switch

configures a global unicast address and selects the advertising router as the default IPv6 router.

• If multiple IPv6 routers on a VLAN send RAs advertising the same network, the switch configures

one global unicast address and selects one router as the default router, based on the router's

relative reachability, using factors such as router priority and route cost.

• If multiple IPv6 routers on a VLAN send RAs advertising different subnets, the switch configures

a corresponding global unicast address for each RA and selects one of the routers as the

default IPv6 router, based on route cost. When multiple RAs are received on a VLAN, the

switch uses the router priority and route cost information included in the RAs to identify the

default router for the VLAN.

Router redirection

With multiple routers on a VLAN, if the default (first-hop) router for an IPv6-enabled VLAN on the

switch determines that there is a better first-hop router for reaching a given remote destination, the

default router can redirect the switch to use that other router as the default router. For further

information on routing IPv6 traffic, see the documentation provided for the IPv6 router.

For related information, see RFC 2461: "Neighbor Discovery for IP Version 6."

Router access and default router selection 35