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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 Outdated and invalid LSPs are removed from the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.
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