JUNOSe 7.2.x Policy Management Configuration Guide
176 ! Understanding the Prepended Header
Format of the Mirror Header Attributes
The mirror header values are determined by the value that you configure in VSA
26-59. VSA 26-59 is declared as a hexadecimal string that can be either 8 bytes or 4
bytes long. The 8-byte format enables you to further specify the value that is used
for the Session-ID field. If you use the 4-byte format, the router automatically
determines the Session-ID field. The value in the 2-bit version field specifies the
format that is used—0 indicates the 8-byte format, and 1 indicates the 4-byte
format.
8-Byte Format
The 8-byte format of VSA 26-59 enables you to manually specify the Session-ID
value in addition to the Mirror Identifier value. To use the 8-byte format, you
configure the first two most significant bits of the first word of the VSA to a value of
0, which indicates two words in the VSA. The remaining 30 bits of the first word
form the Mirror Identifier value, and the second word is the Session-ID field. You
cannot change the order of these two words.
For example, a value of 0000030000000090 in VSA 26-59 configures the following
fields in the mirror header, as shown in Figure 12:
! MHV = 0
! Mirror Identifier = 0x300
! Session-ID = 0x90
Figure 12: 8-Byte Format of VSA 26-59
4-Byte Format
To use the 4-byte format of VSA 26-59, you configure the first two most significant
bits of the VSA to a value of 1, which indicates a single word in the VSA. The
remaining 30 bits of the word form the Mirror Identifier value. The router then
creates the Session-ID value based on the least significant 32 bits of the
Acct-Session-ID (RADIUS attribute 44).
Mirror Header
MHV (mirror header value) 0 2
Mirror Identifier See Format of the Mirror
Header Attributes on
page 176 for details
30
Session-ID See Format of the Mirror
Header Attributes on
page 176 for details
32
Table 33: Prepended Header Field Descriptions (continued)
Field Value Length (Bits)
g014394
0 0 0 0 0 3 0 0 0 0 0 0 0 0 9 0
MHV
(2 bits)
Mirror
identifier Session ID
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