IP Routing

Document ID: 21324

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Contents

Introduction
Prerequisites
      Requirements
      Components Used
      Conventions
Main Troubleshooting Flowchart
Neighbor Check
Redistribution Check
Route Check
Reasons for Neighbor Flapping
      EIGRP Neighbors are not Recognized
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Related Information

Introduction

This document provides troubleshooting information for common problems with Enhanced Interior Gateway Routing Protocol (EIGRP). For more information, or to go to the next flowchart, refer to the links provided in this section.

If you have the output of a show interfaces serial , show ip eigrp neighbors , show tech-support , or a show ip eigrp topology command from your Cisco device, you can use Output Interpreter ( registered customers only) to display potential issues and fixes.

Prerequisites

Requirements

Readers of this document should have a good understanding of how EIGRP works and a good knowledge of Configuring EIGRP.

Components Used

This document is not restricted to specific software and hardware versions.

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.

Conventions

For more information on document conventions, refer to the Cisco Technical Tips Conventions.

Main Troubleshooting Flowchart

In order to troubleshoot EIGRP, use this flowchart, starting at the box marked Main. Depending on the symptoms, the flowchart might refer to one of the three flowcharts later in this document or to other relevant documents on Cisco.com. There are some problems that might not be resolvable here. In these cases, links are provided to Cisco Technical Support. In order to open a service request, you must have a valid service contract.

Neighbor Check

Redistribution Check

Route Check

Reasons for Neighbor Flapping

The stability of the neighbor relationship is of primary concern. A failure in the neighbor relationship is accompanied by increased CPU and bandwidth utilization. EIGRP neighbors can flap for these reasons:

• Underlying link flaps. When an interface goes down, EIGRP takes down the neighbors that are reachable through that interface and flushes all routes learned through that neighbor.

• Misconfigured hello and hold intervals. The EIGRP hold interval can be set independently of the hello interval if you issue the ip hold-time eigrp command. If you set a hold interval smaller than the hello interval, it results in the neighbors flapping continuously. Cisco recommends that the hold time be at least three times the hello interval. If the value is set less than 3 times the hello interval, there is the chance for link flapping or neighborship flapping.

• Loss of hello packets: Hello packets can be lost on overly congested links or error-prone links (CRC errors, Frame errors, or excessive collisions).

• Existence of unidirectional links. A router on a unidirectional link can be able to receive hello packets, but the hello packets sent out are not received at the other end. The existence of this state is usually indicated by the retry limit exceeded messages on one end. If the routers generating retry limit exceeded messages has to form neighborship, then make the link bidirectional for both unicast and multicast. In case tunnel interfaces are used in the topology make sure that the interfaces are advertised properly.

• Route goes stuck-in-active. When a router enters the stuck-in-active state, the neighbors from which the reply was expected are reinitialized, and the router goes active on all routes learned from those neighbors.

• Provision of insufficient bandwidth for the EIGRP process. When sufficient bandwidth is not available, packets can be lost, which causes neighbors to go down.

• Bad serial lines.

• Improperly set bandwidth statements.

• One-way multicast traffic.

• Stuck in active routes.

• Query storms.

EIGRP Neighbors are not Recognized

The EIGRP neighbor relationship is not established over the multipoint GRE tunnel if there is an incorrect NHRP association in the spoke. Next Hop Resolution Protocol (NHRP) is used to discover the addresses of other routers and networks behind the routers that are connected to a nonbroadcast multiaccess (NBMA) network. When a network statement under Eigrp covers both the physical interface and tunnel interface (tunnel interface ip address and physical interface ip address belong to the same major class) and if the phyiscal interface is the source of the tunnel, then the both interfaces have to be separately advertised in the Eigrp to avoid issues with DMVPN. The best practice is to advertise the interfaces using specific subnet advertisements.

This issue can be resolved when you clear the NHRP associations with this command:

Router#clear ip nhrp

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Related Information

• EIGRP Technology Support Page
• Technical Support & Documentation - Cisco Systems