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ISIS Neighbor Relationship failed

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Hi there,

I'll introduce you a case about the IS-IS neighbor relationship establishing failure caused by the MTU value.

Hope it could provide you a method during the trouble shooting.

Problem Description

A CE12800 and another vendor's router are connected and have IS-IS configured. After the configuring completed, the two devices cannot establish an IS-IS neighbor relationship. The corresponding configuration list as below.

CE12800 configuration:

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.146
  isis enable 1

Other vendor router's configuration:

router isis XXXX
net  49.X.X.X.X.00
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

Root Cause

The MTU value configured on a VLANIF interface of a CE switch applies to Layer 3 packets sent on the VLANIF interface, whereas the MTU value configured on an interface of a Cisco device applies to Layer 2 packets by default.

In this example, the MTU value configured on VLANIF 1001 of a CE12800 is set to 9126 bytes. Counting an Ethernet frame header, the maximum length of a Layer 2 Ethernet frame will be 9140 bytes, exceeding the MTU value of the remote interface. As a result, the two devices cannot exchange IS-IS Hello packets, in which situation the IS-IS neighbor relationship cannot be establised correctly.


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

  • Set the MTU value on the CE12800's VLANIF 1001 to 9112 bytes, namely, 9126 (MTU for Layer 2 Ethernet frames on the remote peer) - 14 (Ethernet frame header length) = 9112.

  • Run the isis small-hello command on the CE12800's VLANIF 1001 to configure VLANIF 1001 to send Hello packets without the padding field and run the hello-padding disable level 2 command on the  remote interface. Subsequently, the two devices do not check the MTU values of exchanged IS-IS Hello packets and then can establish an IS-IS neighbor relationship.

That's all for this case.

If you have any further questions, kindly comment below, and I'll help to resovle it.


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