Every area must connect to the backbone, because the backbone is responsible for the distribution of routing information between areas. The backbone itself has all the properties of an area. Its topology is separate from that of other areas.
•Subarea
A subarea has only one area border router, which means that there is only one route out of the area. In this case, the area border router does not need to advertise external routes to the other routers within the subarea. It can simply advertise itself as the default route.
•The sequence of operations performed by OSPF routers is as follows:
1.Discovering OSPF neighbors
2.Electing the Designated Router
3.Forming adjacencies
4.Synchronizing databases
5.Calculating the routing table
6.Advertising Link States
Routers go through these steps when they first come up, and repeat them in response to the events that occur in the network. Each router must perform each of these steps for each network it is connected to, except for the calculation of the routing table. Each router generates and maintains a single routing table for all networks.
•Multicast
•IP Multicast
The GRF supports IP multicast routing per RFC 1112 and some components of RFCs 1301 and 1469. The implementation includes the IP multicast kernel modifications, dynamic route support and mrouted (multicast route daemon).
Data that arrives at a GRF interface is duplicated and forwarded to multiple destination interfaces. The multicast packet’s destination address is a Class D address. A lookup of the multicast route table returns a list of Virtual Interfaces (VIFs) to which the packet is sent.
•OSPF Multicast
The GRF uses the multicast capability of OSPF Version 2, as described in RFC 1583 and RIP Version 2, for communications between routers.
