Supported Platforms
hello-padding
Syntax
Hierarchy Level
Release Information
Statement introduced in Junos OS Release 8.0.
Statement introduced in Junos OS Release 9.0 for EX Series switches.
Statement introduced in Junos OS Release 12.1 for the QFX Series.
Description
Configure padding on hello packets to accommodate asymmetrical maximum transfer units (MTUs) from different hosts.
This helps to prevent a premature adjacency Up state when one routing device’s MTU does not meet the requirements to establish the adjacency.
As an OSI Layer 2 protocol, IS-IS does not support data fragmentation. Therefore, maximum packet sizes must be established and supported between two routers. During adjacency establishment, the IS-IS protocol makes sure that the link supports a packet size of 1492 bytes by padding outgoing hello packets up to the maximum packet size of 1492 bytes.
This is the default behavior of the Junos OS IS-IS implementation. However, Junos OS provides an option to disable hello padding that can override this behavior.
There are four types of hello padding:
- Adaptive padding—On point-to-point connections, the hello packets are padded from the initial detection of a new neighbor until the neighbor verifies the adjacency as Up in the adjacency state type, length, and value (TLV) tuple. If the neighbor does not support the adjacency state TLV, then padding continues. On LAN connections, padding starts from the initial detection of a new neighbor until there is at least one active adjacency on the interface. Adaptive padding has more overhead than loose padding and is able to detect MTU asymmetry from one side of the connection. This one-sided detection can result in generation of extra link-state PDUs that are flooded throughout the network. Specify the adaptive option to configure enough padding to establish an adjacency to neighbors.
- Disabled padding—Padding is disabled on all types of interfaces for all adjacency states. Specify the disable option to accommodate interfaces that support less than the default packet size of 1492 bytes.
- Loose padding (the default)—The hello packet is padded from the initial detection of a new neighbor until the adjacency transitions to the Up state. Loose padding might not be able to detect certain situations such as asymmetrical MTUs between the routing devices. Specify the loose option to configure enough padding to initialize an adjacency to neighbors.
- Strict padding—Padding is done on all interface types and for all adjacency states, and is continuous. Strict padding has the most overhead. The advantage is that strict padding detects MTU issues on both sides of a link. Specify the strict option to configure padding to allow all adjacency states with neighbors.
Options
adaptive—Configure padding until the neighbor adjacency is established and active.
disable—Disable padding on all types of interfaces for all adjacency states.
loose—Configure padding until the state of the adjacency is initialized.
strict—Configure padding for all adjacency states.
Required Privilege Level
routing—To view this statement in the configuration.
routing-control—To add this statement to the configuration.
