WAAS WCCP v2 mask assignment calculation method

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Mon, 11/11/2013 - 22:25
Nov 7th, 2013
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Introduction:

This document describes how the mask is used for calculation when the mask assignment for WCCP version 2 is used.

Core Issue:


There is currently no information available on how the mask is used for calculation when the mask assignment for WCCP version 2 is used.

Resolution:


For WAAS 4.1 or earlier, the mask value 0x1741 is used for source ip by default.

For this mask value, the leading 0 is omitted, with this value equivalent to 0x00001741; red is the mask for the 1st octet, brown is for the 2nd, green is for the 3rd and blue is for the 4th. The binary expression is as follows.

1st octet   2nd octet   3rd octet   4th octet

   0 0         0 0         1 7         4 1

0000:0000 . 0000:0000 . 0001:0111 . 0100:0001


The bucket is allocated based on the logical product of this mask and source ip.

For example, if source ip is 192.168.1.1, its binary expression is as follows.

1100:0000 . 1010:1000 . 0000:0001 . 0000:0001

The logical product of this source ip and 0x1741 mask is as follows.

0000:0000 . 0000:0000 . 0000:0001 . 0000:0001 = 0x101


Similarly, from 192.168.1.1 through 192.168.1.63 , the logical product is 0x101 if the 4th octet is an odd number, and 0x100 if it is an even number. From 192.168.1.64 through 192.168.1.254, the logical product is 0x141 if the 4th octet is an odd number, and 0x140 if it is an even number.


From WAAS 4.2.1, the default mask value has been changed to 0xf00, with binary expression as follows.

1st octet   2nd octet   3rd octet   4th octet

   0 0         0 0         0 f         0 0

0000:0000 . 0000:0000 . 0000:1111 . 0000:0000


You can see that the 4th octet of this mask can be ignored.If the subnet mask is /24, all the clients belonging to the same subnet are redirected to the same WAE and not load-balanced even if there is more than one WAE.


If mask assignment is used on the WAAS branch side, changing the mask value to 0xf, etc. will allow clients to be load-balanced to more than one WAE.


Original Document: Cisco Support Community Japan DOC-13186

Author: Yosuke Kamimura

Posted on September 26, 2010

https://supportforums.cisco.com/docs/DOC-13186

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