IS IS Hello Padding
Table of Contents
Introduction
MTU of adj ISIS routes have to match to be fully adj. Padding is a method to ensure there is not MTU mismatch.
Hello Padding
- By default each hello PDUs are padded to the max MTU-1.
- By padding hello message, MTU mismatch issue can be detected before the adj is established and prevent synchronization issue later on.
- To disable use the command no hello padding
- Padding can be disabled for a multi-point, point-to-point or both circuit types in isis router configuration mode or interface.
- Cisco recommends to disable hello padding for performance improvement.
- On IOS, when padding is disabled, the initial hello PDUs are still padded to find out the MTU values, but subsequent hellos are not padded.
- On IOS-XR, padding can be disabled or set to sometimes. Sometimes will use hello padding for adj formation only, while disabled will not used it at all.
- One way to fix MTU issue is my changing the interface MTU or the clns MTU.
4#sh clns neighbors
System Id Interface SNPA State Holdtime Type Protocol
R5 Gi1/0 ca04.14ee.001c Init 27 L1 IS-IS <--init issue
R3 Gi0/0 ca03.14ed.0008 Up 9 L1 IS-IS
Relevant IOS Commands
no hello padding
Disables hello padding, can be disable per specific circuit type of both under the isis config mode.
R1(config-router)# no hello padding [multi-point | point-to-point ]
no isis hello padding
Disables hello padding, can be disable per specific circuit type of both under the interface config mode.
R1(config-router)# no hello padding
clns mtu
Modifies the ISIS mtu that it used for padding and initial adj.
R1(config-if)# clns mtu
show clns interface
show clns interface
Additional Resources
MTU Mismatch Problem in IS-IS - Cisco