tailieunhanh - The Complete IS-IS Routing Protocol- P26

The Complete IS-IS Routing Protocol- P26: IS-IS has always been my favourite Interior Gateway Protocol. Its elegant simplicity, its well-structured data formats, its flexibility and easy extensibility are all appealing – IS-IS epitomizes link-state routing. Whether for this reason or others, IS-IS is the IGP of choice in some of the world’s largest networks. | 214 8. Synchronizing Databases the SRM Send Routing Message and the SSN Send Sequence Numbers flag and are defined in ISO 10589. If the SRM is set on a link this means that the corresponding LSP has to be sent out on that link. If the SSN flag is set the corresponding LSP should be included in the next PSNP PDU. Note that these two flags are kept strictly internal to the router. They do not show up in any PDU that the router generates. However what the flags do is influence the link on which LSPs and PSNPs are being sent. Getting back to the header fields proper the DIS extracts this header information from its link-state database and packages up to 15 LSP-IDs in a single TLV. Given an IS-IS MTU of 1497 bytes over Ethernet LANs a DIS can package up to 6 times an LSP Entry TLV 9 resulting in up to 90 LSP-IDs in a single CSNP. So even in the largest networks in the world there are just a few CSNP packets going over the wire every 10 seconds. Next each router on the LAN compares its own link-state database to the CSNP received from the DIS. If the DIS reports the same sequence number for an LSP-ID then everything is fine. If not then there are three basic mismatch conditions that can occur The CSNP reporting an older version of a LSP The CSNP reporting a more recent version of a LSP The CSNP reporting an unknown LSP If the CSNP received is an older version then the action is simple. Because it appears that the DIS is not up to date just tell the DIS about the new version of the LSP by re-flooding the most recent version of the LSP onto the LAN. Figure illustrates the chain of events. Router B notices that Router A is still carrying an older version of the LSP in its link-state database. So Router B floods the LSP with the most recent sequence number 0x7a . Note however that no receiver acknowledges the re-flooded LSP. This principle is sometimes referred to as implicit acknowledgement. So how can the update be made more reliable Just .