The OSPF routing protocol is a typical link-state routing protocol, generally used within a single routing domain. Here, a routing domain refers to an Autonomous System (AS), which is a set of networks that exchange routing information through a unified routing policy or routing protocol.
Within this AS, all OSPF routers maintain an identical database describing the structure of the AS. This database stores the link-state information of the corresponding links in the routing domain, and it is through this database that OSPF routers calculate their OSPF routing table.
1. After configuring the IP addresses and OSPF protocol on each interface, we configure the router as a Frame Relay switch. This helps us better understand Frame Relay. Note: On a router interface, you can define the interface type using ip OSPF network broadcast, or you can manually specify neighbors for two interfaces. Otherwise, the neighbor relationship will not establish.
2. Then, we can enter each router’s interface to manually specify the cost value. The command is entered under the interface: ip OSPF cost. Note: The default cost value for an Ethernet interface is 1. For a WAN interface, the default is 64. OSPF’s default reference value of 100 divided by the interface bandwidth of 1.544 equals 64. You can check this using the show interface query. When a router’s source interface connects to another network, it uses the cost value of that interface to determine the path preference. For example, on R3, if the cost value of S1/0 is not manually specified and S1/1 is, S1/0 defaults to 64. In that case, R3 will not choose the S1/0 interface to reach other routes.
3. Based on common analysis, since the cost is the smallest, it should take path 2. However, this is not the case in reality. We find that after traffic goes to R2, it is directly forwarded to R1, instead of going through the 172.16.255.0 network segment with a cost value of 10 towards R4. This path is also calculated by the SPF algorithm.
4. When R3 forwards a data packet to R2, R2 discovers that this packet is destined for another network.
Therefore, R2 will not forward the packet back to other routers in the same area. Because R2 itself is an ABR, it believes that packets destined for other areas need to be forwarded directly to the backbone area. Since it happens to be connected to the backbone area, R2 directly forwards the packet to R1, the router in the backbone area.
It is for this reason that the route we see is inconsistent with the