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Troubleshooting Guide

CloudEngine 16800, 12800, 12800E, 8800, 7800, 6800, and 5800 Series Switches

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Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).
CE12800 and Cisco ASR9000 Fail to Set Up an IS-IS Neighbor Relationship Due to Improper MTU Configuration

CE12800 and Cisco ASR9000 Fail to Set Up an IS-IS Neighbor Relationship Due to Improper MTU Configuration

Keywords

MTU, IS-IS, failure to set up a neighbor relationship, CE12800, switch

Abstract

CE12800 and Cisco ASR9000 fail to set up an IS-IS neighbor relationship due to improper MTU configuration.

Problem Description

Figure 14-15 Networking diagram

The CE12800 and ASR9000 are connected through a bundled link, and IS-IS is configured on their interconnected interfaces.

Fault Symptom

The CE12800 and ASR9000 are connected through a bundled link, and IS-IS is configured on their interconnected interfaces. The two devices fail to set up an IS-IS neighbor relationship.

Procedure

  1. Check the status of the interconnected interfaces between the CE12800 and ASR9000. Their aggregate interfaces have negotiated with each other successfully and can ping each other. This indicates that the interfaces are working normally.
  2. Check the IS-IS configuration on the two devices, including the cost type and network entity name. All the parameters are set correctly, so the problem is not caused by the IS-IS parameter settings.
  3. Obtain Hello packet information on the interconnected interfaces. The packet information proves that the problem is caused by the MTU settings on the two ends. Check the configuration files on the two devices.

    # IS-IS configuration on the CE12800

    #
    isis 1
     is-level level-2
     cost-style wide
     network-entity 49.X.X.X.X.00
    #
    interface Vlanif1001
     mtu 9126
     ip address X.X.X.150 255.255.255.252
     isis enable 1
    #
    return

    # IS-IS configuration on the ASR9000

    #
    router isis XXXX
     net 49. X.X.X.X.00
     log adjacency changes
     address-family ipv4 unicast
     metric-style wide
     ispf level 2
    #
    interface TenGigE0/1/0/0
     mtu 9126
     circuit-type level-2-only
     address-family ipv4 unicast

The MTU value configured on the CE12800 applies to Layer 3 packets, whereas the MTU value configured on the ASR9000 applies to Layer 2 packets by default. On the CE12800, the MTU is set to 9126 bytes. Counting the 14-byte Ethernet frame header, the maximum length of a Layer 2 Ethernet frame will be 9140 bytes, exceeding the MTU value on the remote interface. Therefore, the IS-IS Hello packets are dropped on the remote ASR9000 router.

Solution

This problem can be solved using either of the following methods:

  1. Set the MTU value on the CE12800's interface to 9126 (MTU for Layer 2 Ethernet frames on the ASR9000) – 14 (Ethernet frame header length) = 9112.
  2. Configure the isis small-hello command on the CE12800 and the hello-padding disable level 2 command on the ASR9000 to disable MTU check for the Hello packets exchanged between the two devices.

Suggestion

MTU configuration commands provided on devices from different vendors define MTU differently. Consider the difference when you connect a Huawei device to a non-Huawei device or making a network plan.

Translation
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Updated: 2020-01-07

Document ID: EDOC1000060766

Views: 608608

Downloads: 2951

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