User's Manual

Configuration Guide of XGS-5240-Series
61-2
rminal user systems.
In IPv6 environment, the hosts in a LAN usually learn the default gateway via neig
hbor discovery protocol (NDP), which is implemented based on regularly receiving adver
tisement messages from routers. The NDP of IPv6 has a mechanism called Neighbor
Unreachability Detection, which checks whether a neighbor node is failed by sending un
icast neighbor request messages to it. In order to reduce the overheads of sending nei
ghbor 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 route
r in a period of time. In Neighbor Unreachability Detection, if adopting default paramete
rs, 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 t
ake 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.
61.1.1 The Format of VRRPv3 Message
VRRPv3 has its own message format, VRRP messages are used to communicate t
he priority of routers and the state of Master in the backup group, they are encapsulat
ed in IPv6 messages to send, and are sent to the specified IPv6 multicast address. Th
e format of VRRPv3 message is shown in Graph 1. The source address of the IPv6 m
essage encapsulating the VRRPv3 message is the local address of the outbound interf
ace 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, an
d 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 mini
mum 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 I
Pv6 pseudo head (please refer to RFC2460 for details);
IPv6 Address(es)：one or more IPv6 addresses related to the virtual router, the nu
mber of which is the same with ”Count IPv6 Addr”, and the first one of which should b