tailieunhanh - The Complete IS-IS Routing Protocol- P25

The Complete IS-IS Routing Protocol- P25: 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. | IS-IS Application Level Fragmentation 245 Summary Contrary to IP routing protocols IS-IS cannot rely on a network layer to do fragmentation for it. IS-IS runs directly on the link-layer which has no possibility of fragmenting frames. IS-IS therefore needs to apply a few techniques to get around the too-small MTU problem if it has to transmit a message that is larger than the MTU. The two techniques IS-IS uses fall under the category of fragmentation avoidance and application level fragmentation. IS-IS assumes a minimum MTU that every link has to support. This limit today is theoretically 1492 and in practice 1497 bytes. Additionally in several packet types in IS-IS there is support for multi-packet messages like CSNPs and LSPs. There are also details of how the application IS-IS should fragment in order to avoid network-wide churn. Cluey implementations think in terms of fragments and try only to rebuild fragments that have been affected by an adjacency change. In current network architectures the distributed LSP storage space is typically only utilized at 10 per cent and even if the space could become exhausted the IS-IS working group has come up with a solution that is similar to modelling adjacencies on a large LAN. There is also the possibility of raising the 1492 byte limit of LSP buffer size as all modern interface cards especially in core environments support MTUs up to 4474 bytes. Although not needed today these developments are a good proof that the IS-IS infrastructure will exist for a long time to come in networks. 8 Synchronizing Databases Link-state protocols rely fundamentally on the fact that each router in a given area has the same view of the topology. Sharing the same view is the foundation for computing converged routes. Convergence means that each router computes routes in a way that moves packets one hop closer to the destination. If routes are not convergent packets can take extra hops in the network to reach the final destination. The .