WCCP uses either generic routing encapsulation (GRE) or Layer 2 (L2) to redirect or return IP traffic. When WCCP forwards traffic via GRE, the redirected packets are encapsulated within a GRE header. The packets also have a WCCP redirect header. When WCCP forwards traffic using L2, the original MAC header of the IP packet is overwritten and replaced with the MAC header for the WCCP client.
Using L2 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, on the same IP subnetwork.
When WCCP returns traffic via GRE, the returned packets are encapsulated within a GRE header. The destination IP address is the address of the router and the source address is the address of the WCCP client. When WCCP returns traffic via L2, the original IP packet is returned without any added header information. The router to which the packet is returned will recognize the source of the packet and prevent redirection.
The WCCP redirection method does not have to match the return method.
L2 forwarding, return, or redirection are typically used for hardware accelerated platforms. In Cisco IOS Release 12.4(20)T and later releases, L2 forwarding, return, and redirection can also be used for software switching platforms.
On Cisco Catalyst 6500 Switches with a PFC4, GRE decapsulation is supported in hardware.
On Cisco ASR 1000 Series Aggregation Services Routers, both the GRE and L2 forward/return methods use the hardware, so there is not any significant performance degradation between them.
For content engines running Application and Content Networking System (ACNS) software, use the wccp custom-web-cache command with the l2-redirect keyword to configure L2 redirection. For content engines running Cisco Wide Area Application Services (WAAS) software, use the wccp tcp-promiscuous command with the l2-redirect keyword to configure L2 redirection.
For more information on Cisco ACNS commands used to configure Cisco Content Engines, see the Cisco ACNS Software Command Reference, Release 5.5.13.
For more information on WAAS commands used to configure Cisco Content Engines, see the Cisco Wide Area Application Services Command Reference (Software Versions 4.2.1).