The Anycast Process
Anycast is a mechanism used to route packets to one of many identically addressed nodes The identically addressed nodes might be a group of servers offering a well known service to clients or a group of routers belonging to an ISP which requires that traffic pass through one of its anycast addressed routers A node addresses the IP packet to the single anycast address of the group The node learns the next hop for the address just as it would for a unicast address If the anycast address is on link the node performs the address resolution process The first response is added to the neighbor cache If the address is off link the packet is forwarded to the nearest destination based on the routing protocol s measure of distance There will be a prefix that contains the set of anycast nodes in a domain For instance all nodes using the anycast address FECO A FDFF FFFF FFFF FFFE/64 reside within the FECO 0 0 A /64 prefix All these anycast nodes must be advertised as host routes within the domain addressed with this prefix A node uses the metric of the host route to determine the closest anycast node You can see that if there are a lot of anycast groups and anycast nodes within the groups and the containing domains are very large routing tables within the domain could get very large
Although anycasting is specified in IPv6 its use is currently very restricted There is little experience using widespread anycasting services and there are some known complications such as ensuring all packets for a session reach the same anycast node or requiring the anycast nodes to share state information4 More issues need to be resolved as experience is gained The only defined anycast group other than all anycast subnet routers is the Mobile IPv6 home agent address Until solutions to the problems are agreed upon use of anycasting is restricted to routers only
Continue reading here: Simple Network Management Protocol
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