Hi Juan,
Network devices are classified as PAgP physical learners or aggregate-port learners. A device is a physical learner if it learns addresses by physical ports and directs transmissions based on that knowledge. A device is an aggregate-port learner if it learns addresses by aggregate (logical) ports.
When a device and its partner are both aggregate-port learners, they learn the address on the logical port-channel. The device sends packets to the source by using any of the interfaces in the EtherChannel. With aggregate-port learning, it is not important on which physical port the packet arrives.
PAgP cannot automatically detect when the partner device is a physical learner and the local device is an aggregate-port learner. Therefore, you must manually set the learning method on the local device or source-based distribution by using the pagp learn-method interface configuration command. With source-based distribution, any given source MAC address is always sent on the same physical port.
2-
The Etherchannel does learn address across the physical port, or by the Po Port?
According to the above stated it clearly indicates that the learning should/will happen through the physical port.
More info;
http://books.google.co.in/books?id=3CaYld1w5-8C&pg=PA25&lpg=PA25&dq=Physical+learners+vs+Aggregate&source=bl&ots=Z1zVsaEsPF&sig=530quM1uS-AhnrNExGBHpwt4J-g&hl=en&sa=X&ei=vhFuUefLJovOrQfW5oDYAw&sqi=2&ved=0CEkQ6AEwBA#v=onepage&q=Physical%20learners%20v...
HTH
Regards
Inayath
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