Sequence Number Packets

Link-state packets and their relevance to the process of gathering and disseminating routing information within the IS-IS protocol is covered, in detail, in the preceding section. This section discusses SNPs, which are used in auxiliary mechanisms that ensure integrity of the LSP-based routing information distribution process. You might recollect that hello packets establish and maintain adjacencies between neighbor routers, whereas LSPs are the vehicles for sharing routing information. In a link-state routing environment, routers in an area receive the LSPs from all other routers in the area through their directly connected neighbors in a process called flooding . Through flooding, routers in an area build identical (synchronized) Level 1 Link-State databases. The Level 1 Link-State databases on the routers in an area are synchronized by means of explicit mechanisms in which SNPs play a key role as control packets. The two kinds of SNPs follow:

• Complete sequence number packets (CSNP)

• Partial sequence number packets (PSNP)

CSNPs and PSNPs share the same 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. Separate Level 1 and Level 2 CSNPs and PSNPs are generated to support the Level 1 and Level 2 Link-State databases, respectively. For example, a Level 1 CSNP contains summaries of all the LSPs in the Level 1 Link-State database.

Each of the LSP summaries packed into CSNPs and PSNPs consists of four key pieces of information extracted from the LSP header adequate for unique LSP identification. The following are contained in an LSP summary:

• Link-State Packet Identifier (LSPID)

• Sequence number

• Checksum

• Remaining lifetime

Complete Sequence Number Packets

IS-IS provides mechanisms that allow routers to use CSNP on both point-to-point and broadcast links to check consistency of their Link-State databases. This way, they can determine whether they have current copies of all LSPs generated by the other routers in the local area or in the backbone. As you might recall, intra-area routing is Level 1 and interarea routing is Level 2. Routers that discover they do not have current copies are missing some LSPs, a result of the CSNP exchange (on point-to-point links) or broadcast (on LANs), use PSNPs to request current copies. This process is referred to as Link-State database synchronization . The database synchronization process over point-to-point links differs a little from how it is performed over broadcast links. In the case of point-to-point links, CSNPs are sent only once when the IS-IS adjacency is initialized , preceding the exchange of LSPs over the link. LSPs are exchanged reliably over point-to-point links in a way that ensures all LSPs sent over the link from one end are received at the other end.

On broadcast links, CSNPs are broadcast periodically by the DIS to compensate for an LSP exchange process that is not inherently reliable. As explained in Chapter 2, the DIS plays the role of pseudonode, which is an abstraction for representing broadcast links as network nodes. This reduces the number of one-to-one communications in a broadcast environment and, consequently, reduces the amount of information that is exchanged when many nodes interconnect in such environments.

Routers connecting to the same broadcast link form and maintain Level 1 and Level 2 adjacencies with each other by periodically sending hellos to the multicast addresses known as AllL1IS and AllL2IS, respectively. Adjacency maintenance and database synchronization on broadcast links are independent processes. On broadcast links, LSPs are also broadcast to the same broadcast address mentioned before: AllL1IS for Level 1 LSPs and AllL2IS for Level 2 LSPs. The Level 1- and Level 2-designated routers coordinate Link-State database sychronization by also periodically multicasting Level 1 and Level 2 CSNPs to these same broadcast addresses.

In conformity to the generic IS-IS packet format, CSNPs consist of headers and variable-length fields (TLVs). Figure 5-11 shows the CSNP packet format.

Figure 5-11. CSNP format.

Most of the fields in the CSNP header are discussed in Chapter 2. The following fields are of interest:

• 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 LSP 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.

Table 5-5 lists the types of TLVs in CSNPs: the LSP Entries TLV and a TLV for authentication of the CSNP packet.

Table 5-5. TLVs Supported in CSNPs

TLV	Type	Source
LSP Entries	9	ISO 10589
Authentication Information	10	ISO 10589

• LSP Entries ” This is collection of LSP summaries of all known LSPs in the corresponding Level 1 or Level 2 Link-State databases of the advertising router, sorted in order of ascending LSP ID.

  - Type (1 byte) ” 9
  
  

  - Length (1 byte) ” Total length of the Value field
  
  

  - Value ” Multiples of LSP summaries, each consisting of the remaining lifetime (2 bytes), LSP ID (ID length + 2 bytes), LSP sequence number(4 bytes), and LSP checksum (2 bytes)
  
  

• Authentication Information ” TLV Type 10. As defined for Level 1 and Level 2 LSPs (see Tables 5-3 and 5-4).

Partial Sequence Number Packets

As indicated in the preceding section, PSNPs normally do not contain summaries of all the LSPs in the originating router's database. Instead, PNSPs complement CSNPs in the database synchronization process and perform the following two key functions:

• Routers use PSNPs to acknowledge receipt of one of more LSPs over point-to-point links.

• Routers request transmission of current or missing LSPs by using PSNPs. This applies to both point-to-point and broadcast links.

Because the list of LSP summaries entered into a PSNP is not consistent as in the case of CSNPs, which list summaries of all known LSPs, the Start LSP ID and End LSP ID fields are irrelevant to the application and purpose of PSNPs. These fields are therefore not present in the PSNP header (see Figure 5-12).

Figure 5-12. PSNP format.

Table 5-6 lists the TLVs that can be found in PSNP. These are the same as those supported by CSNPs.

Table 5-6. TLVs Supported in PSNPs

TLV	Type	Source
LSP Entries	9	ISO 10589
Authentication Information	10	ISO 10589