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OSPF and IS-IS: Choosing an IGP for Large-Scale Networks by Jeff Doyle

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8.3. Fragmentation

Sections 7.3.2 and 7.4.2 discuss the issue of large LSAs and LSPs and the effect they have on the scalability of an area in terms of bandwidth usage during flooding and memory usage as they are stored in the link state databases. Another issue with the scaling of LSAs and LSPs concerns the ability of link MTUs to accommodate them. That issue is the question of how to handle an LSA or LSP that is larger than the MTU of a link it is supposed to traverse. When there is a low-MTU link along the flooding path, an OSPF or IS-IS implementation can do one of three things:

  • It can limit the information units it generates to a very small size to ensure that they never exceed the lowest possible MTU of any link. This is not a very practical ...

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