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sequence_number_pakete [2009/09/13 14:37] (current)
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 +===== Sequence number packets =====
 +The third category of IS-IS PDUs are sequence number packets (SNPs). SNPs are used as auxiliary mechanism and ensure integrity of the LSP-based routing information distribution process. The major task of a sequence number packet is to ensure with explicit mechanisms a level 1 or level 2 link-state database. You can realise this goal with two different packet types.
 +
 +  * Complete sequence number packets (CSNP)
 +  * Partial sequence number packets (PSNP) 
 +
 +Both packet types share a similar packet format and each carries a collection of LSP summaries. The basic difference between them is that a CSNP advertised by a router contains summaries of all the known LSPs in its database, whereas the PSNP contains only a subset.
 +
 +----
 +
 +These two types of packets are used in level 1 link-state databases and in level 2 link-state databases. That means that four types of sequence number packets are used.
 +
 +==== Complete sequence number packets (CSNP) ====
 +
 +Most of the fields of a CSNP header are discussed in an earlier chapter.Yet untreated or very important fields are:
 +
 +  * Source ID: The source ID refers to the SysID of the router that generated the CSNP. On a point-to-point link, it is the SysID of the nodes on either side of the link. On a broadcast link, it is the SysID of the designated router.
 +  * Start LSP ID: The start LSP ID is the LSP ID of the first LDP in the LSP entries TLV attached to the header.
 +  * End LSP ID: This is the LSP ID of the last LSP in the LSP entries TLV field.
 +  * LSP Entries: This is a collection of LSP summaries of all known LSPs in the corresponding level 1 or level 2 link-state database of the advertising router, sorted in order of ascending LSP ID.
 +  * Authentication Information: As defined for level 1 and level 2 LSPs. TLV type 10.   
 +
 +==== Partial sequence number packets (PSNP)    ====
 +
 +Partial sequence number packets normally have two key functions. First of all PSNPs are used by routers to acknowledge receipt of one or more LSPs over point-to-point link. The second key function is that routers request transmission of current or missing LSPs by using PSNPs. That’s true to both network types, point-to-point and broadcast links. 
 +
 +
 +
 +
 +----
 +__Sources:__
 +
 +
 +Abe Martey, IS-IS Network Design Solution, Kap. 3
 +
 +CCNP 1 Ver. 3.1 Kap.8
 
sequence_number_pakete.txt · Last modified: 2009/09/13 14:37 (external edit)
 
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