A packet is delivered to the nearest member of a group, in anycast. Anycast routing finds these paths. Sometimes nodes provide a service, such as time of day or content distribution for which it is getting the right information all that matters, not the node that is contacted; any node will do. For example, anycast is used in the internet as part of DNS.
Regular distance vector and link state routing can produce anycast routes because there is no need to devise new routing schemes for anycast. Suppose we want to anycast to the members of group 1. They will be given the address “1”, instead of different addresses. Distance vector routing will distribute vectors as usual, and nodes will choose the shortest path to destination 1. This will result in nodes sending to the nearest instance of destination 1. That is, it believes that all the instances of node 1 are the same node, as in the topology shown in figure below.
This procedure works for link state routing as well, although there is the added consideration that the routing protocol must not find seemingly short paths that pass through node 1. This would result in jumps through hyperspace, since the instances of node 1 are really nodes located in different parts of the network. However, link state protocols already make this distinction between routers and hosts. We glossed over this fact earlier because it was not needed for our discussion.
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