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92 Chapter 4: Network Integration and Interception

The return method deﬁnes how trafﬁc should be returned from the WAE to the redirecting

router for normal forwarding. Like the forwarding method, there are two different return

methods:

• GRE return: Egress trafﬁc from the WAE using GRE return are encapsulated using

IP GRE, with a destination IP address of the WCCPv2 router ID and a source IP

address of the WAE itself. When the WCCPv2-enabled router receives the returned

packet, the IP GRE header is removed and the packet is forwarded normally. WCCPv2

in IOS knows not to re-intercept trafﬁc returned to it using GRE return.

• L2 return: The L2 return method returns trafﬁc to the WCCPv2-enabled router by

rewriting the destination MAC address of the packet to equal the MAC address of the

WCCPv2-enabled router.

Load Distribution

When multiple WAEs exist in a service group, WCCPv2 automatically distributes redirected

trafﬁc across all WAEs in the service group. When trafﬁc passes through an IOS device with

WCCPv2 redirection conﬁgured, the IOS device assigns trafﬁc for that connection to a

bucket. Each bucket is assigned to a speciﬁc WAE. The method that determines to which

bucket trafﬁc is assigned, which determines how trafﬁc is distributed across multiple WAEs

within a service group, is called the assignment method. The bucket assignments are com-

municated from the lead WAE to all of the IOS devices in the service group. The assignment

method can use either a hashing or masking scheme, and is negotiated between IOS and

WAE during the formation of the service group.

Hash assignment, which is the default assignment method, performs a bitwise hash on a key

identiﬁed as part of the service group. In WAAS, the hash key used for service group 61 is

the source IP address, while the hash key used for service group 62 is the destination IP

address. The hash is not conﬁgurable, and is deterministic in nature. This means that all

of the routers within the same service group will make the same load-balancing decision

given the same hash key. This deterministic behavior is what allows WCCPv2 to support

asymmetric trafﬁc ﬂows, so long as both directions of the ﬂow pass through WCCPv2-

enabled IOS devices in the same service group. Hash assignment uses 256 buckets.

Figure 4-8 shows an example of the hash assignment method and bucket-based distribution

model used by WCCPv2.

The second assignment method is called mask assignment. With mask assignment, the

source IP address, destination IP address, source port, and destination port are concatenated

and ANDed with a 96-bit mask to yield a value. The resulting 96-bit value is compared to

a list of mask/value pairs. Each mask/value pair is associated with a bucket, and each bucket

is in turn assigned to a WAE. Unlike hash assignment, the number of buckets used with

mask assignment depends on the number of bits used in the mask. By default, WAAS uses

a mask of 0x1741. This results in 2

buckets that can be assigned across the WAEs in a

service group. With current Catalyst WCCPv2 implementations, up to 7 bits can be deﬁned

for the mask. Figure 4-9 shows an example of the mask assignment method and bucket-

based distribution model used by WCCPv2.

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