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.
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:
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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.)
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Building a list of default (reachable) routers, along with router lifetime and prefix lifetime data
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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.)
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Learning the hop limit for traffic leaving the VLAN interface
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Learning the MTU (Maximum Transmission Unit) to apply to frames intended to be routed
When an IPv6 interface becomes operational on the switch, a router solicitation is automatically sent to trigger an RA from any IPv6 routers reachable on the VLAN. (Router solicitations are sent to the All-Routers multicast address; ff02::2. 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, IPv6 traffic on that VLAN cannot be routed, and the only usable unicast IPv6 address on the VLAN is the link-local address.
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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. |
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If IPv6 is enabled on a VLAN where there is at least one accessible IPv6 router, the switch selects a default IPv6 router.
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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.
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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.
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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.
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."