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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:
-
hello-padding (adaptive | loose | strict);
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
configure this statement, see the statement summary sections for this
statement.
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