Q&A: CloudEngine - A CE12800 Cannot Establish an IS-IS Neighbor Relationship with a Cisco ASR9000

250 0 0 0

Problem Description

A CE12800 and a Cisco ASR9000 are connected and have IS-IS configured. After the configurations are complete, the two devices cannot establish an IS-IS neighbor relationship. The following lists configurations on the two devices.

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.146 255.255.255.252
  isis enable 1
  ...
#
ASR9000 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 ASR9000's interface. As a result, the two devices cannot exchange IS-IS Hello packets and so cannot establish an IS-IS neighbor relationship.

Solution

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 ASR9000) - 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 ASR9000's TenGigE0/1/0/0. 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.
  • x
  • convention:

Reply

Reply
You need to log in to reply to the post Login | Register

Notice Notice: To protect the legitimate rights and interests of you, the community, and third parties, do not release content that may bring legal risks to all parties, including but are not limited to the following:
  • Politically sensitive content
  • Content concerning pornography, gambling, and drug abuse
  • Content that may disclose or infringe upon others ' commercial secrets, intellectual properties, including trade marks, copyrights, and patents, and personal privacy
Do not share your account and password with others. All operations performed using your account will be regarded as your own actions and all consequences arising therefrom will be borne by you. For details, see " Privacy."
If the attachment button is not available, update the Adobe Flash Player to the latest version!
Login and enjoy all the member benefits

Login and enjoy all the member benefits

Login