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[diya]

From cisco definition, this command is used to send summary rip prefixes , but rip always advertising the major networks!. so no need to that summary, right?. the ios even rejected summaries beyond classuful networks such as 10.0.0.0/6!


Even in the case in which a router (router A for example) has two interfaces with two subnets belong to one classuful network such as 10.2.2.0 , 10.1.1.0 "in this case the router is enforced to advertise thses subnets as they are "without summary" to other routers from the same major network, i tried to configure summary-address rip 10.0.0.0 in router A it didn't work, it did not advertised this summary.

so why we need this command??

[ThermalWinter]

I'm assuming you are talking about RIPv1. The RIPv1 doesn't pass the subnet mask in its advertisements, and therefore assumes the subnet mask of any local interface that is in the same major network as the received update. For example, if the router receives an update 10.2.2.0, and has a configured interface of 10.1.1.0/24, then the router assumes that 10.2.2.0 is also a /24. If the router does not have an interface in the same major network as the update, then the major network subnet mask is assumed (/8) in the previous example. Since RIPv1 does not pass the subnet mask in its update, it doesn't support manual summarization (ip summary-address rip command) and auto-summary cannot be disabled.

RIPv2 on the hand, does carry the subnet mask in its updates and therefore supports manual summaration and the turning off of auto summary.