EIGRP supports IPv4 classless addressing and utilizes the DUAL algorithm in order to create the routing table. Both the algorithm and data structure (Neighbor Table & Topology Table) will be analyzed below:
EIGRP routers obtain information about the state of the adjacent neighbors and their IP addresses. Every time new neighbors are discovered their IP address and interface are recorded and stored in the neighbors’ table (data structure). While the neighbor sends Hello packets, it also advertises the Hold Time to determine whether the neighbor is operational and reachable. It must be noted that the ASN (Autonomous System Number), Subnet Number and K values must be identical in order for the neighbor adjacency to be formed. Hello packets are sent to the multicast address every 5 seconds on LAN interfaces & every 60 seconds on WAN interfaces to verify that the neighbor relationship is still active. If the Hold Time Interval passes (hold-down timer by default is 15 seconds) due to the fact that a Hello packet wasn’t heard within this, the DUAL algorithm is forced to run taking into account the topology changes. Furthermore, the neighbor table contains essential information for the RTP (Reliable Transport Protocol) mechanism in order to pair acknowledgements with their corresponding data packets. It must be stressed that round trip timers are stored in the neighbor table in order to evaluate an optimal retransmission interval (Graziani & Jonson, 2008).
EIGRP uses the DUAL (Diffusing Update ALgorithm) or else DUAL FSM (finish-state machine) which ensures that each route will be loop-free calculated in order for routing loops to be avoided. This algorithm responds promptly in changes that might occur in the routing topology and adjusts dynamically the routing tables. The factors that contribute in the loop-free routes mechanism are being analyzed below (Xu, Dai & Garcia-Luna-Aceves, 1997):
EIGRP topology table includes all learned routes to a destination advertised by neighboring routers. Specifically, the topology table stores routes and their metrics, Successors and Feasible Successors as well as locally connected subnets. It must be noticed that routes in the topology table are usable by the router only when they are active and inserted into the routing table or have a higher AD than an equivalent path. For every reachable network, the topology table contains the total delay, reliability and path loading, the lowest bandwidth on the path (the weakest link), the feasible and reporting distance and finally the route source (Graziani & Jonson, 2008).
Convergence starts when two routers become neighbors. This dynamic learning happens through the exchange of hello packets (default hello timer is 5 seconds on high-bandwidth links and 60 seconds on slower links). The outcome of this neighbor discovery is the creation of the neighboring tables with all the additional features as described in previous sections.
At that point the neighboring routers exchange routing information and build their corresponding topology tables. In a next step they employ the DUAL algorithm to calculate the feasible and reported distances, and of course the successor and feasible successor routers. The latter routes may exist in the case the feasibility condition is met, thus providing loop-free alternatives to the successor route.
The feasible successor routes and their existence is utmost significant to the EIGRP convergence process. When a successor (primary route) fails, then the EIGRP process (Sankar & Lancaster, 2014):
The aforementioned convergence process poses a threat to the scaling of an EIGRP network in an arbitrary way. When the number of routers in an EIGRP network grows to the number of hundreds, then the stuck in active situation may bring the network to its knees. In that case, a strict design must be implemented both in organizational structure and in route summarization.
Summarizing all the above points, in order for a network designer to make EIGRP convergence quicker he must: