b)generate an adjacencyStateChange (Down) notifi
cation.
8.4.2 Insufficient Space in Adjacency Database
If an IS needs to create a new Intermediate system adja
cency, but there is insufficient space in the adjacency data
base, the adjacency of neighbourSystemType Ln Inter
mediate System with lowest lANPriority (or if more than
one adjacency has the lowest priority, the adjacency with
the lowest lANAddress, from among those with the lowest
priority) shall be purged from the database. If the new adja
cency would have the lowest priority, it shall be ignored,
and a rejectedAdjacency notification generated.
If an old adjacency must be purged, the IS shall generate an
adjacencyStateChange (Down) notification for that adja
cency. After the Subnetwork Independent Functions issue
an adjacency down complete, the IS may create a new ad
jacency.
8.4.3 Transmission of LAN IIH PDUs
A Level 1 IS shall transmit a Level 1 LAN IIH PDU imme
diately when any circuit whose externalDomain attribute
is False has been enabled. A Level 2 Intermediate Sys
tem shall transmit a Level 2 LAN IIH PDU. A Level 2 In
termediate System shall also transmit a Level 1 LAN IIH
PDU unless the circuit is marked as manualL2OnlyMode
True.
The IS shall also transmit a LAN IIH PDU when at least 1
second has transpired since the last transmission of a LAN
IIH PDU of the same type on this circuit by this system
and:
a)iSIS
Hello
Timer seconds have elapsed1010Jitter is applied as described in 10.1.
since the last
periodic LAN IIH PDU transmission
The Holding Time is set to ISISHoldingMultiplier W
iSIS
Hello
Timer. For a Designated Intermediate Sys
tem the value of dRISIS
Hello
Timer1111 In this case jitter is not applied, since it would result in
intervals of less than one second.
is used instead
of iSISHelloTimer. The Holding Time for this PDU
shall therefore be set to ISISHoldingMultiplier W
dR
ISIS
Hello
Timer seconds. This permits failing
Designated Intermediate Systems to be detected more
rapidly,
or
b)the contents of the next IIH PDU to be transmitted
would differ from the contents of the previous IIH
PDU transmitted by this system, or
c)this system has determined that it is to become or re
sign as LAN Designated Intermediate System for that
level.
To minimise the possibility of the IIH PDU transmissions
of all Intermediate systems on the LAN becoming synchro
nised, the Hello Time timer shall only be reset when a IIH
PDU is transmitted as a result of timer expiration, or on be
coming or resigning as Designated Intermediate System.
Where an Intermediate system is transmitting both Level 1
and Level 2 LAN IIH PDUs, it shall maintain a separate
timer (separately jittered) for the transmission of the
Level 1 and Level 2 IIH PDUs. This avoids correlation be
tween the Level 1 and Level 2 IIH PDUs and allows the re
ception buffer requirements to be minimised.
If the value of the circuitTransmitPassword for the cir
cuit is non-null, then the IS shall include the Authentica
tion Information field in the transmitted IIH PDU, indicat
ing an Authentication Type of Password and contain
ing the circuitTransmitPassword as the authentication
value.
8.4.4 LAN Designated Intermediate Systems
A LAN Designated Intermediate System is the highest pri
ority Intermediate system in a particular set on the LAN,
with numerically highest MAC source lANAddress break
ing ties. (See 7.1.5 for how to compare LAN addresses.)
There are, in general, two LAN Designated Intermediate
Systems on each LAN, namely the LAN Level 1 Desig
nated Intermediate System and the LAN Level 2 Desig
nated Intermediate System. They are elected as follows.
a)Level 1 Intermediate systems elect the LAN Level 1
Designated Intermediate System.
b)Level 2 Only Intermediate systems (i.e. Level 2 Inter
mediate Systems which have the Circuit manual
L2
Only
Mode characteristic set to the value True)
elect the LAN Level 2 Designated Intermediate Sys
tem.
c)Level 2 Intermediate systems (with manu
alL2OnlyMode False) elect both the LAN Level 1
and LAN Level 2 Designated Intermediate Systems.
The set of Intermediate systems to be considered includes
the local Intermediate system, together with all Intermedi
ate systems of the appropriate type as follows.
a)For the LAN Level 1 Designated Intermediate System,
it is the set of Intermediate systems from which LAN
Level 1 IIH PDUs are received and to which Level 1
adjacencies exist in state Up. When the local sys
tem either becomes or resigns as LAN Level 1 Desig
nated Intermediate System, the IS shall generate a lan
Level1
Designated
Inter
mediate
Sys
tem
Change
notification. In addition, when it becomes LAN
Level 1 Designated Intermediate System, it shall per
form the following actions.
1)Generate and transmit Level 1 pseudonode LSPs
using the existing End system configuration.
2)Purge the Level 1 pseudonode LSPs issued by the
previous LAN Level 1 Designated Intermediate
System (if any) as described in 7.2.3.
3)Solicit the new End system configuration as de
scribed in 8.4.5.
b)For the LAN Level 2 Designated Intermediate System,
it is the set of Intermediate systems from which LAN
Level 2 IIH PDUs are received and to which Level 2
adjacencies exist in state Up. When the local sys
tem either becomes or resigns as LAN Level 2 Desig
nated Intermediate System, the IS shall generate a lan
Level2
Designated
Inter
mediate
System
Change
notification. In addition, when it becomes LAN
Level 2 Designated Intermediate System, it shall per
form the following actions.
1)Generate and transmit a Level 2 pseudonode LSP.
2)Purge the Level 2 pseudonode LSPs issued by the
previous LAN Level 2 Designated Intermediate
System (if any) as described in 7.2.3.
When an Intermediate system resigns as LAN Level 1 or
Level 2 Designated Intermediate System it shall perform
the actions on Link State PDUs described in 7.2.3.
When the broadcast circuit is enabled on an Intermediate
system the IS shall perform the following actions.
a)Commence sending IIH PDUs with the LAN ID field
set to the concatenation of its own systemID and its
locally assigned one octet Local Circuit ID.
b)Solicit the End system configuration as described in
8.4.5.
c)Start listening for ISO 9542 ISH PDUs and ESH
PDUs and acquire adjacencies as appropriate. Do not
run the Designated Intermediate System election proc
ess.
d)After waiting iSIS
Hello
Timer * 2 seconds, run the
Level 1 and or the Level 2 Designated Intermediate
System election process depending on the Intermedi
ate system type. This shall be run subsequently when
ever an IIH PDU is received or transmitted as de
scribed in 8.4.3. (For these purposes, the transmission
of the system's own IIH PDU is equivalent to receiv
ing it). If there has been no change to the information
on which the election is performed since the last time
it was run, the previous result can be assumed. The
relevant information is
1)the set of Intermediate system adjacency states,
2)the set of Intermediate System priorities (including
this system's) and
3)the existence (or otherwise) of at least one Up
End system (not including Manual Adjacencies) or
Intermediate system adjacency on the circuit.
An Intermediate system shall not declare itself to be a LAN
Designated Intermediate system of any type until it has at
least one Up End system (not including Manual Adjacen
cies) or Intermediate system adjacency on the circuit. (This
prevents an Intermediate System which has a defective re
ceiver and is incapable of receiving any PDUs from errone
ously electing itself LAN Designated Intermediate System.)
The LAN ID field in the LAN IIH PDUs transmitted by this
system shall be set to the value of the LAN ID field reported
in the LAN IIH PDU (for the appropriate level) received
from the system which this system considers to be the Des
ignated Intermediate System. This value shall also be
passed to the Update Process, as the pseudonode ID, to en
able Link State PDUs to be issued for this system claiming
connectivity to the pseudonode.
If this system, as a result of the Designated Intermediate
System election process, considers itself to be designated
Intermediate System, the LAN ID field shall be set to the
concatenation of this system's own system ID and the lo
cally assigned one octet Local Circuit ID.
8.4.5 Soliciting the ES configuration
When there is a change in the topology or configuration of
the LAN (for example the partitioning of a LAN into two
segments by the failure of a repeater or bridge), it is desir
able for the (new) Designated Intermediate System to ac
quire the new End system configuration of the LAN as
quickly as possible in order that it may generate Link State
PDUs which accurately reflect the actual configuration.
This is achieved as follows.
When the circuit is enabled, or the Intermediate system de
tects a change in the set of Intermediate systems on the
LAN, or a change in the Designated Intermediate System
ID, the IS shall initiate a poll of the ES configuration by
performing the following actions.
a)Delay a random interval between 0 and iSIS
Hello
Timer seconds. (This is to avoid synchronisation with
other Intermediate systems which have detected the
change.)
b)If (and only if) an Intermediate System had been re
moved from the set of Intermediate systems on the
LAN, reset the entryRemainingTime field in the
endSystemIDs adjacency database record of all adja
cencies on this circuit to the value
(iSIS
Hello
Timer + pollESHelloRate) W
HoldingMultiplier
or the existing value whichever is the lower. (This
causes any End systems which are no longer present
on the LAN to be rapidly timed out, but not before
they have a chance to respond to the poll.)
c)Transmit HoldingMultiplier ISH PDUs for each NET
possessed by the Intermediate system with a Sug
gested ES Configuration Timer value of poll
ES
Hello
Rate at an interval between them of iSIS
Hello
Timer seconds and a holding time of hello
Timer *
HoldingMultiplier.
d)Resume sending ISH PDUs at intervals of hello
Timer seconds with a Suggested ES Configuration
Timer value of defaultESHelloTimer.
8.4.6 Receipt of ESH PDUs Database of End
Systems
An IS shall enter an End system into the adjacency database
when an ESH PDU is received from a new data link ad
dress. If an ESH PDU is received with the same data link
address as a current adjacency, but with a different NSAP
address, the new address shall be added to the adjacency,
with a separate timer. A single ESH PDU may contain more
than one NSAP address. When a new data link address or
NSAP address is added to the adjacency database, the IS
shall generate an adjacencyStateChange (Up) notifica
tion on that adjacency.
The IS shall set a timer for the value of the Holding Time
field in the received ESH PDU. If another ESH PDU is not
received from the ES before that timer expires, the ES shall
be purged from the database, provided that the Subnetwork
Independent Functions associated with initialising the adja
cency have been completed. Otherwise the IS shall clear the
adjacency as soon as those functions are completed.
When the adjacency is cleared, the Subnetwork Independ
ent Functions shall be informed of an adjacencyState
Change (Down) notification, and the adjacency can be re-
used after the Subnetwork Independent Functions associ
ated with bringing down the adjacency have been com
pleted.
9 Structure and Encoding of PDUs
This clause describes the PDU formats of the Intra-Domain
Routeing protocol.
9.1 General encoding Rules
Octets in a PDU are numbered starting from 1, in increasing
order. Bits in a octet are numbered from 1 to 8, where bit 1
is the least significant bit and is pictured on the right. When
consecutive octets are used to represent a number, the lower
octet number has the most significant value.
Fields marked Reserved (or simply R) are transmitted as
zero, and ignored on receipt, unless otherwise noted.
Values are given in decimal. All numeric fields are un
signed integers, unless otherwise noted.
9.2 Encoding of Network Layer
Addresses
Network Layer addresses (NSAP addresses, NETs, area ad
dresses and Address Prefixes) are encoded in PDUs accord
ing to the preferred binary encoding specified in
ISO 8348/Add.2; the entire address, taken as a whole is rep
resented explicitly as a string of binary octets. This string is
conveyed in its entirety in the address fields of the PDUs.
The rules governing the generation of the preferred binary
encoding are described in ISO 8348/Add.2. The address so
generated is encoded with the most significant octet (i.e. the
AFI) of the address being the first octet transmitted, and the
more significant semi-octet of each pair of semi-octets in
the address is encoded in the more significant semi-octet of
each octet (i.e. in the high order 4 bits). Thus the address
/371234 is encoded as
Figure 6 - 111No. of Octets3
7
1
2
3
4
Address encoding example
9.3 Encoding of SNPA Addresses
SNPA addresses (e.g. lANAddress) shall be encoded ac
cording to the rules specified for the particular type of
subnetwork being used. In the case of an ISO 8802
subnetwork, the SNPA address is the MAC address defined
in ISO 10039, which is encoded according to the binary
representation of MAC addresses specified in ISO 10039.
9.4 PDU Types
The types of PDUs are:
-Level 1 LAN IS to IS Hello PDU
-Level 2 LAN IS to IS Hello PDU
-Point-to-Point IS to IS Hello PDU
-Level 1 Link State PDU
-Level 2 Link State PDU
-Level 1 Complete Sequence Numbers PDU
-Level 2 Complete Sequence Numbers PDU
-Level 1 Partial Sequence Numbers PDU
-Level 2 Partial Sequence Numbers PDU
These are described in the following subclauses.
9.5 Level 1 LAN IS to IS Hello PDU
This PDU is multicast by Intermediate systems on broad
cast circuits to the multi-destination address AllL1ISs.
The purpose of this PDU is for Intermediate systems on
broadcast circuits to discover the identity of other Level 1
Intermediate systems on that circuit. Trailing Pad options
are inserted to make PDU Length equal to at least maxsize
- 1 where maxsize is the maximum of
-dataLinkBlocksize
-originating
L1
LSP
Buf
fer
Size
(see 8.4.1). 11No. of Octets1111111ID Length2ID Length +
121VARIABLEIntradomain Routeing
Protocol Discriminator
Length Indicator
Version/Protocol ID Extension
ID Length
PDU Type
R
R
R
Version
ECO
User ECO
Reserved/Circuit Type
Source ID
Holding Time
LAN ID
PDU Length
Priority
R
VARIABLE LENGTH FIELDS
-Intradomain Routeing Protocol Discriminator
architectural constant
-Length Indicator Length of the fixed header in oc
tets
-Version/Protocol ID Extension 1
-ID Length Length of the ID field of NSAP ad
dresses and NETs used in this routeing domain. This
field shall take on one of the following values:
7An integer between 1 and 8, inclusive, indicating
an ID field of the corresponding length
7The value zero, which indicates a 6 octet ID field
length
7The value 255, whhich means a null ID field (i.e.
zero length)
All other values are illegal and shall not be used.
-PDU Type (bits 1 through 5) 15. Note bits 6, 7 and
8 are Reserved, which means they are transmitted as 0
and ignored on receipt.
-Version 1
-ECO transmitted as zero, ignored on receipt
-User ECO transmitted as zero, ignored on receipt
-Reserved/Circuit Type Most significant 6 bits re
served (Transmitted as zero, ignored on receipt). Low
order bits (bits 1 and 2) indicate:
70 reserved value (if specified the entire PDU
shall be ignored)
71 Level 1 only
72 Level 2 only (sender is Level 2 Intermediate
system with manualL2OnlyMode set True for
this circuit, and will use this link only for Level 2
traffic)
73 both Level 1 and Level 2 (sender is Level 2 In
termediate system, and will use this link both for
Level 1 and Level 2 traffic)
NOTE In a LAN Level 1 IIH PDU the Circuit
Type shall be either 1 or 3.
-Source ID the system ID of transmitting Intermedi
ate system
-Holding Time Holding Timer to be used for this In
termediate system
-PDU Length Entire length of this PDU, in octets,
including header
-Reserved/Priority Bit 8 reserved (Transmitted as
zero, ignored on receipt). Bits 1 through 7 priority
for being LAN Level 1 Designated Intermediate Sys
tem. Higher number has higher priority for being LAN
Level 1 Designated Intermediate System. Unsigned
integer.
-LAN ID a field composed the system ID (18 octets)
of the LAN Level 1 Designated Intermediate System,
plus a low order octet assigned by LAN Level 1 Des
ignated Intermediate System. Copied from LAN
Level 1 Designated Intermediate System's IIH PDU.
-VARIABLE LENGTH FIELDS fields of the form:11No. of OctetsLENGTHCODE
LENGTH
VALUE
Any codes in a received PDU that are not recognised
shall be ignored.
Currently defined codes are:
7Area Addresses the set of manual
Area
Addresses of this Intermediate System.
xCODE 1
xLENGTH total length of the value field.
xVALUE 1Address Length1Address LengthNo. of OctetsAddress Length
Area Address
Address Length
Area Address
7Address Length Length of Area Ad
dress in octets.
7Area Address Area address.
7Intermediate System Neighbours This option
field can occur multiple times. The set of Interme
diate systems on this LAN to which adjacencies of
neighbourSystemType L1 Intermediate Sys
tem exist in state Up or Initialising (i.e.
those from which Level 1 IIH PDUs have been
heard).
xCODE 6
xLENGTH total length of the value field.
xVALUE 66No. of OctetsLAN Address
LAN Address
7LAN Address 6 octet MAC Address of
Intermediate System neighbour.
7Padding This option may occur multiple times.
It is used to pad the PDU to at least maxsize - 1.
xCODE 8.
xLENGTH total length of the value field (may
be zero).
xVALUE LENGTH octets of arbitrary value.
7Authentication Information information for
performing authentication of the originator of the
PDU.
xCODE 10.
xLENGTH variable from 1254 octets
xVALUE 1VARIABLENo. of OctetsAuthentication Type
Authentication Value
7Authentication Type a one octet iden
tifier for the type of authentication to be
carried out. The following values are de
fined:
0 RESERVED
1 Cleartext Password
2254 RESERVED
255 Routeing Domain private
authentication method
7Authentication Value determined by
the value of the authentication type. If
Cleartext Password as defined in this Inter
national Standard is used, then the authenti
cation value is an octet string.
9.6 Level 2 LAN IS to IS Hello PDU
This PDU is multicast by Intermediate systems on broad
cast circuits to the multi-destination address AllL2ISs.
The purpose of this PDU is for Intermediate systems on
broadcast circuits to discover the identity of other Level 2
Intermediate systems on that circuit. Trailing Pad options
are inserted to make PDU Length equal to at least maxsize
- 1 where
-dataLinkBlocksize
-originatingL2LSPBufferSize
(see 8.4.1). 11No. of Octets1111111ID Length2ID Length +
121VARIABLEIntradomain Routeing
Protocol Discriminator
Length Indicator
Version/Protocol ID Extension
ID Length
PDU Type
R
R
R
Version
ECO
User ECO
Reserved/Circuit Type
Source ID
Holding Time
LAN ID
PDU Length
Priority
R
VARIABLE LENGTH FIELDS
-Intradomain Routeing Protocol Discriminator ar
chitectural constant
-Length Indicator Length of fixed header in octets
-Version/Protocol ID Extension 1
-ID Length Length of the ID field of NSAP ad
dresses and NETs used in this routeing domain. This
field shall take on one of the following values:
7An integer between 1 and 8, inclusive, indicating
an ID field of the corresponding length
7The value zero, which indicates a 6 octet ID field
length
7The value 255, whhich means a null ID field (i.e.
zero length)
All other values are illegal and shall not be used.
-PDU Type (bits 1 through 5) 16. Note bits 6, 7 and
8 are Reserved, which means they are transmitted as 0
and ignored on receipt.
-Version 1
-ECO transmitted as zero, ignored on receipt
-User ECO transmitted as zero, ignored on receipt
-Reserved/Circuit Type Most significant 6 bits re
served (Transmitted as zero, ignored on receipt). Low
order bits (bits 1 and 2) indicate:
70 reserved value (if specified the entire PDU
shall be ignored)
71 Level 1 only
72 Level 2 only (sender is Level 2 Intermediate
System with manualL2OnlyMode set True for
this circuit, and will use this link only for Level 2
traffic)
73 both Level 1 and Level 2 (sender is Level 2 In
termediate System, and will use this link both for
Level 1 and Level 2 traffic)
NOTE In a LAN Level 2 IIH PDU the Circuit Type
shall be either 2 or 3.
-Source ID the system ID of transmitting Intermedi
ate System
-Holding Time Holding Timer to be used for this In
termediate System
-PDU Length Entire length of this PDU, in octets,
including header
-Reserved/Priority Bit 8 reserved (Transmitted as
zero, ignored on receipt). Bits 1 through 7 priority
for being LAN Level 2 Designated Intermediate Sys
tem. Higher number has higher priority for being LAN
Level 2 Designated Intermediate System. Unsigned
integer.
-LAN ID a field composed the system ID (18 octets)
of the LAN Level 1 Designated Intermediate System,
plus a low order octet assigned by LAN Level 1 Des
ignated Intermediate System. Copied from LAN
Level 1 Designated Intermediate System's IIH PDU.
-VARIABLE LENGTH FIELDS fields of the form:11No. of OctetsLENGTHCODE
LENGTH
VALUE
Any codes in a received PDU that are not recognised
shall be ignored.
Currently defined codes are:
7Area addresses the set of manual
Area
Addresses of this Intermediate system.
xCODE 1
xLENGTH total length of the value field.
xVALUE 1Address Length1Address LengthNo. of OctetsAddress Length
Area Address
Address Length
Area Address
7Address Length Length of area address
in octets.
7Area Address Area address.
7Intermediate System Neighbours This option
can occur multiple times. The set of Intermediate
systems on this LAN to which adjacencies of
neighbourSystemType L2 Intermediate Sys
tem exist in state Up or Initialising (i.e.
those from which Level 2 IIH PDUs have been
heard).
xCODE 6
xLENGTH total length of the value field.
xVALUE 66No. of OctetsLAN Address
LAN Address
xLAN Address 6 octet MAC Address of In
termediate System neighbour
7Padding This option may occur multiple times.
It is used to pad the PDU to at least maxsize 1.
xCODE 8.
xLENGTH total length of the value field (may
be zero).
xVALUE LENGTH octets of arbitrary value.
7Authentication Information information for
performing authentication of the originator of the
PDU.
xCODE 10.
xLENGTH variable from 1254 octets
xVALUE 1VARIABLENo. of OctetsAuthentication Type
Authentication Value
7Authentication Type a one octet iden
tifier for the type of authentication to be
carried out. The following values are de
fined:
0 RESERVED
1 Cleartext Password
2254 RESERVED
255 Routeing Domain private
authentication method
7Authentication Value determined by
the value of the authentication type. If
Cleartext Password as defined in this Inter
national Standard is used, then the authenti
cation value is an octet string.
9.7 Point-to-Point IS to IS Hello PDU
This PDU is transmitted by Intermediate systems on non-
broadcast circuits, after receiving an ISH PDU from the
neighbour system. Its purpose is to determine whether the
neighbour is a Level 1 or a Level 2 Intermediate System.
Trailing pad options are inserted to make PDU Length
equal to at least maxsize 1 where maxsize is the maxi
mum of
-dataLinkBlocksize
-originating
L1
LSP
Buf
fer
Size
-originatingL2LSPBufferSize
(see 8.2.3).11No. of Octets1111111ID Length212VARIABLEIntradomain Routeing
Protocol Discriminator
Length Indicator
Version/Protocol ID Extension
ID Length
PDU Type
R
R
R
Version
ECO
User ECO
Reserved/Circuit Type
Source ID
Holding Time
Local Circuit ID
PDU Length
VARIABLE LENGTH FIELDS
-Intradomain Routeing Protocol Discriminator
architectural constant
-Length Indicator Length of fixed header in octets
-Version/Protocol ID Extension 1
-ID Length Length of the ID field of NSAP ad
dresses and NETs used in this routeing domain. This
field shall take on one of the following values:
7An integer between 1 and 8, inclusive, indicating
an ID field of the corresponding length
7The value zero, which indicates a 6 octet ID field
length
7The value 255, whhich means a null ID field (i.e.
zero length)
All other values are illegal and shall not be used.
-PDU Type (bits 1 through 5) 17. Note bits 6, 7
and 8 are Reserved, which means they are transmitted
as 0 and ignored on receipt.
-Version 1
-ECO transmitted as zero, ignored on receipt
-User ECO transmitted as zero, ignored on receipt
-Reserved/Circuit Type Most significant 6 bits re
served (Transmitted as zero, ignored on receipt). Low
order bits (bits 1 and 2) indicate:
70 reserved value (if specified the entire PDU
shall be ignored)
71 Level 1 only
72 Level 2 only (sender is Level 2 Intermediate
system with manualL2OnlyMode set True for
this circuit, and will use this link only for Level 2
traffic)
73 both Level 1 and Level 2 (sender is Level 2 In
termediate system and will use this link both for
Level 1 and Level 2 traffic)
-Source ID the system ID of transmitting Intermedi
ate system
-Holding Time Holding Timer to be used for this In
termediate system
-PDU Length Entire length of this PDU, in octets,
including header
-Local Circuit ID 1 octet unique ID assigned to this
circuit when it is created by this Intermediate system.
The actual ID by which the circuit is known to both
ends of the link is determined by the Intermediate sys
tem with the lower Source ID.
-VARIABLE LENGTH FIELDS fields of the form:11No. of OctetsLENGTHCODE
LENGTH
VALUE
Any codes in a received PDU that are not recognised
shall be ignored.
Currently defined codes are:
7Area addresses the set of manual
Area
Addresses of this Intermediate system
xCODE 1
xLENGTH total length of the value field.
xVALUE 1Address Length1Address LengthNo. of OctetsAddress Length
Area Address
Address Length
Area Address
7Address Length Length of area address
in octets.
7Area Address Area address.
7Padding This option may occur multiple times.
It is used to pad the PDU to at least maxsize 1.
xCODE 8.
xLENGTH total length of the value field (may
be zero).
xVALUE LENGTH octets of arbitrary value.
7Authentication Information information for
performing authentication of the originator of the
PDU.
xCODE 10.
xLENGTH variable from 1254 octets
xVALUE 1VARIABLENo. of OctetsAuthentication Type
Authentication Value
7Authentication Type a one octet iden
tifier for the type of authentication to be
carried out. The following values are de
fined:
0 RESERVED
1 Cleartext Password
2254 RESERVED
255 Routeing Domain private
authentication method
7Authentication Value determined by
the value of the authentication type. If
Cleartext Password as defined in this Inter
national Standard is used, then the authenti
cation value is an octet string.
9.8 Level 1 Link State PDU
Level 1 Link State PDUs are generated by Level 1 and
Level 2 Intermediate systems, and propagated throughout
an area. The contents of the Level 1 Link State PDU indi
cates the state of the adjacencies to neighbour Intermediate
Systems, or pseudonodes, and End systems of the Interme
diate system that originally generated the PDU.11No. of
Octets11111122ID Length + 214VARIABLE2Intradomain Routeing
Protocol Discriminator
Length Indicator
Version/Protocol ID Extension
ID Length
PDU Type
R
R
R
Version
ECO
User ECO
PDU Length
Remaining Lifetime
LSP ID
P
Sequence Number
VARIABLE LENGTH FIELDS
LSPDBOL
IS Type
Checksum
ATT
-Intradomain Routeing Protocol Discriminator ar
chitectural constant
-Length Indicator Length if fixed header in octets
-Version/Protocol ID Extension 1
-ID Length Length of the ID field of NSAP ad
dresses and NETs used in this routeing domain. This
field shall take on one of the following values:
7An integer between 1 and 8, inclusive, indicating
an ID field of the corresponding length
7The value zero, which indicates a 6 octet ID field
length
7The value 255, whhich means a null ID field (i.e.
zero length)
All other values are illegal and shall not be used.
-PDU Type (bits 1 through 5) 18. Note bits 6, 7 and
8 are Reserved, which means they are transmitted as 0
and ignored on receipt.
-Version 1
-ECO transmitted as zero, ignored on receipt
-User ECO transmitted as zero, ignored on receipt
-PDU Length Entire Length of this PDU, in octets,
including header
-Remaining Lifetime Number of seconds before
LSP considered expired
-LSP ID the system ID of the source of the Link
State PDU. It is structured as follows:ID Length1No. of Octets1Source ID
Pseudonode ID
LSP Number
-Sequence Number sequence number of LSP
-Checksum Checksum of contents of LSP from
Source ID to end. Checksum is computed as de
scribed in 7.3.11.
-P/ATT/LSPDBOL/IS Type
-P Bit 8, indicates when set that the issuing Interme
diate System supports the Partition Repair optional
function.
7ATT - Bits 7-4 indicate, when set, that the issuing
Intermediate System is `attached' to other areas
using:
xBit 4 - the Default Metric
xBit 5 - the Delay Metric
xBit 6 - the Expense Metric
xBit 7 - the Error Metric.
7LSPDBOL Bit 3 A value of 0 indicates no
LSP Database Overload, and a value of 1 indicates
that the LSP Database is Overloaded. An LSP with
this bit set will not be used by any decision proc
ess to calculate routes to another IS through the
originating system.
7IS Type Bits 1 and 2 indicate the type of Inter
mediate System One of the following values:
x0 Unused value
x1 ( i.e. bit 1 set) Level 1 Intermediate system
x2 Unused value
x3 (i.e. bits 1 and 2 set) Level 2 Intermediate
system.
-VARIABLE LENGTH FIELDS fields of the form:11No. of OctetsLENGTHCODE
LENGTH
VALUE
Any codes in a received LSP that are not recognised
are ignored and passed through unchanged.
Currently defined codes are:
7Area Addresses the set of manual
Area
Addresses of this Intermediate system. For
LSPs not generated on behalf of the pseudonode
this option shall always be present in the LSP with
LSP number zero, and shall never be present in an
LSP with non-zero LSP number. It shall appear
before any Intermediate System Neighbours or
End System Neighbours options. This option
shall never be present in pseudonode LSPs.
xCODE 1
xLENGTH total length of the value field.
xVALUE 1Address Length1Address LengthNo. of OctetsAddress Length
Area Address
Address Length
Area Address
7Address Length Length of area address
in octets.
7Area Address Area address.
7Intermediate System Neighbours Intermedi
ate system and pseudonode neighbours.
This is permitted to appear multiple times, and in
an LSP with any LSP number. However, all the
Intermediate System Neighbours options
shall precede the End System Neighbours op
tions. i.e. they shall appear before any End system
Neighbour options in the same LSP and no End
system Neighbour options shall appear in an LSP
with lower LSP number.
xCODE 2.
xLENGTH 1. plus a multiple of 11.
xVALUE No. of Octets11ID Length + 11111ID Length + 1111Virtual Flag
Default Metric
Neighbour ID
Delay Metric
Expense Metric
Error Metric
I/E
0
I/E
S
I/E
S
I/E
S
Default Metric
Neighbour ID
Delay Metric
Expense Metric
Error Metric
I/E
0
I/E
S
I/E
S
I/E
S
7Virtual Flag is a Boolean. If equal to 1, this
indicates the link is really a Level 2 path to
repair an area partition. (Level 1 Intermedi
ate Systems would always report this octet
as 0 to all neighbours).
7Default Metric is the value of the default
metric for the link to the listed neighbour.
Bit 8 of this field is reserved. Bit 7 of this
field (marked I/E) indicates the metric type,
and shall contain the value 0, indicating
an Internal metric.
7Delay Metric is the value of the delay met
ric for the link to the listed neighbour. If
this IS does not support this metric it shall
set the bit S to 1 to indicate that the met
ric is unsupported. Bit 7 of this field
(marked I/E) indicates the metric type, and
shall contain the value 0, indicating an
Internal metric.
7Expense Metric is the value of the ex
pense metric for the link to the listed neigh
bour. If this IS does not support this metric
it shall set the bit S to 1 to indicate that
the metric is unsupported. Bit 7 of this field
(marked I/E) indicates the metric type, and
shall contain the value 0, indicating an
Internal metric.
7Error Metric is the value of the error metric
for the link to the listed neighbour. If this
IS does not support this metric it shall set
the bit S to 1 to indicate that the metric is
unsupported. Bit 7 of this field (marked
I/E) indicates the metric type, and shall
contain the value 0, indicating an Internal
metric.
7Neighbour ID. For Intermediate System
neighbours, the first ID Length octets are
the neighbour's system ID, and the last oc
tet is 0. For pseudonode neighbours, the
first ID Length octets is the LAN Level 1
Designated Intermediate System's ID, and
the last octet is a non-zero quantity defined
by the LAN Level 1 Designated Intermedi
ate System.
7End System Neighbours End system neigh
bours
This may appear multiple times, and in an LSP
with any LSP number. See the description of the
Intermediate System Neighbours option
above for the relative ordering constraints. Only
adjacencies with identical costs can appear in the
same list.
xCODE 3.
xLENGTH 4. plus a multiple of 6.
xVALUE ID LengthNo. of Octets1ID Length111Neighbour ID
Default Metric
Neighbour ID
Delay Metric
Expense Metric
Error Metric
I/E
0
I/E
S
I/E
S
I/E
S
7Default Metric is the value of the default
metric for the link to each of the listed
neighbours. Bit 8 of this field is reserved.
Bit 7 of this field (marked I/E) indicates the
metric type, and shall contain the value 0,
indicating an Internal metric.
7Delay Metric is the value of the delay met
ric for the link to each of the listed neigh
bours. If this IS does not support this met
ric it shall set the bit S to 1 to indicate
that the metric is unsupported. Bit 7 of this
field (marked I/E) indicates the metric type,
and shall contain the value 0, indicating
an Internal metric.
7Expense Metric is the value of the ex
pense metric for the link to each of the
listed neighbours. If this IS does not sup
port this metric it shall set the bit S to 1
to indicate that the metric is unsupported.
Bit 7 of this field (marked I/E) indicates the
metric type, and shall contain the value 0,
indicating an Internal metric.
7Error Metric is the value of the error metric
for the link to each of the listed neighbour.
If this IS does not support this metric it
shall set the bit S to 1 to indicate that the
metric is unsupported. Bit 7 of this field
(marked I/E) indicates the metric type, and
shall contain the value 0, indicating an
Internal metric.
7Neighbour ID system ID of End system
neighbour.
7Authentication Information information for
performing authentication of the originator of the
PDU.
xCODE 10.
xLENGTH variable from 1254 octets
xVALUE 1VARIABLENo. of OctetsAuthentication Type
Authentication Value
7Authentication Type a one octet iden
tifier for the type of authentication to be
carried out. The following values are de
fined:
0 RESERVED
1 Cleartext Password
2254 RESERVED
255 Routeing Domain private
authentication method
7Authentication Value determined by
the value of the authentication type. If
Cleartext Password as defined in this Inter
national Standard is used, then the authenti
cation value is an octet string.
9.9 Level 2 Link State PDU
Level 2 Link State PDUs are generated by Level 2 Interme
diate systems, and propagated throughout the level 2 do
main. The contents of the Level 2 Link State PDU indicates
the state of the adjacencies to neighbour Level 2 Intermedi
ate Systems, or pseudonodes, and to reachable address pre
fixes of the Intermediate system that originally generated
the PDU.11No. of Octets11111122ID Length + 214VARIABLE2Intradomain Routeing
Protocol Discriminator
Length Indicator
Version/Protocol ID Extension
ID Length
PDU Type
R
R
R
Version
ECO
User ECO
PDU Length
Remaining Lifetime
LSP ID
P
Sequence Number
VARIABLE LENGTH FIELDS
LSPDBOL
IS Type
Checksum
ATT
-Intradomain Routeing Protocol Discriminator ar
chitectural constant
-Length Indicator Length of fixed header in octets
-Version/Protocol ID Extension 1
-ID Length Length of the ID field of NSAP ad
dresses and NETs used in this routeing domain. This
field shall take on one of the following values:
7An integer between 1 and 8, inclusive, indicating
an ID field of the corresponding length
7The value zero, which indicates a 6 octet ID field
length
7The value 255, whhich means a null ID field (i.e.
zero length)
All other values are illegal and shall not be used.
-PDU Type (bits 1 through 5) 20. Note bits 6, 7 and
8 are Reserved, which means they are transmitted as 0
and ignored on receipt.
-Version 1
-ECO transmitted as zero, ignored on receipt
-User ECO transmitted as zero, ignored on receipt
-PDU Length Entire Length of this PDU, in octets,
including header.
-Remaining Lifetime Number of seconds before
LSP considered expired
-LSP ID the system ID of the source of the Link
State PDU. It is structured as follows:ID Length1No. of Octets1Source ID
Pseudonode ID
LSP Number
-Sequence Number sequence number of LSP
-Checksum Checksum of contents of LSP from
Source ID to end. Checksum is computed as de
scribed in 7.3.11.
-P/ATT/LSPDBOL/IS Type
7P Bit 8, indicates when set that the issuing Inter
mediate System supports the Partition Repair op
tional function.
7ATT - Bits 7-4 indicate, when set, that the issuing
Intermediate System is `attached' to other areas
using:
xBit 4 - the Default Metric
xBit 5 - the Delay Metric
xBit 6 - the Expense Metric
xBit 7 - the Error Metric.
7LSPDBOL Bit 3 A value of 0 indicates no
LSP Database Overload, and a value of 1 indicates
that the LSP Database is Overloaded. An LSP with
this bit set will not be used by any decision proc
ess to calculate routes to another IS through the
originating system.
7IS Type Bits 1 and 2 indicate the type of Inter
mediate System One of the following values:
x0 Unused value
x1 ( i.e. bit 1 set) Level 1 Intermediate system
x2 Unused value
x3 (i.e. bits 1 and 2 set) Level 2 Intermediate
system.
NOTE In a Level 2 Link State PDU, IS Type
shall be 3.
-VARIABLE LENGTH FIELDS fields of the form:11No. of OctetsLENGTHCODE
LENGTH
VALUE
Any codes in a received LSP that are not recognised
are ignored and passed through unchanged.
Currently defined codes are:
7Area Addresses the set of partition
Area
Addresses of this Intermediate system. For non-
pseudonode LSPs this option shall always be pre
sent in the LSP with LSP number zero, and shall
never be present in an LSP with non-zero LSP
number. It shall appear before any Intermediate
System Neighbours or Prefix Neighbours op
tions. This option shall never be present in
pseudonode LSPs.
xCODE 1
xLENGTH total length of the value field.
xVALUE 1Address Length1Address LengthNo. of OctetsAddress Length
Area Address
Address Length
Area Address
7Address Length Length of area address
in octets.
7Area Address Area address.
7Partition Designated Level 2 Intermediate
System ID of Designated Level 2 Intermediate
System for the partition. For non-pseudonode
LSPs issued by Intermediate Systems which sup
port the partition repair optional function this op
tion shall always be present in the LSP with LSP
number zero, and shall never be present in an LSP
with non-zero LSP number. It shall appear before
any Intermediate System Neighbours or Prefix
Neighbours options. This option shall never be
present in pseudonode LSPs.
xCODE 4.
xLENGTH 6
xVALUE ID of Partition Designated Level 2
Intermediate System for the partition.
7Intermediate System Neighbours Intermedi
ate system and pseudonode neighbours.
This is permitted to appear multiple times, and in
an LSP with any LSP number. However, all the
Intermediate System Neighbours options
shall precede the Prefix Neighbours options.
i.e. they shall appear before any Prefix Neighbour
options in the same LSP and no Prefix Neighbour
options shall appear in an LSP with lower LSP
number.
xCODE 2.
xLENGTH 1. plus a multiple of 11.
xVALUE No. of Octets11ID Length + 11111ID Length + 1111Virtual Flag
Default Metric
Neighbour ID
Delay Metric
Expense Metric
Error Metric
I/E
0
I/E
S
I/E
S
I/E
S
Default Metric
Neighbour ID
Delay Metric
Expense Metric
Error Metric
I/E
0
I/E
S
I/E
S
I/E
S
7Virtual Flag is a Boolean. If equal to 1, this
indicates the link is really a Level 2 path to
repair an area partition. (Level 1 Intermedi
ate Systems would always report this octet
as 0 to all neighbours).
7Default Metric is the value of the default
metric for the link to the listed neighbour.
Bit 8 of this field is reserved. Bit 7 of this
field (marked I/E) indicates the metric type,
and shall contain the value 0, indicating
an Internal metric.
7Delay Metric is the value of the delay met
ric for the link to the listed neighbour. If
this IS does not support this metric it shall
set bit S to 1 to indicate that the metric is
unsupported. Bit 7 of this field (marked
I/E) indicates the metric type, and shall
contain the value 0, indicating an Internal
metric.
7Expense Metric is the value of the ex
pense metric for the link to the listed neigh
bour. If this IS does not support this metric
it shall set bit S to 1 to indicate that the
metric is unsupported. Bit 7 of this field
(marked I/E) indicates the metric type, and
shall contain the value 0, indicating an
Internal metric.
7Error Metric is the value of the error metric
for the link to the listed neighbour. If this
IS does not support this metric it shall set
bit S to 1 to indicate that the metric is un
supported. Bit 7 of this field (marked I/E)
indicates the metric type, and shall contain
the value 0, indicating an Internal metric.
7Neighbour ID. For Intermediate System
neighbours, the first ID Length octets are
the neighbour's system ID, and the last oc
tet is 0. For pseudonode neighbours, the
first ID Length octets is the LAN Level 1
Designated Intermediate System's ID, and
the last octet is a non-zero quantity defined
by the LAN Level 1 Designated Intermedi
ate System.
7Prefix Neighbours reachable address prefix
neighbours
This may appear multiple times, and in an LSP
with any LSP number. See the description of the
Intermediate System Neighbours option
above for the relative ordering constraints. Only
adjacencies with identical costs can appear in the
same list.
xCODE 5.
xLENGTH Total length of the VALUE field.
xVALUE 1iAddress Prefix Length /2y1No. of OctetsiAddress Prefix Length
/2y1111Address Prefix Length
Address Prefix
Address Prefix Length
Address Prefix
Default Metric
Delay Metric
Expense Metric
Error Metric
I/E
0
I/E
S
I/E
S
I/E
S
7Default Metric is the value of the default
metric for the link to each of the listed
neighbours. Bit 8 of this field is reserved.
Bit 7 (marked I/E) indicates the metric
type, and may be set to zero indicating an
internal metric, or may be set to 1 indicat
ing an external metric.
7Delay Metric is the value of the delay met
ric for the link to each of the listed neigh
bours. If this IS does not support this met
ric it shall set the bit S to 1 to indicate
that the metric is unsupported. Bit 7
(marked I/E) indicates the metric type, and
may be set to zero indicating an internal
metric, or may be set to 1 indicating an ex
ternal metric.
7Expense Metric is the value of the ex
pense metric for the link to each of the
listed neighbours. If this IS does not sup
port this metric it shall set the bit S to 1
to indicate that the metric is unsupported.
Bit 7 (marked I/E) indicates the metric
type, and may be set to zero indicating an
internal metric, or may be set to 1 indicat
ing an external metric.
7Error Metric is the value of the error metric
for the link to each of the listed neighbour.
If this IS does not support this metric it
shall set the bit S to 1 to indicate that the
metric is unsupported. Bit 7 (marked I/E)
indicates the metric type, and may be set to
zero indicating an internal metric, or may
be set to 1 indicating an external metric.
7Address Prefix Length is the length in
semi-octets of the following prefix. A
length of zero indicates a prefix that
matches all NSAPs.
7Address Prefix is a reachable address pre
fix encoded as described in 7.1.4. If the
length in semi-octets is odd, the prefix is
padded out to an integral number of octets
with a trailing zero semi-octet.
Note that the area addresses listed in the Area Ad
dresses option of Level 2 Link State PDU with
LSP number zero, are understood to be reachable
address neighbours with cost 0. They are not listed
separately in the Prefix Neighbours options.
7Authentication Information information for
performing authentication of the originator of the
PDU.
xCODE 10.
xLENGTH variable from 1254 octets
xVALUE 1VARIABLENo. of OctetsAuthentication Type
Authentication Value
7Authentication Type a one octet iden
tifier for the type of authentication to be
carried out. The following values are de
fined:
0 RESERVED
1 Cleartext Password
2254 RESERVED
255 Routeing Domain private
authentication method
7Authentication Value determined by
the value of the authentication type. If
Cleartext Password as defined in this Inter
national Standard is used, then the authenti
cation value is an octet string.
9.10 Level 1 Complete Sequence
Numbers PDU11No. of Octets1111112ID Length + 1ID Length + 2ID Length +
2VARIABLEIntradomain Routeing
Protocol Discriminator
Length Indicator
Version/Protocol ID Extension
ID Length
PDU Type
R
R
R
Version
ECO
User ECO
PDU Length
Source ID
Start LSP ID
End LSP ID
VARIABLE LENGTH FIELDS
-Intradomain Routeing Protocol Discriminator ar
chitectural constant
-Length Indicator Length of fixed header in octets
-Version/Protocol ID Extension 1
-ID Length Length of the ID field of NSAP ad
dresses and NETs used in this routeing domain. This
field shall take on one of the following values:
7An integer between 1 and 8, inclusive, indicating
an ID field of the corresponding length
7The value zero, which indicates a 6 octet ID field
length
7The value 255, whhich means a null ID field (i.e.
zero length)
All other values are illegal and shall not be used.
-PDU Type (bits 1 through 5) 24. Note bits 6, 7 and
8 are Reserved, which means they are transmitted as 0
and ignored on receipt.
-Version 1
-ECO transmitted as zero, ignored on receipt