DUAL (diffusion update algorithm) of VPN)

DUAL (diffusion update algorithm) of VPN)
I. Diffusing Update Algorithm (DUAL) is a method for determining the optimal no-ring routing and no-ring standby path by using the concept diffusion Update Algorithm. DUAL terminology: feasible distance (FD) of the successor router (FS) report distance (RD) or advertised distance (AD) of the successor router (FC) II. Detailed description of the successor router: it refers to an adjacent router used to forward data packets. This router is the route with the lowest network overhead to the destination. Feasible successor router (FS): it is the Standby Router to the destination network, and the overhead is second only to the overhead of the successor router. Feasible distance (FD): The measurement value that reaches the destination CIDR block. Advertised distance (AD): A metric value that is measured by the network to which the network of the same destination is located by the neighbor of the network. Feasible condition (FC): The advertised distance of the local route is smaller than that of the optimal route. 3. Example 1. Experiment topology 2. show ip Address: the network topology on R1 to view the route to 4.4.4.0/24. P -- the route is passive. When DUAL does not perform diffusion computing to determine the path to a network, the route entry is in a stable state, that is, passive state. If DUAL is re-calculating and searching for A new path, the path will be active and the ID will be displayed as. 4.4.4.0/24 -- this is the destination network. 1 successors, which is used to display the number of successor routers to this network. If there are multiple equal paths to this network, there will be multiple successors. FD is 435200 -- this is a feasible distance, that is, the OSPF measurement to the destination network. Via 13.1.1.3 (435200/409600), FastEthernet0/0--13.1.1.3 is the next hop address of the successor router, 435200 is the feasible distance to the destination network, 409600 is the advertised distance, and FastEthernet0/0 is the outbound interface. Via 12.1.1.2 (2323456/409600), FastEthernet0/0--12.1.1.2 is the next hop address of the feasible successor router, 2323456 is the feasible distance to the destination network, 409600 is the advertised distance, and Serial1/0 is the outbound interface. 3. The core of DUAL finite state machine (FSM) is DUAL and DUAL's replicaset routing computing engine. The exact name of this technique is DUAL finite state machine (FSM ). The finite state machine contains all the logics used to calculate and compare routes in the network of the network. The simplified version of dual fsm is shown in the figure. Iv. DUAL Algorithm Summary 1. Record all routes advertised by neighbors and write them to the topology table. 2. Select the smallest FD as the successor and write it into the IP route table. 3. Select a feasible successor based on the principle that the route AD is smaller than the optimal route FD. 4. If the optimal route fails, view the topology table. If a feasible successor exists, you can directly use it as the new optimal route (the route remains in the Passive state). If no feasible successor exists, query the route from all the OSPF neighbors (the route changes to Active ). If the router does not receive all the Query Reply packets, it will not be able to calculate the new route, and the route will remain Active for a long time. This status is called Stuck in Active (Stuck in active State) -- SIA status (3 minutes by default). The processing of the SIA status is improved in Versions later than IOS 12.3, that is, an SIA Query is sent in the Active Timer 90S to confirm the connectivity with this neighbor. If the connection confirmation message is received but the Reply message is not received, after 90 s, re-Send the SIA Query confirmation. After 7 consecutive times, the message will not be sent. After Active timer (3 minutes), delete the route.