Content

| 7.3 |  | IS-IS Operation |
| 7.3.2 |  | OSI PDUs |

The OSI stack defines a unit of data as a PDU. A frame is therefore regarded by OSI as a data-link PDU. There are three types of PDUs with 802.2 Logical Link Control encapsulation. From these it can be seen that the IS-IS and ES-IS PDUs are encapsulated directly in a data-link PDU while CLNP data packets contain a full CLNP header between the data-link header and any higher layer CLNS information. The IS-IS and ES-IS PDUs contain variable-length fields, depending on the function of the PDU. Each field contains a type code, a length, and then the appropriate values. For this reason, the abbreviation TLV is used for Type, Length, and Value fields. The TLV fields contain the following information:

- The neighbor ISs for the router are used to build the map of the network
- The neighbor ESs for the router
- Authentication information, used to secure routing updates
- Attached IP subnets, if running Integrated IS-IS.

IS-IS PDUs are encapsulated directly into an OSI data-link layer frame. There is no CLNP header and no IP header. The IS-IS protocol family is OSI, and values such as 0xFE and 0xFEFE are used by the data-link protocol to identify the Layer 3 protocol as OSI. This module focuses on the OSI PDUs specific to IS-IS.

There are four categories of IS-IS PDUs:

- Hello PDU (ESH, ISH, IS-IS Hello [IIH]) – Used to establish and maintain adjacencies. ESHs are sent from ESs to ISs. ISHs are sent from ISs to ESs. IIHs are sent between ISs. Note that ESH and ISH PDUs are ES-IS PDUs, not IS-IS PDUs.
- LSP – Used by IS-IS to distribute link-state information. There are independent pseudonode and non-pseudonode LSPs for both Level 1 and Level 2.
- Complete Sequence Number PDU (CSNP) – Used to distribute a complete link-state database on the router. CSNPs are used to inform other routers of LSPs that may be outdated or missing from their own database. This ensures that all routers have the same information and are synchronized. The packets are similar to an OSPF database description packet.
- Partial Sequence Number PDU (PSNP) – Used to acknowledge and request link-state information.

There are nine IS-IS PDU types. The value codes 1 through 10 are defined in ISO 10589, and 128 through 133 are defined in RFC 1195. TLV Code 133, for authentication information, is specified in RFC 1195, but Cisco technology uses the ISO Code of 10 instead. TLV Code 4 is used for partition repair and is not supported by Cisco technology.