When selected dynamically, the designated router is elected arbitrarily. The selection is made on the basis of the highest router ID or IP address. It is wise to be aware that the highest IP address is the numerically highest number, not the class ranking of the addresses. Therefore, a remote, small router with a Class C address may end up as a designated router. This may not be the optimal choice.
If you are manually determining which routers are to be the designated and backup designated routers, it is easier to design your network to the optimum.
After the designated and backup designated routers have been elected, all routers on the broadcast medium will communicate directly with the designated routers. They will use the multicast address to all designated routers. The backup router will listen but will not respond; remember, it is the understudy waiting in the wings. The designated router will send out multicast messages if it receives any information pertinent to the connected routers for which it is responsible.
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