ISO 10589
Intermediate System to Intermediate System Protocol
Level-2 Link State
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IS-IS is a routing protocol used between intermediate systems (IS = router) / router exchange protocol.
IS-IS is used in nine different PDU types:
> Point-to-point IS-IS hello
> Level-1 LAN IS-IS hello
> Level-2 LAN IS-IS hello> Level-1 link state
> Level-2 link state
> Level-1 complete sequence numbers
> Level-2 complete sequence numbers
> Level-1 partial sequence numbers
> Level-2 partial sequence numbers
All PDU types use the same 'common part' at the beginning of the PDU; the parameters after 'common part' differ.
There are four different routing-level:
Level-0 Routing: between ESs and ISs in the same subnet.
Level-1 Routing: between ISs within the same ares.
Level-2 Routing: between different areas within the same routing domain.
Level-3 Routing: between different routing domains.
Level-2 Link State uses a fixed part first and a variable part behind it.
PDU Length (2 bytes)
Protocol: ISO 10589 / IS-IS
PDU Part: Level-2 Link State / Fixed Part
Field: PDU Length
Length: 2 Bytes
Contents: length of PDU in bytes.
Remaining Lifetime (2 bytes)
Protocol: ISO 10589 / IS-IS
PDU Part: Level-2 Link State / Fixed Part
Field: Remaining Lifetime
Length: 2 Bytes
Contents: remaining lifetime of PDU.
This parameter's value indicates the remaining lifetime in seconds. If this time has passed, the LSP will be discarded.
LSP ID (n bytes)
Protocol: ISO 10589 / IS-IS
PDU Part: Level-2 Link State / Fixed Part
Field: link state PDU ID
Length: n bytes
Contents: addresse of sender of the link state PDU.
The LSP ID length is calculated as: [ID Length] + 2 bytes.
Sequence Number (4 bytes)
Protocol: ISO 10589 / IS-IS
PDU Part: Level-2 Link State / Fixed Part
Field: sequence number
Length: 4 bytes
Contents: sequence number of current link state PDU.
Checksum (2 bytes)
Protocol: ISO 10589 / IS-IS
PDU Part: Level-2 Link State / Fixed Part
Field: checksum
Length: 2 bytes
Contents: checksum.
The checksum covners the part from LSP ID to end of PDU.
P Bit (1 bit)
Protocol: ISO 10589 / IS-IS
PDU Part: Level-2 Link State / Fixed Part
Field: P bit = partition repair bit
Length: 1 bit
Contents: indicates partition repair support.
P=1 indicates that the sending IS supports the optional function 'partition repair'.
ATT (3 bits)
Protocol: ISO 10589 / IS-IS
PDU Part: Level-2 Link State / Fixed Part
Field: ATT
Length: 3 bits
Contents: indicates the kind of use of routing metric.
The ATT bits indicates which kind of routing metrics is used:
> default
> delay
> expense
> error
L Bit (1 bit)
Protocol: ISO 10589 / IS-IS
PDU Part: Level-2 Link State / Fixed Part
Field: L bit
Length: 1 bit
Contents: overload indication.
L=1 indicates that the LSP (link state PDU) database is overloaded.
Type (2 bits)
Protocol: ISO 10589 / IS-IS
PDU Part: Level-2 Link State / Fixed Part
Field: type
Length: 2 bits
Contents: IS type.
This parameter inidicates the current IS type:
> Level-1 IS
> Level 2 IS
Area Addresses (n bytes)
Protocol: ISO 10589 / IS-IS
PDU Part: Level-2 Link State / Variable Part
Field: area addresses
Length: n bytes
Contents: one or more area addresses of this IS.
Partition Designated Level-2 IS (n bytes)
Protocol: ISO 10589 / IS-IS
PDU Part: Level-2 Link State / Variable Part
Field: partition designated Level-2 IS
Length: n bytes
Contents: ID of the designated IS within the current partition.
IS Neighbors (n bytes)
Protocol: ISO 10589 / IS-IS
PDU Part: Level-2 Link State / Variable Part
Field: IS neighbors
Length: n bytes
Contents: one ore more addresses of neighbor ISs.
IS neighbor entries contain the corresponding neighbor ID as well as the current values of all four kind of metric.
Prefix Neighbors (n bytes)
Protocol: ISO 10589 / IS-IS
PDU Part: Level-2 Link State / Variable Part
Field: Prefix Neighbors
Length: n bytes
Contents: one ore more addresses of prefix neighbors.
Prefix neighbor entries contain the corresponding prefix neighbor ID as well as the current values of all four kind of metric.
Authentication Information (n bytes)
Protocol: ISO 10589 / IS-IS
PDU Part: Level-2 Link State / Variable Part
Field: authentication information
Length: n bytes
Contents: password for source station authentication.
This parameter is optional used for protection of the routing information.