IPv6 VRRPv3 Configuration
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systems.
In IPv6 environment, the hosts in a LAN usually learn the default gateway via
neighbor discovery protocol (NDP), which is implemented based on regularly receiving
advertisement messages from routers. The NDP of IPv6 has a mechanism called
Neighbor Unreachability Detection, which checks whether a neighbor node is failed by
sending unicast neighbor request messages to it. In order to reduce the overheads of
sending neighbor request messages, these messages are only sent to those neighbor
nodes which are sending flows, and are only sent if there is no instruction of UP state of
the router in a period of time. In Neighbor Unreachability Detection, if adopting default
parameters, it will take about 38 seconds to detect an unreachable router, which is a delay
not ignorable for users and might cause a time-out in some transport protocols. Compared
with NDP, VRRP provides a fast default gateway switch. In VRRP, backup routers can
take up the unavailable master router in about 3 seconds (default parameter), and this
process needs no interaction with hosts, which means being transparent to hosts.
60.1.1
The Format of VRRPv3 Message
VRRPv3 has its own message format, VRRP messages are used to communicate
the priority of routers and the state of Master in the backup group, they are encapsulated
in IPv6 messages to send, and are sent to the specified IPv6 multicast address. The
format of VRRPv3 message is shown in Graph 1. The source address of the IPv6
message encapsulating the VRRPv3 message is the local address of the outbound
interface of the message, and the destination address of it is the IPv6 multicast
address(the multicast allocated to VRRPv3 is FF02:0:0:0:0:0:0:12). The number of hops
should be limited to 255, and the next message head is 112(representing a VRRP
message).
The meaning of each field in a VRRPv3 message is shown as follows:
Version: The version of VRRPv3, whose value is 3;
Type: The type of VRRP messages. There is only one type: ADVERTISEMENT, and
its value is 1;
Virtual Rtr ID
：
The ID of the virtual router;
Priority
：
Priority, ranging from 0 to 255;
Count IPv6 Addr
：
The number of IPv6 addresses in a VRRPv3 message, the
minimum of which is 1;
Rsvd
：
Reserved field, whose value is 0;
Adver Int
：
The advertisement interval of VRRPv3 messages, in seconds;
Checksum
：
The checksum, taking account of the whole VRRPv3 message and an
IPv6 pseudo head (please refer to RFC2460 for details);
IPv6 Address(es)
：
one or more IPv6 addresses related to the virtual router, the