Operation
DUAL uses three separate tables for the route calculation. These tables are created using information exchanged between the EIGRP routers. The information is different than that exchanged by link-state routing protocols. In EIGRP, the information exchanged includes the routes, the "metric" or cost of each route, and the information required to form a neighbor relationship (such as AS number, timers, and K values). The three tables and their functions in detail are as follows:
- Neighbor table contains information on all other directly connected routers. A separate table exists for each supported protocol (IP, IPX, etc.). Each entry corresponds to a neighbour with the description of network interface and address. In addition, a timer is initialized to trigger the periodic detection of whether the connection is alive. This is achieved through "Hello" packets. If a "Hello" packet is not received from a neighbor for a specified time period, the router is assumed down and removed from the neighbor table.
- Topology table contains the metric (cost information) of all routes to any destination within the autonomous system. This information is received from neighboring routers contained in the Neighbor table. The primary (successor) and secondary (feasible successor) routes to a destination will be determined with the information in the topology table. Among other things, each entry in the topology table contains the following:
- "FD (Feasible Distance)": The calculated metric of a route to a destination within the autonomous system.
- "RD (Reported Distance)": The metric to a destination as advertised by a neighboring router. RD is used to calculate the FD, and to determine if the route meets the "feasibility condition".
- Route Status: A route is marked either "active" or "passive". "Passive" routes are stable and can be used for data transmission. "Active" routes are being recalculated, and/or not available.
- Routing table contains the best route(s) to a destination (in terms of the lowest "metric"). These routes are the successors from the topology table.
DUAL evaluates the data received from other routers in the topology table and calculates the primary (successor) and secondary (feasible successor) routes. The primary path is usually the path with the lowest metric to reach the destination, and the redundant path is the path with the second lowest cost (if it meets the feasibility condition). There may be multiple successors and multiple feasible successors. Both successors and feasible successors are maintained in the topology table, but only the successors are added to the routing table and used to route packets.
For a route to become a feasible successor, its RD must be smaller than the FD of the successor. If this feasibility condition is met, there is no way that adding this route to the routing table could cause a loop.
If all the successor routes to a destination fail, the feasible successor becomes the successor and is immediately added to the routing table. If there is no feasible successor in the topology table, a query process is initiated to look for a new route.
Read more about this topic: Diffusing Update Algorithm
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