The exact purpose of this command is to force the router to adopt a classless behavior as its name indicates. And it is correct that it doesn't applt the routing protocols as such.
Here's an example of when classfull behavior is configured (no ip classless). The router receives a packet with a destination address of 172.16.19.1 and there is already a route in the RIB for another subnet belonging to the same classfull network, let's say 172.16.18.0/24. The router tries to resolv the destination address using the classfull network and drops the packet if there is not matching subnet even if a default route exists in the RIB. In other words, the classfull behavior will not allow resolution outside the classfull network.
On the other hand, if a classless behavior is configured (ip classless) the router sear for the best match in the RIB and will use the default route if no better router exists.
Hope this helps,
Harold Ritter Sr. Technical Leader CCIE 4168 (R&S, SP) firstname.lastname@example.org México móvil: +52 1 55 8312 4915 Cisco México Paseo de la Reforma 222 Piso 19 Cuauhtémoc, Juárez Ciudad de México, 06600 México
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