Diffusing Update Algorithm

The Diffusing Update Algorithm ( DUAL short ) is a component of proprietary routing protocol EIGRP by Cisco. He is responsible for the route calculation. The specified Cisco full name of the algorithm is "DUAL finite - state machine" (DUAL FSM).

DUAL agent is created when using EIGRP a loop-free (loop- free ) routing within an autonomous system. DUAL responds to changes in the routing topology dynamically and automatically adjusts the routing tables of the router to changing circumstances on.

Operation

DUAL uses for route calculation from three EIGRP in each router tables created separately. These tables are created based on EIGRP exchanged between the routers information. This is similar to the exchange of information by means of a link-state routing protocol. In contrast to link-state transfers, with each router the "Link States " - ie the status ( active / inactive) ships as well as the range of its interfaces on all routers of an autonomous system, are called in EIGRP "neighborhood relations " established. This neighborhoods only the information is transmitted from router to router, necessary for the remote station.

The three tables and their functions in detail:

• Neighbor table = contains information about all other routers directly connected. For each protocol (IP, IPX, etc.) that support EIGRP, there is a separate neighbor table. For each neighbor entry with a description of the network interface and address is applied. In addition, a timer is initialized to checked in regular intervals, whether the connection with the neighbor is still active. This is realized by so-called Hello packets. If a hello packet is not answered during a certain period, it is assumed that the route is no longer available and marked as " active". (See below)
• = Topology table contains the information on the cost of each of the possible connections to each target in the autonomous system. In the topology table, the primary and secondary route will be determined at a target using this information. Among other things, the topology table contains the following entries for each target:

• Routing table contains each = best ( metric- based, so in this case the " cost-effective " ) connection to a destination

DUAL evaluates the received data from other routers in the topology table, and calculates the primary and redundant network path. Of the primary path is typically the path with the lowest cost to reach the destination, the redundant path is the path with the second lowest cost. All paths are kept, but only one of these paths is also actively used. Thus, loops are avoided automatically.

In order to handle requests for a specific destination on the primary path, DUAL carries the neighbors on the primary path as the gateway of all requests to the destination in the routing table. This router is referred to by Cisco as Successor. In the topology table of EIGRP DUAL also maintains the neighbors of the second best connection to a target as a so-called Feasible Successor. If the route to the designated successor as router, it Feasible Successor is entered instead of the Successors in the routing table. These must be the RD of the Feasible Successor less than the FD of the Successors. This is not the case, a possible successor using a query process is sought. This behavior, these terms are used for loop avoidance.

Example

Legend:

A (2 ) B ( 1) C       ----- -----                   | |                ( 2) | | ( 3)                   | |                   -----                   D ( 1) E Now a client wants in a network to Router E to communicate with a client in a network connected to router A start, it means that router E must provide a route to Router A is available.

This route has been calculated as follows:

The direct neighbors of E are D and C.

So DUAL queries the Reported Distance (RD ) of C and D to A. This produces the following results:

The route via C so if DUAL would include only the distance of the neighbors in the routing choices, the best route. In the next step, in addition, the distance between neighboring and the router itself is still involved with in the calculation. The sum of Reported Distance plus the distance to neighbors is set as Feasible Distance ( FD) and used for the prioritization of routes as a basis:

Thus DUAL notes that the route via D is the total cost route. The route via D is thus marked as Successor, provided with passive status and entered into the routing table for use. The route via C is kept as a Feasible Successor.