ISO 10589
Intermediate System to Intermediate System Protocol
Common Part
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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.
Short explanationn of the different PDU types:
Level-1 LAN IS-IS hello PDU
Used by Level-1 ISs in broadcast LANs. The PDU is a multicast to all ISs within the same broadcast subnet.
Level-2 LAN IS-IS hello PDU
Used by Level-2 ISs within broadcast LANs. The PDU is a multicast to all ISs within the same broadcast subnet.
Point-to-point IS-IS hello PDU
Used on non-broadcast connections (point-to-point links).
Level-1 complete-sequence-numbers PDU
Sent by designated Level-1 ISs periodically into broadcast subnetworks.
Level-2 complete-sequence-numbers PDU
Sent by designated Level-2 ISs periodically into broadcast subnetworks.
Level-1 partial-sequence-numbers PDU
Sent by Level-1 ISs after recept of a complete-sequence-numbers PDU that contains current link state information and which replaces the Level-1 IS's outdated information. If the designated IS receives a partial-sequence-numbers PDU, it send the outstanding information via link-state PDU to the requesting Level-1 IS.
Level-2 partial-sequence-numbers PDU
Sent by Level-2 ISs after recept of a complete-sequence-numbers PDU that contains current link state information and which replaces the Level-2 IS's outdated information. If the designated IS receives a partial-sequence-numbers PDU, it send the outstanding information via link-state PDU to the requesting Level-2 IS.