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The Complete IS-IS Routing Protocol- Part 19

The Complete IS-IS Routing Protocol- P19: 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. | Flooding 167 Figure 6.13. ATM overlay networks and flooding stress Each router in the Figure 6.13 setup forms an adjacency with the other routers effectively forming a full-mesh. So far so good. Now consider the following scenario the ATM virtual circuit between Seattle and Los Angeles breaks for some reason as indicated by the dotted gray line. Both Seattle and LA notice the break and therefore generate a new LSP incrementing the Sequence Number and removing the adjacency between Seattle and LA . The new LSP is sent according to the flooding rule on all interfaces where there are adjacencies in the Up state. Thus both Seattle and LA send four copies gray arrows of their new LSPs into the network. Next the four other routers will receive the two LSPs white arrows . Here is where the trouble starts because the flooding algorithm is so simple the algorithm does not yet know that all the other routers already been have updated and know that the adjacency between Seattle and LA is down. What follows is a multiplication of LSPs due to the simplicity of the flooding algorithm. All of the routers receive the two new LSPs and re-send the LSP to all the logical interfaces except on the ones on which they got the LSP gray arrows . What results is that 32 LSPs are sent for a single broken ATM VC. This does not sound too stressful for a modern router s control plane however just think if there are not six routers but 100 routers in the network. The problem is that the number of LSPs grows by the square of the number of routers or in mathematical speak O N2 . Thus a single failing VC in the network may generate up to 10 000 LSP updates all flying around in a relatively short amount of time. This is an awful lot of stress for the control plane of a router no matter how powerful. 168 6. Generating Flooding and Ageing LSPs Things get even worse with another failure scenario what if not a single VC but an entire router is going down due to a reboot for example The amount of LSPs .