Content

| 7.3 |  | IS-IS Operation |
| 7.3.4 |  | IS-IS link-state PDU (LSP) formats |

The contents of the TLV fields include the following:

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

The complete list of LSP fields for Level 1 and Level 2 PDUs, is as follows:

- Intradomain Routing Protocol discriminator – This is the network layer identifier assigned to IS-IS in ISO 10589. Its binary value is 10000011, hexadecimal 0x83.
- Length indicator – Length of the fixed header in octets.
- Protocol ID Ext – Currently has value of one (1).
- ID length – Length of the system ID field. Must be the same for all nodes in the domain. If set to zero, it implies six (6) octets.
- PDU types – Assumes decimal value. For example, values of 17, 18, and 20 are for point-to-point, Level 1, and Level 2 LSPs, respectively.
- Version – Value is one (1).
- Maximum area addresses – Number of area addresses permitted for this IS area. Values are between 1 and 254 for actual number. 0 (zero) implies a maximum of three.
- PDU length – Length of the entire PDU, fixed header, and TLVs.
- Remaining lifetime – Time in seconds before LSP expires. Used to age out LSPs. Outdated and invalid LSPs are removed from the topology table after a suitable period. It is a count-to-zero operation with a default 1200 second start value, or MaxAge. If the remaining lifetime expires, the first router that notices purges the LSP, removes the LSP body, keeps the LSP header, and sets the age to zero. It floods this modified LSP in the usual way throughout the network. Zero lifetime LSPs are newer than non-zero lifetime LSPs. After awhile, all routers remove the purged LSP from their LSP database.
- LSP ID – Consists of three components. There are the system ID, pseudonode ID, and the LSP fragmentation number. The pseudonode ID is 0 (zero) for a router LSP. Length is ID length plus 2 bytes.
- Sequence number – Used for synchronization. Higher sequence number indicates newer LSP. Enables receiving routers to ensure they use only the latest LSPs in their route calculations. Used to avoid duplicate LSPs being entered into the topology tables.
  - When there is a change, the sequence number is incremented and a new version of the LSP is generated with the new sequence number.
  - When a router reloads, the sequence number is set initially to one (1). The router may then receive its own old LSPs back from its neighbors, which will have the last good sequence number before the router reloaded. It records this number and reissues its own LSPs with the next highest sequence number.
- Checksum – Checksum is computed from Source ID to end of PDU. Used to detect LSP corruption during flooding. It may be that the Layer 2 CRC is not sufficient for error checking. Corruption happens in routers and switches. The checksum is computed upon receipt of LSP and checked against the checksum inside the LSP. If corrupt, LSP is dropped and sender retransmits. If two LSPs have the same LSP ID, the same sequence number, and the same remaining lifetime, the LSP with the highest checksum is kept. This guarantees consistent LSP databases across the network. This scenario can happen after a router reboots or is reconnected to the network.
- Partition (P) – Bit 8 of the octet. When set, means originator of LSP supports partition repair.
- Attached Bit (ATT) – Bits 4 through 7 of the octet. When any of these bits is set, it indicates the originator is attached to another area using the referred metric. For example, bit 4 set implies attached using the default metric. Set in the Level 1 LSP by an L1L2 router if it has connectivity to another area, it will indicate to the area routers, Level 1, that it is a potential exit point of the area. Level 1 routers select the closest, best metric Level 2 router with the ATT-bit set.
- LSPDBOL (Overload Bit) – Bit 3. When set, it indicates the originator's LSP database is overloaded and should be circumvented in path calculations to other destinations. Indicates that the router has an incomplete LS database, and therefore cannot be trusted to computer any correct routes. Used in the LSP database, but topology behind it is not calculated. Therefore, other routers do not compute routes that would require the PDU to pass through the overloaded router. An exception to this is ES neighbors since these paths are guaranteed to be non-looping.
- IS type – Bits 1 and 2 used to indicate Level 1 or Level 2 LSP type. When only bit 1 is set it indicates Level 1 IS. If both are set, it indicates Level 2 IS.

Section 9 of RFC 1142, a rewrite of ISO 10589, gives details about the packet layouts for each type of IS-IS PDU. It also gives the TLV information supported for each type. The first eight octets of all IS-IS PDUs are header fields that are common to all PDU types. The Level 1 and Level 2 LAN Hello PDUs are identical, except for the PDU type, which differentiates them as either Level 1 or Level 2. - It also shows that the point-to-point Hello PDU is very similar to the Level 1 and Level 2 LAN Hello PDUs.

The lengths for the various ID fields in the PDUs, the LSP ID, source ID, and so on, all assume that the length of the system ID is fixed at six (6) bytes. Under the column for the number of octets in Figure , an 8 would mean ID length + 2, a 7 would mean ID length + 1, and a 6 would mean ID length. Try not to confuse the value of the ID length variable with the size of the ID length field, which is fixed at one byte. The CLNS protocol allows the system ID, part of the NSAP address, to vary from three to eight bytes. However, in practice a six-byte system ID is always used, ID length = 0. If the ID length field is 0 (zero), it means that the system ID is using the default length of six bytes.