diff --git a/src/conf_mode/protocols_isis.py b/src/conf_mode/protocols_isis.py
index d4c82249b..4cf0312e9 100755
--- a/src/conf_mode/protocols_isis.py
+++ b/src/conf_mode/protocols_isis.py
@@ -1,248 +1,252 @@
 #!/usr/bin/env python3
 #
 # Copyright (C) 2020-2021 VyOS maintainers and contributors
 #
 # This program is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License version 2 or later as
 # published by the Free Software Foundation.
 #
 # This program is distributed in the hope that it will be useful,
 # but WITHOUT ANY WARRANTY; without even the implied warranty of
 # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 # GNU General Public License for more details.
 #
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 import os
 
 from sys import exit
 from sys import argv
 
 from vyos.config import Config
 from vyos.configdict import dict_merge
 from vyos.configdict import node_changed
 from vyos.configverify import verify_common_route_maps
 from vyos.configverify import verify_interface_exists
 from vyos.ifconfig import Interface
 from vyos.util import dict_search
 from vyos.util import get_interface_config
 from vyos.template import render_to_string
 from vyos.xml import defaults
 from vyos import ConfigError
 from vyos import frr
 from vyos import airbag
 airbag.enable()
 
 def get_config(config=None):
     if config:
         conf = config
     else:
         conf = Config()
 
     vrf = None
     if len(argv) > 1:
         vrf = argv[1]
 
     base_path = ['protocols', 'isis']
 
     # eqivalent of the C foo ? 'a' : 'b' statement
     base = vrf and ['vrf', 'name', vrf, 'protocols', 'isis'] or base_path
     isis = conf.get_config_dict(base, key_mangling=('-', '_'),
                                 get_first_key=True)
 
     # Assign the name of our VRF context. This MUST be done before the return
     # statement below, else on deletion we will delete the default instance
     # instead of the VRF instance.
     if vrf: isis['vrf'] = vrf
 
     # As we no re-use this Python handler for both VRF and non VRF instances for
     # IS-IS we need to find out if any interfaces changed so properly adjust
     # the FRR configuration and not by acctident change interfaces from a
     # different VRF.
     interfaces_removed = node_changed(conf, base + ['interface'])
     if interfaces_removed:
         isis['interface_removed'] = list(interfaces_removed)
 
     # Bail out early if configuration tree does not exist
     if not conf.exists(base):
         isis.update({'deleted' : ''})
         return isis
 
     # We have gathered the dict representation of the CLI, but there are default
     # options which we need to update into the dictionary retrived.
     # XXX: Note that we can not call defaults(base), as defaults does not work
     # on an instance of a tag node. As we use the exact same CLI definition for
     # both the non-vrf and vrf version this is absolutely safe!
     default_values = defaults(base_path)
     # merge in default values
     isis = dict_merge(default_values, isis)
 
     # We also need some additional information from the config, prefix-lists
     # and route-maps for instance. They will be used in verify().
     #
     # XXX: one MUST always call this without the key_mangling() option! See
     # vyos.configverify.verify_common_route_maps() for more information.
     tmp = conf.get_config_dict(['policy'])
     # Merge policy dict into "regular" config dict
     isis = dict_merge(tmp, isis)
 
     return isis
 
 def verify(isis):
     # bail out early - looks like removal from running config
     if not isis or 'deleted' in isis:
         return None
 
     if 'net' not in isis:
         raise ConfigError('Network entity is mandatory!')
 
     # last byte in IS-IS area address must be 0
     tmp = isis['net'].split('.')
     if int(tmp[-1]) != 0:
         raise ConfigError('Last byte of IS-IS network entity title must always be 0!')
 
     verify_common_route_maps(isis)
 
     # If interface not set
     if 'interface' not in isis:
         raise ConfigError('Interface used for routing updates is mandatory!')
 
     for interface in isis['interface']:
         verify_interface_exists(interface)
         # Interface MTU must be >= configured lsp-mtu
         mtu = Interface(interface).get_mtu()
         area_mtu = isis['lsp_mtu']
-        if mtu < int(area_mtu):
-            raise ConfigError(f'Interface {interface} has MTU {mtu}, minimum ' \
-                              f'area MTU is {area_mtu}!')
+        # Recommended maximum PDU size = interface MTU - 3 bytes
+        recom_area_mtu = mtu - 3
+        if mtu < int(area_mtu) or int(area_mtu) > recom_area_mtu:
+            raise ConfigError(f'Interface {interface} has MTU {mtu}, ' \
+                              f'current area MTU is {area_mtu}! \n' \
+                              f'Recommended area lsp-mtu {recom_area_mtu} or less ' \
+                              '(calculated on MTU size).')
 
         if 'vrf' in isis:
             # If interface specific options are set, we must ensure that the
             # interface is bound to our requesting VRF. Due to the VyOS
             # priorities the interface is bound to the VRF after creation of
             # the VRF itself, and before any routing protocol is configured.
             vrf = isis['vrf']
             tmp = get_interface_config(interface)
             if 'master' not in tmp or tmp['master'] != vrf:
                 raise ConfigError(f'Interface {interface} is not a member of VRF {vrf}!')
 
     # If md5 and plaintext-password set at the same time
     for password in ['area_password', 'domain_password']:
         if password in isis:
             if {'md5', 'plaintext_password'} <= set(isis[password]):
                 tmp = password.replace('_', '-')
                 raise ConfigError(f'Can use either md5 or plaintext-password for {tmp}!')
 
     # If one param from delay set, but not set others
     if 'spf_delay_ietf' in isis:
         required_timers = ['holddown', 'init_delay', 'long_delay', 'short_delay', 'time_to_learn']
         exist_timers = []
         for elm_timer in required_timers:
             if elm_timer in isis['spf_delay_ietf']:
                 exist_timers.append(elm_timer)
 
         exist_timers = set(required_timers).difference(set(exist_timers))
         if len(exist_timers) > 0:
             raise ConfigError('All types of delay must be specified: ' + ', '.join(exist_timers).replace('_', '-'))
 
     # If Redistribute set, but level don't set
     if 'redistribute' in isis:
         proc_level = isis.get('level','').replace('-','_')
         for afi in ['ipv4', 'ipv6']:
             if afi not in isis['redistribute']:
                 continue
 
             for proto, proto_config in isis['redistribute'][afi].items():
                 if 'level_1' not in proto_config and 'level_2' not in proto_config:
                     raise ConfigError(f'Redistribute level-1 or level-2 should be specified in ' \
                                       f'"protocols isis {process} redistribute {afi} {proto}"!')
 
                 for redistr_level, redistr_config in proto_config.items():
                     if proc_level and proc_level != 'level_1_2' and proc_level != redistr_level:
                         raise ConfigError(f'"protocols isis {process} redistribute {afi} {proto} {redistr_level}" ' \
                                           f'can not be used with \"protocols isis {process} level {proc_level}\"')
 
     # Segment routing checks
     if dict_search('segment_routing.global_block', isis):
         high_label_value = dict_search('segment_routing.global_block.high_label_value', isis)
         low_label_value = dict_search('segment_routing.global_block.low_label_value', isis)
 
         # If segment routing global block high value is blank, throw error
         if (low_label_value and not high_label_value) or (high_label_value and not low_label_value):
             raise ConfigError('Segment routing global block requires both low and high value!')
 
         # If segment routing global block low value is higher than the high value, throw error
         if int(low_label_value) > int(high_label_value):
             raise ConfigError('Segment routing global block low value must be lower than high value')
 
     if dict_search('segment_routing.local_block', isis):
         high_label_value = dict_search('segment_routing.local_block.high_label_value', isis)
         low_label_value = dict_search('segment_routing.local_block.low_label_value', isis)
 
         # If segment routing local block high value is blank, throw error
         if (low_label_value and not high_label_value) or (high_label_value and not low_label_value):
             raise ConfigError('Segment routing local block requires both high and low value!')
 
         # If segment routing local block low value is higher than the high value, throw error
         if int(low_label_value) > int(high_label_value):
             raise ConfigError('Segment routing local block low value must be lower than high value')
 
     return None
 
 def generate(isis):
     if not isis or 'deleted' in isis:
         isis['frr_isisd_config'] = ''
         isis['frr_zebra_config'] = ''
         return None
 
     isis['protocol'] = 'isis' # required for frr/vrf.route-map.frr.tmpl
     isis['frr_zebra_config'] = render_to_string('frr/vrf.route-map.frr.tmpl', isis)
     isis['frr_isisd_config'] = render_to_string('frr/isisd.frr.tmpl', isis)
     return None
 
 def apply(isis):
     isis_daemon = 'isisd'
     zebra_daemon = 'zebra'
 
     # Save original configuration prior to starting any commit actions
     frr_cfg = frr.FRRConfig()
 
     # The route-map used for the FIB (zebra) is part of the zebra daemon
     frr_cfg.load_configuration(zebra_daemon)
     frr_cfg.modify_section(r'(\s+)?ip protocol isis route-map [-a-zA-Z0-9.]+$', '', '(\s|!)')
     frr_cfg.add_before(r'(ip prefix-list .*|route-map .*|line vty)', isis['frr_zebra_config'])
     frr_cfg.commit_configuration(zebra_daemon)
 
     # Generate empty helper string which can be ammended to FRR commands, it
     # will be either empty (default VRF) or contain the "vrf <name" statement
     vrf = ''
     if 'vrf' in isis:
         vrf = ' vrf ' + isis['vrf']
 
     frr_cfg.load_configuration(isis_daemon)
     frr_cfg.modify_section(f'^router isis VyOS{vrf}$', '')
 
     for key in ['interface', 'interface_removed']:
         if key not in isis:
             continue
         for interface in isis[key]:
             frr_cfg.modify_section(f'^interface {interface}{vrf}$', '')
 
     frr_cfg.add_before(r'(ip prefix-list .*|route-map .*|line vty)', isis['frr_isisd_config'])
     frr_cfg.commit_configuration(isis_daemon)
 
     # Save configuration to /run/frr/config/frr.conf
     frr.save_configuration()
 
     return None
 
 if __name__ == '__main__':
     try:
         c = get_config()
         verify(c)
         generate(c)
         apply(c)
     except ConfigError as e:
         print(e)
         exit(1)