How to troubleshoot the following:

EIGPR operations

The Interior Gateway Routing Protocol is progressed to a new version known as Enhanced Interior Gateway Routing Protocol (EIGRP). This protocol mainly focuses on the routing of networks used. It is a faster convergence protocol which is generally operated by the Diffusing Update Algorithm. The invention of the DUAL Algorithm was attained by Dr. J.J. Gracia-Luna Aceves of SRI International. EIGRP can easily perform the partial updates that are required for routing. EIGRP integrates seamlessly with IGRP that is EIGRP was designed in such a way that it can be translatable with IGRP. This is so because the metrics of EIGRP are 256 times larger than those of IGRP. Thus, EIGRP can be considered as an improvement to the IGRP routing protocols. No prior broadcast or any periodic updates are required as EIGRP consumes less bandwidth. The leading features of EIGRP are Distance Vector routing protocol and a Link State routing protocol which makes EIGRP a hybrid protocol. Automatic redistribution of routes can be achieved using EIGRP as it can redistribute its routes and metrics into other routing protocols and moreover it can receives redistribution from other routing protocols also. EIGRP supports multiple network layer protocols and VLSM subnets.

Components of EIGRP

The main components of EIGRP can be numbered as four namely, Neighbor Tables, Topology Tables, Route States, and Route Tagging. The neighboring process, dynamically find other routers that are running EIGRP and forms neighbor relationships with them. When the unreachable or inoperative state of neighbors is discovered by the devices, small hello packets are sent periodically to each other to maintain neighbor relationship. To deliver these EIGRP packets, reliable transport protocols are established which uses DUAL Finite State machine.

EIGRP routers have knowledge about only those routes which are advertised by directly adjacent neighbors as EIGRP is a distant vector routing protocol. Thus, in case if a network determining the destination of the router is eliminated from the EIGRP then a different route has to be traced to locate that destination. In attempt to overcome this problem, a new advancement is done by introducing Feasible Successor Routes as a feature of EIGRP. This feature helps EIGRP topology to selects the best possible path other than the actual path of the router. In other words, feasible successor is a neighbor router used for packet forwarding which is a least- cost path to a destination such that it does not include in routing loop. The lowest sum of reported distance in addition with the cost to get to the advertising route can be considered as the feasible distance. Since, each destination is advertised by EIGRP router which can be reached as a route with an attached metric called route's reported distance. The feasible distance can be related as the lowest sum of reported distance in addition with the cost to get to the advertising router. Also, each destination which can be reached as a route with an attached metric called route's reported distance is advertised by EIGRP router. In short best path can be summarized as sum of best advertised metric from all neighbors and the link cost to the best neighbor. If no feasible successor exists, then queries are sent back to the network and diffusing computation is performed to select another feasible route.


Routing can be classified in three types namely, Summary Routes, Internal Routes, and External Routes. Routing contains RTP information such as sequence number, transmission list of packets, and round trip timers optimize retransmission interval. In routing, the traces of each neighbor in EIGRP are recorded. There exists one neighbor table for each network protocol. A hold down time is send by the neighbors. If neighbor is not heard from within this hold down time, the topology table is changed via DUAL. Each destinations advertised by each neighboring routers is contained in this routing process. Each topology table entry holds destination address, list of neighbors used to reach the destination. The advertised metric for each destination is stored for each neighbor.

The two states available for routing are active and passive. In active state, re-computation is being performed whereas in passive state, there is no re-computation being performed. It is to be noted that a destination never goes onto the active state if there exists a feasible successor always. For the routes in the active state, the routing table information cannot be changed. Once all neighbors have replied the topology table then the entry for the destination returns to the passive state and the router may then select a feasible successor. Re-computation occurs only when there is no feasible successor route. In case if a neighbor which is the only feasible successor to a destination doesn't work out then, the entire neighbor's routes enter the active state and trigger route re-computation. In re-computation process a query packet is sent to all neighboring routes and neighbors sends a reply that it has a feasible successor or a query packet to indicate if it is participating in the re-computation. Internal routes come from neighbors with the same EIGRP AS number or from directly attached interfaces over which EIGRP runs whereas external routes come from other routing protocols or from static routes and are attached with the information such as Router ID of the router that distributed the route , AS number of the destination, Configurable administrator tag, ID of the external protocol, Metric from the external protocol , Bit flags for default routing.

Basic Router configuration:

router(config)#router eigrp <AS number>

It enables EIGRP routing and set the Autonomous System number where running of multiple EIGRP autonomous systems on the same set of interfaces on the router is recommended.


It configures the directly connected networks that will be advertised.

show ip eigrp topology:

It shows only feasible successor routes.

show ip eigrp topology <network>

It shows all entries in the topology table for the given destination network.

show ip eigrp topology all-links

It shows all entries in the topology table.

show ip eigrp topology [active |pending| zero successors]

It shows destinations that are in the active or pending states or have zero successors.

EIGRP Stub Routing

The stub router forwards all traffic to a central or distribution router having one exit path from the routing domain. EIGRP Stub Routing have functions common to that of an OSPF stub area yet it have improved stability, reduces resource utilization and simplifies stub router configuration. The stub network cannot be a transit router for EIGRP instead it has only one EIGRP neighbor. Stub routing is commonly used in a hub and spoke network topology in which one or more end networks are connected to a remote router known as spoke that is connected to one or more distribution routers referred as hub. The only route for IP traffic to follow into the remote router is through a distributor router and the remote router is adjacent only to one or more distributor routers. But the distribution route can be connected to many more remote routers. Generally, the distributor router doesn't require sending anything more than a default route to the remote router. In a hub and spoke topology, all nonlocal traffic is forwarded to a distributor router by remote router, such that the remote router can easily avoid holding a complete routing table. This type of configuration is commonly used in WAN topologies where the distribution router is directly connected to a WAN. When all routers on the multi-access interface except the hub are configured as stub routers such as in Ethernet, ATM, Frame Relay etc. then these are supported by the EIGRP Stub Routing feature.

Only specified routes are propagated from the remote stub router to configure the distribution and remote routers to use EIGRP, and to configure only the remote route as a stub. For summaries, connected routes, redistributed static routes, external routes, and internal routes with the message inaccessible, the router responds to queries. And sends a special peer information packet to all neighboring routers to report its status as a stub router. Any neighbor that receives a packet having information such that the stub status will not query the stub router for any routes, and a router that has a stub peer will not query that peer then to send the proper updates to all peers, the stub router will be dependent on the distribution router. The remote router can access the corporate network and the internet through the distribution router only. Thus, having a full router table on the remote router would be of no use as the path to the corporate network and the internet would always be through the distribution router. In short, the larger route table would only increase the amount of memory required by the remote router. The drawback of the stub feature is that it does not prevent routes from being advertised to the remote router.

Stuck in Active Condition

The EIGRP Stub Routing feature does not automatically enable summarization on the distribution router. Yet, by summarizing and filtering routes on the distribution router, conservation of bandwidth and memory can be achieved. The remote router must send all nonlocal traffic to the distribution router regardless of destination so that the remote router needs not to receive routes that have been discovered from other networks. Only a default route should be send to the remote router if a true stub network is desired, to configure the distribution router. In most cases, the network administrator will need to configure summarization on the distribution routers. If a route is lost somewhere in the corporate network, EIGRP could send a query to the distribution router which will send a query to the remote outer even if routes are being summarized. An EIGRP stuck in active condition could occur, if a problem arrives in the communication over the WAN link between the distribution router and remote router which will cause instability elsewhere in the network. The EIGRP Stub routing feature then comes to rescue as it allows network administrator to prevent the queries from being sent to remote router.

Configuring EIGRP Stub Routing

The following commands can be used to configure a remote or distribution router for EIGRP stub routing:





router(config)#router eigrp as-number

Configures a remote or distributor router to run an EIGRP process.



Specifies the network address of the EIGRP distribution router.


Router(config-router)#eigrp stub[receive-only|connected|static|summary]

Configures a remote router as an EIGRP stub router.

Load Balancing and equal cost balancing

Load sharing occurs only when a route shows up in the forwarding table with multiple paths. EIGRP has the capability to use unequal cost load balancing in a similar way as that by IGRP. EIGRP load shares over four equal cost paths as a standard. To ensure that EIGRP has a steady view of the metrics of the network, bandwidth interface command on serial links is preferred. This can also be a helpful in making the route show up in the IP forwarding table.

Configuring EIGRP Unequal cost load balancing

This can be done in three steps:

  • Configure the bandwidth on both sides of all the interfaces involved in the load sharing group.
  • Define the lower cost metric and the highest cost metric and then compute the variance multiplier and add it to the EIGRP routing process.
  • Set the maximum paths or the traffic share variables.

The metric multiplier of the routes to be used in unequal cost load balancing is defined by the variance command whose default variance is 1 which is equal cost load balancing. With the maximum paths command to share traffic across, the router uses up to six paths. The use of maximum paths command is required to limit this number. The multiple paths that make up a single hop transport to a common destination are called load-sharing group whose default value is approximately 4. EIGRP will use equal cost load balancing if there are multiple cost paths and traffic share min is configured. By default, the command is set to be balanced where traffic will be distributed proportionally to the ratio of the metrics. For example if variance is set to be 2 and traffic is set to be balanced, the best route will transport traffic 2 times that of the worst route.


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