EIGRP is an advanced distance vector routing protocol that has to establish a neighbor relationship before updates are sent. Because of this the first thing we’ll have to do is check if the neighbor adjacency is working properly. If this is the case we can continue by checking if networks are being advertised or not. In this lesson I’ll show you everything that can go wrong with EIGRP, how to fix it and in what order. Let’s get started with the neighbor adjacency!
There are a number of items that cause problems with Enhanced Interior Gateway Routing Protocol neighbor adjacencies:
- Uncommon subnet: EIGRP neighbors with IP addresses that are not in the same subnet.
- K value mismatch: By default bandwidth and delay are enabled for the metric calculation. We can enable load and reliability as well but we have to do it on all EIGRP routers.
- AS mismatch: The autonomous system number has to match on both Enhanced Interior Gateway Routing Protocol routers in order to form a neighbor adjacency.
- Layer 2 issues: Enhanced Interior Gateway Routing Protocol works on layer 3 of the OSI-model. If layer 1 and 2 are not working properly we’ll have issues with forming a neighbor adjacency.
- Access-list issues: It’s possible that someone created an access-list that filters out multicast traffic. EIGRP by default uses 18.104.22.168 to communicate with other EIGRP neighbors.
- NBMA: Non Broadcast Multi Access networks like frame-relay will not allow broadcast or multicast traffic by default. This can prevent EIGRP from forming EIGRP neighbor adjacencies.
Enhanced Interior Gateway Routing Protocol (EIGRP) is an interior gateway protocol suited for many different topologies and media. In a well designed network, EIGRP scales well and provides extremely quick convergence times with minimal network traffic.
Enhanced Interior Gateway Routing Protocol Theory of Operation
Some of the many advantages of EIGRP are:
- very low usage of network resources during normal operation; only hello packets are transmitted on a stable network
- when a change occurs, only routing table changes are propagated, not the entire routing table; this reduces the load the routing protocol itself places on the network
- rapid convergence times for changes in the network topology (in some situations convergence can be almost instantaneous)
Enhanced Interior Gateway Routing Protocol is an enhanced distance vector protocol, relying on the Diffused Update Algorithm (DUAL) to calculate the shortest path to a destination within a network.
Major Revisions of the Protocol
There are two major revisions of Enhanced Interior Gateway Routing Protocol, versions 0 and 1. Cisco IOS versions earlier than 10.3(11), 11.0(8), and 11.1(3) run the earlier version of EIGRP; some explanations in this paper may not apply to that earlier version. We highly recommend using the later version of Enhanced Interior Gateway Routing Protocol , as it includes many performance and stability enhancements.
A typical distance vector protocol saves the following information when computing the best path to a destination: the distance (total metric or distance, such as hop count) and the vector (the next hop). For instance, all the routers in the network in Figure 1 are running Routing Information Protocol (RIP). Router Two chooses the path to Network A by examining the hop count through each available path.