TAILIEUCHUNG - The Complete IS-IS Routing Protocol- Part 17

The Complete IS-IS Routing Protocol- P17: 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. | LSPs and Revision Control 147 that the link is down. The other routers then update their link-state databases schedule an SPF calculation and remove that broken link from any transit paths in the network that might use the failed link. Now assume that a remote router gets two conflicting messages at the same time. That is messages arrive almost at the same time claiming that a link is up and down. Consider Figure . There are three routers connected by point-to-point links. Unfortunately the link between Router B and Router C is constantly going down then up and a short time later it goes down again. In network-speak this a flapping link. Next assume that Router B is a busy router with its CPU being loaded close to the ceiling. Therefore Router B is slower in processing the link-down link-up events. In addition the subsequent regeneration of its LSPs that report the link as down or up to other routers is slower. Figure shows the LSP messages from Router A s perspective. Both Router B and Router C find out first that there is a link-up event. However Router C processes that event far faster than the overloaded Router B. The trouble occurs as the link flaps again this time transitioning from the Up to the Down state. Router B still did not send the previous LSP out because it was too busy. Router B s LSP is now outdated information because the link is now in the Down state. So both LSPs Router B s old Up one and Router C s accurate Down one arrive at the same time at Router A. Now Router A needs to decide which is the most accurate report - the Up or Down state message. The LSP to use should be the most recent one but in the example the most recent rule would fail because the propagation delay from Router B to Router A made that inaccurate LSP arrive after Router C s up-to-date LSP. The conclusion here is that LSPs need to carry some sort of version information to explicitly tell the receiving router what is current and what is outdated LSP information. .

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