Troubleshooting EIGRP Chapter Summary

The following section is a summary of the major points you should be aware of in this chapter.

Enhanced Interior Gateway Protocol Overview

1. EIGRP is a Cisco-proprietary advanced Distance Vector routing protocol
2. EIGRP runs directly over IP using protocol number 88
3. EIGRP uses several different types of packets, which include:

1. Hello Packets
2. Acknowledgement Packets
3. Update Packets
4. Query Packets
5. Reply Packets

1. EIGRP supports both dynamic and static (manually configured) neighbor discovery
2. Dynamically discovered neighbors use Multicast for communication
3. Statically defined neighbors use Unicast for communication
4. EIGRP uses different Hello and Hold timers for different types of media
5. Hello timers are used to determine the interval rate EIGRP Hello packets are sent
6. The Hold timer determines the time before a router considers a neighbor down
7. By default, the Hold time is three (3) times the Hello interval
8. The EIGRP Neighbor Table is used to maintain state information about EIGRP neighbors
9. RTP is used to ensure that Update, Query and Reply packets are sent reliably
10. EIGRP uses a composite metric, which includes different variables, called K values
11. The default values for the K values are K1 = K3 = 1 and K2 = K4 = K5 = 0
12. The Diffusing Update Algorithm is at the crux of the EIGRP routing protocol
13. The best route, which is the route with the lowest Feasible Distance, is the Successor route
14. Feasible successor routes must first meet the Feasibility Condition
15. The Topology Table allows all EIGRP routers to have a consistent view of the entire network
16. The stub routing feature is most commonly used in hub-and-spoke networks
17. EIGRP supports two types of route summarization. These two types are:

3. Automatic route summarization
4. Manual route summarization

Troubleshooting Neighbor Relationships

1. The EIGRP neighbor relationship may not establish due to any of the following:

1. The neighbor routers are not on a common subnet
2. Mismatched primary and secondary subnets
3. Mismatched K Values
4. Mismatched AS Number
5. Access Control Lists are filtering EIGRP packets
6. Physical Layer Issues
7. Data Link Layer Issues
8. Mismatched Authentication Parameters

Troubleshooting Route Installation

1. Some common reasons for route installation failure include the following:

1. The same route is received via another protocol with a  lower AD
2. EIGRP summarization
3. Duplicate Router IDs are present within the EIGRP domain
4. The routes do not meet the Feasibility Condition

Troubleshooting Route Advertisement

1. There are several reasons why EIGRP might not advertise a network. These reasons include:

1. Distribute Lists
2. Split Horizon
3. Summarization

Troubleshooting Stub Routing Issues

1. Stub routing issues are typically due to basic router misconfigurations
2. Ensure that you understand what needs to be advertised
3. By default, stub routers advertise only connected and summary routes

Troubleshooting SIA Issues

1. Within the EIGRP Topology Table, entries may either be marked as Passive or as Active
2. Routes go Active when the successor is lost and a query is sent to find a feasible successor
3. If a response to a query is not received in three minutes, the route becomes SIA
4. Some of these reasons include the following:

1. The neighbor routers CPU is overloaded and it cannot respond in time
2. The neighbor router itself has no information about the lost route
3. The neighbor cannot allocate memory to process the query or build the reply packet
4. Low bandwidth links are congested and packets are being delayed
5. Queries are not received, i.e. this may due to a bad circuit or unidirectional link

1. Whenever you troubleshoot SIA errors, answer the following two questions to troubleshoot:

1. Why is the route active?
2. Why is the route stuck?

1. Use the show ip eigrp topology active command to troubleshoot SIA issues
2. Use the timers active-time command to increase route waiting time

Troubleshooting Route Redistribution Issues

1. By default, external routes redistributed into EIGRP are assigned a default metric of infinity
2. The only three exceptions to this rule are:

1. When redistributing between two EIGRP autonomous systems
2. When redistributing static routes into EIGRP
3. When redistributing connected interfaces (subnets) into EIGRP

1. If routes are not being redistributed into EIGRP, check the following while troubleshooting

1. Verify that the routes are not incorrectly filtered
2. Verify that the metric has been specified