WCCP uses Layer 2 to redirect or return traffic. In addition to Layer 2 redirect, the C9500-32C, C9500-32QC, C9500-48Y4C,
and C9500-24Y4C models of the Cisco Catalyst 9500 Series Switches support generic routing encapsulation (GRE) redirection
too.
When WCCP forwards traffic through GRE, the redirected packets are encapsulated within a GRE header. The packets also have
a WCCP redirect header. When WCCP forwards traffic using Layer 2, the original MAC header of the IP packet is overwritten
and replaced with the MAC header for the WCCP client.
Using Layer 2 as a forwarding method allows direct forwarding to the content engine without further lookup. Layer 2 redirection
requires that the router and content engines are directly connected, that is, located on the same IP subnetwork.
When WCCP returns traffic using GRE, the returned packets are encapsulated within a GRE header. When WCCP returns traffic
through Layer 2, the original IP packet is returned without any added header information. The router to which the packet is
returned recognizes the source of the packet and prevents redirection.
Layer 2 forwarding, return, or redirection are typically used for hardware-accelerated platforms. Depending on the Cisco IOS
XE release running on your device, Layer 2 forwarding, return, and redirection are also used on software-switching platforms.