CISCO EIGRP DUAL Algorithm

Enhanced Interior Gateway Routing Protocol (EIGRP) is a advanced distance vector routing protocol proprietary to CISCO. Highly valued for its ease of deployment and fast convergence, EIGRP is used in many large Enterprise networks. EIGRP maintains all of the advantages of distance-vector protocols and having good features set for selecting loop free paths too.

EIGRP scales effectively in a well-designed network and provides extremely quick convergence times with minimal network traffic. EIGRP advantages include:
• Consumes low network resources as compared to OSPF
• Transmits only partial updates not the full routing table
• Rapid convergence times for changes in the network topology

Deep dive to get better understanding of Cisco EIGRP DUAL Algorithm

What is Reported Distance?
As depicted in Figure 1, A has three directly connected neighbors to reach E. The distance reported by A’s neigbors to reach E is known as Reported Distance. So A has three reported distance from it’s neighbors as below mentioned:-
• B can reach E with a cost of 10
• C can reach E with a cost of 10
• D can reach E with a cost of 30

What is Feasible Distance?
The total cost of each link to reach from A to E is known as feasible distance. As depicted in figure 2, A has three feasible distance to reach E with below mentioned cost.
• A can reach to E via B with cost of 20
• A can reach to E via C with cost of 25
• A can reach to E via D with cost of 45
The best out of the three feasible distances will become the successor. So as per the above output, B is nominated as the successor.

How to select the feasible successor or loop free alternate? After selecting successor, DUAL will look for the Reported Distance of the neighbors and check which one satisfies the Feasibility Condition; Reported Distance < Feasible Distance. As per the above two outputs we can conclude that
• C can reach E with a cost of 10(Reported Distance), so C reported distance (10) < feasible distance (20). This path is a loop free.
• D can reach E with a cost of 30(Reported Distance), so D reported distance (30) > feasible distance (20). This doesn’t satisfies the condition of selecting feasible successor (Reported Distance < Feasible Distance). So DUAL will mark this path as loop.

So now A has marked B as its successor and C has its feasible successor. Let’s assume now link between A and B is down as per figure 3. Now EIGRP will examine the available paths to E and declared C as a successor (best path to reach E) which was earlier selected as feasible successor (Loop Free Path). Now EIGRP will again look for its feasible successor and as per the Feasibility Condition, Reported Distance from D is 30 which is greater than the Feasible Distance. So EIGRP will consider D as loop path and didn’t qualify it for the feasible successor.

Now assume, link between A and C is down as shown in Figure 4. Now EIGRP will look for loop free path but unfortunately, as shown in Figure 3, no loop free path is available. However, A has a neighbor D might have a loop free path to E. So A will put E into Active State and query to D, D sends an reply to its query that it has a loop free path available to E. Once A will receive the reply from D, it beings start using that path for E.

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