WCCP2 allows the setting of return methods between the
router/switch and the cache for packets that the cache
decides not to handle. Valid values are as follows:
gre - GRE encapsulation (forward the packet in a GRE/WCCP tunnel)
l2 - L2 redirect (forward the packet using Layer 2/MAC rewriting)
Currently (as of IOS 12.4) cisco routers only support GRE.
Cisco switches only support the L2 redirect assignment.
If the "ip wccp redirect exclude in" command has been
enabled on the cache interface, then it is still safe for
the proxy server to use a l2 redirect method even if this
option is set to GRE.