Enhanced Interior Gateway Routing Protocol (EIGRP) is a unique protocol considering it uses a hybrid approach, combining distance vector and link-state characteristics. Combining these features makes EIGRP very robust and allows for fast convergence, even in large topologies.
The first thing a network designer should consider is that EIGRP is a Cisco proprietary protocol, so it can be used only in environments that contain Cisco devices. This protocol is not suitable in a multi-vendor architecture. Similar to RIPv2, EIGRP is a classless protocol that allows for VLSM. Another similarity between the two protocols is their automatic summarization behavior, but this can be disabled easily.
The algorithm that EIGRP uses is called the Diffusing Update Algorithm (DUAL). DUAL is the engine that makes EIGRP such a powerful protocol. DUAL operates based on a topology table that contains all the possible prefixes and information about how to reach those prefixes. The topology table is used to identify the best prefix, called the successor, and puts this route in the routing table. After determining the best route in the topology table, EIGRP identifies the second best route, called the feasible successor. Feasible successors are not installed in the routing table until the best route is lost. At that moment, the next best successor in the topology table is installed in the routing table almost immediately because there is no need for other computations. This is the reason EIGRP provides such fast convergence times.
EIGRP is the only IGP that can perform unequal cost load balancing across different paths, as illustrated in Figure 7.6 below. This is accomplished using the “variance” command, which allows defining a tolerance multiplier that can be applied to the best metric and that will result in the maximum allowed metric.
Figure 7.6 – EIGRP Unequal Cost Load Balancing
In Figure 7.6, there are two routes with a cumulative metric of 100 to a destination and a route with a cumulative metric of 200 to the same destination. By default, EIGRP performs only equal cost load balancing, so it will send traffic across only the first two links, which have the best metric of 100. If you wanted to send traffic over the third link as well, you would set the variance to 2, meaning the maximum allowed metric is 2 times the lowest metric, which equals 200. Traffic will be sent proportionally to the metric, meaning for each packet sent over the third link, two packets are sent over the first two links, because their metric is better.
EIGRP creates neighbor relationships with adjacent routes and exchanges information with them using the Reliable Transport Protocol (RTP). This protocol ensures that neighbors can exchange information in a reliable manner.
Note: Do not confuse the EIGRP-specific RTP with the Real-time Transport Protocol used in VoIP environments.
By default, EIGRP calculates route metrics based on bandwidth and delay but it can use other parameters in the calculation, including the following:
- Load
- Reliability
Enabling the metric calculation based on load and reliability is not recommended by Cisco because the network might become unstable.