· LPSs. Using CSNP ensures that all routers

includes a header that
describes the PDU type and length, the LSP ID and a sequence number, and a
remaining lifetime timer. It also includes the TLV which contains links to
neighbor routers, including the metrics on those interfaces. The sequence
number is used to ensure that receiving routers do not use outdated LSPs to
calculate best routes to destinations. This avoids having the same copy of LSPs
that can be added to the topology table. The remaining lifetime timer is used
to determine how long the LSP can be kept valid in the IS–IS LSP database. This
timer is used by the LSP aging process to ensure that LSPs that are not valid
or that are outdated are dropped from

the topology table after every 20 minutes. There is also an LSP
refresh timer specified in ISO 10598 by which routers sending LSPs must use to
update their entries in the topology table.

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Complete Sequence Number
PDU (CSNP): This packet is made up of a list of LSPs that
are stored in the link state database. IS–IS routers use CSNP to inform each
other about outdated or lost LPSs. Using CSNP ensures that all routers have the
same information in their link state database.

Partial Sequence Number
PDU (PSNP): If an LSP becomes outdated or lost, PSNP is
used to request a router to send a new LSP. It is also used to acknowledge the
receipt of an LSP (Lemma et al, 2008). Type Length Values (TLVs)


length value is the main data portion of the IS–IS packet through which the protocol
can be extended. A typical TLV contains routing information that is propagated
throughout the IS–IS routing domain. TLV is divided into three fields. These
fields are identified by one octet of type (T), one octet of length (L) and “L”
octets of value (V). The type field is used to specify the type of data in the
value field. The length field is used to indicate the length of value field
while the value field is the data portion of the IS–IS packet where routing
information is carried for transmission (Cisco.com, 2005).

To support routing in IP, IP information is
encoded in a new TLV 128 defined for the IS–IS PDUs. The purpose of this is to
allow the distribution of IP destinations before they can be reached. Since IP
information resides in the same packet as CLNS information, there is a protocol
supported field included in the IS–IS Hello packet to indicate the type of
protocol supported by a sending router. In addition to that, the Hello packet
also includes the IP address on the interface of a neighboring router because
ICMP redirect messages to end systems must include the next hop address. Thus
each Hello packet contains the IP address of the interface on which