Which statement is true, as relates to classful or classless routing?
A.
Classful routing protocols send the subnet mask in routing updates.
B.
RIPv1 and OSPF are classless routing protocols.
C.
Automatic summarization at classful boundaries can cause problems on discontiguous subnets.
D.
EIGRP and OSPF are classful routing protocols and summarize routes by default.
Classful routing protocols (RIPv1 and IGRP) automatically summarize routes on the classful network boundary and do not support summarization on any other bit boundaries. Classless routing protocols support summarization on any bit boundary.
Consider the following example –
Router R1 and R2 are running EIGRP. Router R1 has the locally connected subnet 10.0.1.0/24 that is advertised to the router R2. Because of the auto summary feature, the router R1 summarizes the network 10.0.1.0/24 before sending the route to R2. With the auto summary feature turned on, R1 sends the classful route 10.0.0.0/8 to R2 instead of the more specific 10.0.1.0/24 route.
On R1, we have configured, but R2 receives the route to the classful network 10.0.0.0/8.
The auto summary feature can cause problems with discontiguous networks. This is why this feature is usually turned off. This is done by using the no auto-summary command
http://study-ccna.com/eigrp-automatic-manual-summarization/
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