tailieunhanh - The Complete IS-IS Routing Protocol- P31

The Complete IS-IS Routing Protocol- P31: 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. | Analysis of IS-IS Extensibility 289 Consider extending OSPF by adding a new hypothetical LSA type. The number could be anything not currently used or defined - LSA type 53 for example. Now what if a router running older OSPF software does not recognize the new hypothetical LSA type 53 Should the router flood the LSA further across the network or should the router simply discard the update To explain what OSPF does with extensions that the router does not understand it is necessary to borrow a term from BGP terminology. The term is called transitivity. Certain BGP attributes are transitive as the attributes flow from BGP router to BGP router. But the term can be applied to protocols other than BGP. Transitivity means that if a router cannot interpret a given message the router will flood the message further across the network anyway. Non-transitive behaviour means that a router does not forward an LSA that the router cannot interpret in terms of the LSA payload. OSPF is a strictly non-transitive protocol. OSPF routers will not flood LSA types that the routers themselves do not understand. The ramifications of that are a bit depressing. Non-transitive OSPF behaviour means that a new feature cannot be rolled out unless all OSPF routers in a given link-state domain are updated with the new software. Given the fact that the role of the IGP is changing from the narrow role of distributing NLRI information towards a wider role with regard to a topology discovery function it might be necessary for new OSPF features of increasing interest to use the flooding sub-system of OSPF for distributing network-wide information. And this information must be spread all over the network information that not each OSPF speaker must necessarily see. I won t flood because I do not understand behaviour is not migration-friendly and therefore causes a lot of headache for preparing migrations. Fortunately the issue of OSPF transitivity has been addressed. RFC 2370 The OSPF Opaque LSA Option .