EIGRP algorithm

EIGRP algorithm
3.1.6

This page will describe the DUAL algorithm, which results in the exceptionally fast convergence of EIGRP.
The sophisticated DUAL algorithm results in the exceptionally fast convergence of EIGRP. To better understand convergence with DUAL, consider the example in Figure . Each router has constructed a topology table that contains information about how to route to destination Network A.
Each topology table identifies the following information:

• The routing protocol or EIGRP
• The lowest cost of the route, which is called feasible distance (FD)
• The cost of the route as advertised by the neighboring router, which is called reported distance (RD)

The Topology column identifies the primary route called the successor route (successor), and, where identified, the backup route called the feasible successor (FS). Note that it is not necessary to have an identified feasible successor.
The EIGRP network follows a sequence of actions to allow convergence between the routers, which currently have the following topology information:

• Router C has one successor route by way of Router B.
• Router C has one feasible successor route by way of Router D.
• Router D has one successor route by way of Router B.
• Router D has no feasible successor route.
• Router E has one successor route by way of Router D.
• Router E has no feasible successor.

The feasible successor route selection rules are specified in Figure .
The following example demonstrates how each router in the topology will carry out the feasible successor selection rules when the route from Router D to Router B goes down:
In Router D: 

• Route by way of Router B is removed from the topology table.
• This is the successor route. Router D has no feasible successor identified.
• Router D must complete a new route computation.

In Router C:

• Route to Network A by way of Router D is down.
• Route by way of Router D is removed from the table.
• This is the feasible successor route for Router C.

In Router D:  

• Router D has no feasible successor. It cannot switch to an identified alternative backup route.
• Router D must recompute the topology of the network. The path to destination Network A is set to Active.
• Router D sends a query packet to all connected neighbors to request topology information.
• Router C does have a previous entry for Router D.
• Router D does not have a previous entry for Router E.

In Router E:

• Route to Network A through Router D is down.
• The route by way of Router D is removed from the table.
• This is the successor route for Router E.
• Router E does not have a feasible route identified.
• Note that the RD cost of routing by way of Router C is 3. That is the same cost as the successor route by way of Router D.

In Router C: 

• Router E sends a query packet to Router C.
• Router C removes Router E from the table.
• Router C replies to Router D with a new route to Network A.

In Router D:

• Route status to destination Network A is still marked as Active. The computation has not been completed yet.
• Router C has replied to Router D to confirm that a route to destination Network A is available with a cost of 5.
• Router D still waits for a reply from Router E.

In Router E:

• Router E has no feasible successor to reach destination Network A.
• Router E, therefore, tags the status of the route to destination network as Active.
• Router E has to recompute the network topology.
• Router E removes the route by way of Router D from the table.
• Router E sends a query to Router C, to request topology information.
• Router E already has an entry by way of Router C. It is at a cost of 3, the same as the successor route.

In Router E: 

• Router C replies with an RD of 3.
• Router E can now set the route by way of Router C as the new successor with an FD of 4 and an RD of 3.
• Router E replaces the Active status of the route to destination Network A with a Passive status. Note that a route will have a Passive status by default as long as hello packets are received. In this example, only Active status routes are flagged.

In Router E: 

• Router E sends a reply to Router D to inform it of the Router E topology information.

In Router D:

• Router D receives the reply packed from Router E.
• Router D enters this data for the route to destination Network A by way of Router E.
• This route becomes an additional successor route as the cost is the same as routing by way of Router C and the RD is less than the FD cost of 5.

Convergence occurs among all EIGRP routers that use the DUAL algorithm.
This page concludes this lesson. The next lesson will discuss the configuration of EIGRP. The first page will explain how EIGRP is configured.