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Cisco Catalyst Blade Switch 3130 and 3032 for Dell Software Configuration Guide
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Chapter 43 Configuring Web Cache Services By Using WCCP
Understanding WCCP
WCCP Message Exchange
This sequence of events describes the WCCP message exchange:
1.
The application engines send their IP addresses to the WCCP-enabled switch by using WCCP,
signaling their presence through a
Here I am
message. The switch and application engines
communicate to each other through a control channel based on UDP port 2048.
2.
The WCCP-enabled switch uses the application engine IP information to create a cluster view (a list
of application engines in the cluster). This view is sent through an
I see you
message to each
application engine in the cluster, essentially making all the application engines aware of each other.
A stable view is established after the membership of the cluster remains the same for a certain
amount of time.
3.
When a stable view is established, the application engine in the cluster with the lowest IP address
is elected as the designated application engine.
WCCP Negotiation
In the exchange of WCCP protocol messages, the designated application engine and the WCCP-enabled
switch negotiate these items:
•
Forwarding method (the method by which the switch forwards packets to the application engine).
The switch rewrites the Layer 2 header by replacing the packet destination MAC address with the
target application engine MAC address. It then forwards the packet to the application engine. This
forwarding method requires the target application engine to be directly connected to the switch at
Layer 2.
•
Assignment method (the method by which packets are distributed among the application engines in
the cluster). The switch uses some bits of the destination IP address, the source IP address, the
destination Layer 4 port, and the source Layer 4 port to determine which application engine receives
the redirected packets.
•
Packet-return method (the method by which packets are returned from the application engine to the
switch for normal forwarding). These are the typical reasons why an application engine rejects
packets and starts the packet-return feature:
–
The application engine is overloaded and has no room to service the packets.
–
The application engine receives an error message (such as a protocol or authentication error)
from the web server and uses the dynamic client bypass feature. The bypass enables clients to
bypass the application engines and to connect directly to the web server.
The application engine returns a packet to the WCCP-enabled switch to forward to the web server
as if the application engine is not present. The application engine does not intercept the reconnection
attempt. In this way, the application engine effectively cancels the redirection of a packet to the
application engine and creates a bypass flow. If the return method is generic-route encapsulation
(GRE), the switch receives the returned packet through a GRE tunnel that is configured in the
application engine. The switch CPU uses Cisco express forwarding to send these packets to the
target web server. If the return method is Layer 2 rewrite, the packets are forwarded in hardware to
the target web server. When the server responds with the requested information, the switch uses
normal Layer 3 forwarding to return the information to the requesting client.
