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Suppose a router R1 is directly connected to the following subnets:

10.1.0.0/24

10.1.1.0/24

10.1.2.0/24

10.1.3.0/24

If it is running RIPv1, it will advertise:

"i have the network 10.0.0.0" (implicitly understood by receiving RIPv1 routers as 10.0.0.0/8 because the protocol is classful)

but suppose we changed the routing protocol to RIPv2 and turned ON auto-summarization. Would it behave in the same way? Would it advertise:

"i have the network 10.0.0.0" (advertised WITHOUT subnet mask, and implicitly understood by other routers as 10.0.0.0/8)

OR would it auto-summarize in a non classful way like:

"i have 10.1.0.0/22" (advertised as network id and subnet mask pair)

In other words, does turning on auto-summarization in RIPv2 (or other classless routing protocols) cause it to auto-summarize in a classful manner or simply auto-summarize classlessly to the best of its ability?

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1 Answer 1

up vote 1 down vote accepted

That would depend on the router, but if you do not define a subnet (if the router allows this) than it would be logical to advertise the class. However if you define the subnetmask it will advertise the subnet.

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That is my understanding as well. It totally depends on the routers firmware. It may even make a difference if you setup these routes straigth in RIPv2 or if you set them up in RIPv1 and then migrate to v2. (v1 may have implicitly stored them as class A which migrates into a /8 in v2 when you upgrade.) This is very implementation depended –  Tonny Oct 14 '12 at 21:11
    
From my experience with cisco routers, you cannot give a layer 3 interface an IP address without giving a subnet mask as well, so assume the router always knows the subnet it is attached to. Then question is: how does the router summarize its attached subnets when RIPv2 auto-summarization is on, classfully or classlessly? (I dont think it should depend on the router since we are referring to the behavior of a standardized routing protocol). –  yorble Oct 14 '12 at 21:15
    
auto-summarize will summarize them classfull IIRC –  Lucas Kauffman Oct 14 '12 at 21:24
    
Classfull would be logical I presume if you upgrade from v1 to v2 and then turn of auto-summary as the OP asks. That would maintain the state of the configuration, so there is no change in behavior of the network as a whole. –  Tonny Oct 15 '12 at 20:04

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