isis hello padding

Intermediate System-to-Intermediate System (IS-IS) hellos are padded to the full maximum transmission unit (MTU) size. The benefit of padding IS-IS hellos to the full MTU is that it allows for early detection of errors that result from transmission problems with large frames or errors that result from mismatched MTUs on adjacent interfaces.

You can disable hello padding in order to avoid wasting network bandwidth in case the MTU of both interfaces is the same or, in case of translational bridging. While hello padding is disabled, Cisco routers still send the first five IS-IS hellos padded to the full MTU size, in order to maintain the benefits of discovering MTU mismatches.
To selectively disable hello padding for a specific interface, enter the no isis hello padding command in interface configuration mode. To disable hello padding for all interfaces on a router for the IS-IS routing process, enter the no hello padding command in router configuration mode.

Defaults
IS-IS hello padding is enabled.

Examples
To turn off hello padding at the interface level for the Ethernet interface 0/0, enter the no isis hello padding command in interface configuration mode:
Router# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)# interface e0/0
Router(config-if)# no isis hello padding
Router(config-if)# end


When the show clns neighbor command is entered for Ethernet interface 0/0, the output confirms that hello padding has been turned off for both Level 1 and Level 2 circuit types:
Router_A# show clns interface e0/0
Ethernet0/0 is up, line protocol is up
Checksums enabled, MTU 1497, Encapsulation SAP
ERPDUs enabled, min. interval 10 msec.
CLNS fast switching enabled
CLNS SSE switching disabled
DEC compatibility mode OFF for this interface
Next ESH/ISH in 47 seconds
Routing Protocol: IS-IS
Circuit Type: level-1-2
Interface number 0x0, local circuit ID 0x1
Level-1 Metric: 10, Priority: 64, Circuit ID: Router_B.01
Level-1 IPv6 Metric: 10
Number of active level-1 adjacencies: 1
Level-2 Metric: 10, Priority: 64, Circuit ID: Router_B.01
Level-2 IPv6 Metric: 10
Number of active level-2 adjacencies: 1
Next IS-IS LAN Level-1 Hello in 2 seconds
! No hello padding
Next IS-IS LAN Level-2 Hello in 2 seconds
! No hello padding
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