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

Publication Date:  2015-06-09 Views:  969 Downloads:  0
Issue Description
Figure 1-1 Networking diagram

As shown in the figure, 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.
Handling Process
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.146
isis enable 1
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.
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.
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.