From: Connary, Julie Ann (jconnary@xxxxxxxxx)
Date: Wed Jan 24 2001 - 15:47:37 GMT-3
Hi Tom,
I got it - the value is a decimal value that is derived in the following
manner:
The IP TOS field is 8 bits, upper 3 are Precedence, next 4 are TOS and the
least significant bit is always 0.
Precedence = 0-7 because it is 3 bits. TOS = 0-15 because it is 4 bits
So what you do is if you want precedence 5 - critical. Then it is binary
101 for the 3 bits. Now map this into the
8 bit field and get the decimal value:
bit
position: 8 7 6 5 4 3 2 1
decimal
value 128 64 32 16 8 4 2 1
binary
value 1 0 1 0 0 0 0 0
resulting in a decimal value of 160. Put this in your extended ping and you
will hit an accesslist that has the following:
access-list 101 permit ip any any precedence critical
If you want to hit TOS = 8 for min-delay - or 1000 for the four bits
bit
position: 8 7 6 5 4 3 2 1
decimal
value 128 64 32 16 8 4 2 1
binary
value 0 0 0 1 0 0 0
0
resulting in a decimal value of 16.
This would hit the access-list
access-list 101 permit ip any any tos min-delay.
Now if you had both tos and precedence in the same access-list watch out -
because if you had the access-list:
access-list 101 deny ip any any precedence routine (binary 000 in upper
three bits)
access-list 101 permit ip any any tos min-delay (binary 1000 in
next four bits)
then any packet with no precedence bits set would not go through even if
the tos bits are set to min-delay.
Julie Ann
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Julie Ann Connary
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c i s c o S y s t e m s Email: jconnary@cisco.com
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