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TCC_2
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Introduction

How to configure EIGRP bandwidth usage over Frame Relay and low speed links.

Core Issue

In order to prevent Enhanced Interior Gateway Routing Protocol (EIGRP) updates from overwhelming the interface, Cisco IOS  Software provides control over how much bandwidth it can consume on the physical interface. By default, EIGRP limits itself to using no more than 50 percent of the interface bandwidth. Although, this is not very significance on high speeds like Ethernet, it becomes critical on low speed links, especially on Nonbroadcast Multiaccess Address (NBMA) networks like Frame Relay. The access interface bandwidth and the Permanent Virtual Connection (PVC) capacity may be very different. A secondary benefit is that it allows the network administrator to ensure that some bandwidth remains for passing user data, even when EIGRP is very busy.

Resolution

You must consider the three important rules while configuring the EIGRP bandwidth percentage on the NBMA interfaces. If these rules are not followed, EIGRP or the data packets can be lost. These are the critical rules:

  • The traffic that EIGRP is allowed to send on a single Virtual Circuit (VC) cannot exceed the capacity of that VC.
  • The total EIGRP traffic for all virtual circuits cannot exceed the access line speed of the interface.
  • The bandwidth allowed for EIGRP on each Virtual Circuit must be the same in each direction.

These recommendations are described in terms of configuring the interface "bandwidth" parameter (with EIGRP being able to use 50 percent of that bandwidth by default). If the interface bandwidth configuration cannot be changed because of routing policy considerations, or for any other reason, the bandwidth-percent command should be used to control the EIGRP bandwidth. On low-speed interfaces, raising the available bandwidth for EIGRP above the default of 50 percent is advisable in order to improve convergence.

As a best practice Auto Summarization feature should be disabled. Configure no auto-summary command in order to disable auto summary.

Example

For example, this configuration allows IP-EIGRP AS 109 to use 42Kbps (75% of 56Kbps) on Serial 0:

interface Serial 0

bandwidth 56

ip bandwidth-percent eigrp 109 75

bandwidth percent {value} - Specifies bandwidth allocation as a percentage of the underlying link rate.

Reference

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