Configuring Hello Packet Padding

You can configure padding on hello packets to accommodate asymmetrical maximum transfer units (MTUs) from different routers. This help prevents a premature adjacency UP state when one router’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 1,492 bytes by padding outgoing hello packets up to the maximum packet size of 1,492 bytes.

To configure padding for hello packets, include the hello-padding statement:

There are three 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 TLV. 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 may result in generation of extra LSPs that are flooded throughout the network. Specify the adaptive option to configure enough padding to establish an adjacency to neighbors.
• 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 may not be able to detect certain situations such as asymmetrical MTUs between the routers. 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.

For a list of hierarchy levels at which you can include this statement, see the statement summary sections for this statement.

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