The article explain the note while adjusting MTU for ISIS interfaces.

The problem only happened after decreasing the interface media MTU from 1514(default) to a less value ,  then ISIS will be fail to UP.

user@R1# show interfaces ge-0/0/0 
mtu 1400;
unit 0 {
    family inet {
        address 10.1.12.1/30;
    }
    family iso;
}

[edit]
user@R1# show protocols isis 
interface ge-0/0/0.0 {
    point-to-point;
}
interface lo0.0 {
    passive;
}
level 1 disable;
level 2 wide-metrics-only;

[edit]
user@R1# run show isis adjacency 
Interface             System         L State         Hold (secs) SNPA
ge-0/0/0.0            R2             2  Initializing          24

user@R1> show isis database extensive | grep "LSP ID|lsp buffer"  
  Header: LSP ID: R1.00-00, Length: 87 bytes
  Packet: LSP ID: R1.00-00, Length: 87 bytes, Lifetime : 1200 secs
    LSP Buffer Size: 1492
  Header: LSP ID: R2.00-00, Length: 122 bytes
  Packet: LSP ID: R2.00-00, Length: 122 bytes, Lifetime : 1198 secs
    LSP Buffer Size: 1492

According to document max-lsp-size-edit-protocols-isis , the max-lsp-size must be less than or equal to the protocol ISO MTU.

2/ The second parameter is max-hello-size , which can be checked as below:

user@R1> show isis interface ge-0/0/0.0 extensive 
IS-IS interface database:
ge-0/0/0.0
  Index: 328, State: 0x6, Circuit id: 0x1, Circuit type: 2
  LSP interval: 100 ms, CSNP interval: disabled, Loose Hello padding, IIH max size: 1492
  Adjacency advertisement: Advertise, Layer2-map: Disabled
  Interface Group Holddown Delay: 20 s, remaining: 0 s
  Level 2
    Adjacencies: 0, Priority: 64, Metric: 10
    Hello Interval: 9.000 s, Hold Time: 27 s

According to document interface-edit-protocols-isis , 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. Hence the max-hello-size must be less than or equal to the protocol ISO MTU too.

By default on MX series, the interface media MTU is 1514 , and the protocol ISO MTU is 1497.
Thus if interface media MTU was changed to 1400 , then protocol ISO MTU should be 1383.

user@R1> show interfaces ge-0/0/0 | grep "MTU:"
  Link-level type: Ethernet, MTU: 1400, MRU: 1408, LAN-PHY mode, Speed: 1000mbps, BPDU Error: None, Loop Detect PDU Error: None, Ethernet-Switching Error: None,
    Protocol inet, MTU: 1386
    Protocol iso, MTU: 1383
    Protocol multiservice, MTU: Unlimited

The default max-lsp-size and max-hello-size is 1492 , so adjust max-lsp-size and max-hello-size to 1383, then ISIS can be UP.

user@R1# show protocols isis 
interface ge-0/0/0.0 {
    point-to-point;
}
interface lo0.0 {
    passive;
}
level 1 disable;
level 2 wide-metrics-only;
max-lsp-size 1383;
max-hello-size 1383;

[edit]
user@R1# run show isis adjacency 
Interface             System         L State         Hold (secs) SNPA
ge-0/0/0.0            R2             2  Up                    26