1.Virtual link is normaly used for temporary purpose while trobleshooting.It is used to connect a non-backbone area to backbone area through another non-backbone area.Also used to avoid partitioning of backbone(area 0).
2.Summarization is normaly used when u want to minimize the size of routing table,hence u can put a single summarized routes summarizing the specific routes.It also helps as the upadtes also will be only this smmarized routes supressing the specific routes.So the link is better utilised.(not much a benifit in eigrp as the updates are not periodic)
3.This is a static entry point towards virtual null interface,ie ,default routes are droped.The redistribution might be done for any lab purpose or something ,ie,when u want a default route to be redistributed into eigrp etc.
Virtual links can also be used, when e.g 2 companies merge each having an area 0 at their premises. The 2 area 0's can be consolidated by connecting them through a non-backbone area (using the virtual link feature), thus connecting their OSPF networks together.
In case of EIGRP question, it appears to me as if there was an untold question: is it possible to summarize only at the interface?
The answer is 'yes'. At least I don't know any possibilities in EIGRP other than summarizing routes at the inteface.
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