Networking
CCNA tutorials, Cisco guides, and enterprise networking insights for IT professionals.
What is DUAL?
The diffusing update algorithm (DUAL) is the algorithm used by EIGRP routing protocol to make sure that a given route is recalculated globally whenever it might cause a routing loop. It is guarantees loop-free and backup paths throughout the routing domain. EIGRP store all available backup routes using diffusing update algorithm and then adapt the...
How to Calculate EIGRP Metric
EIGRP automatically calculates the routing table metric to choose the best path. But it is important that the network administrator understands how to calculate EIGRP metric. The default formula for the composite metric is: [k1 x Bandwidth + k3 x delay] x 256 By default K1 and K3 is equal to 1 so the formula...
Introduction to Delay Metric
A network engineer at a Tier 1 ISP spent three weeks troubleshooting intermittent VoIP call drops before discovering the root cause: an incorrectly weighted delay metric in their OSPF routing tables. The fix took nine minutes. How Delay Metrics Dictate Traffic Flow in Modern Networks Delay metrics measure the time it takes for a packet...
Introduction to Bandwidth Metric
A network engineer stares at a traceroute that defies logic — traffic flows across a 10 Mbps satellite link while a 1 Gbps fiber path sits idle. The root cause isn’t a hardware fault. It’s a misconfigured bandwidth metric. Across every major routing protocol and QoS framework, the bandwidth metric quietly dictates which path packets...
EIGRP Composite Metric – Exclusive Explanation
A composite metric is a number calculated based on several different components that determine the preferred route. By default, EIGRP uses bandwidth and delay to calculate the preferred path to a network. The reliability and load can also be used, but are not recommended, because they typically result in a frequent recalculation of the topology...
What is EIGRP Topology Table
EIGRP uses a neighbor table, topology table, and IP routing table. The neighbor table maintains a state of neighbors. The topology table is used to store information about all known routes received from all neighbors. EIGRP Update messages send the routers’ EIGRP topology tables. The EIGRP topology table is a database of possible routes. Each...
Introduction to EIGRP Neighbor Adjacencies
Before exchanging any EIGRP update packets between routers, EIGRP must first discover its neighbor. EIGRP neighbor is an adjacent router running EIGRP on directly connected networks. EIGRP Hello packets are used to establish and maintain neighbor adjacencies. Several parameters between the two routers must match to become neighbors; for example, the same autonomous system number...
Examine EIGRP in the Routing Table
We can verify the EIGRP in the routing table using the “show ip route” command. It is important to verify the information in the routing table to ensure that it is populated as estimated, based on configurations entered. The automatic summarization is enabled by default before in the ISO 15. It is important to know...
Introduction to EIGRP Neighbors
EIGRP-enabled routers to establish neighbor adjacencies with other EIGRP-enabled routers by exchanging EIGRP Hello packets. Without establishing neighbor adjacencies routers cannot send or receive any updates. Using the “show ip eigrp neighbors” command, we can examine the neighbor’s table and verify EIGRP adjacencies. Figure 1 illustrates the output of the “show ip eigrp neighbors” command....
How to Configure EIGRP Passive-Interface
A passive interface is a feature used across routing protocols that stops routing updates from being sent and received on a specific interface, though the exact mechanics vary from one protocol to another. In EIGRP specifically, configuring the passive-interface command stops the router from sending outgoing hello packets on that interface. Because EIGRP neighbor adjacencies...