From: Nicolai Gersbo Solling (nicolai@xxxxxxxxx)
Date: Thu Mar 21 2002 - 08:27:55 GMT-3
Hi there hot shot SAP'ers
I am sitting here going through the maths behind filtering saps, and that
seems a bit (or should i say a hex) problematic!
It is not that I don't understand it, some of what is happening is just a
bit unclear to me!
For instance can you make SAP filer, which permits or deny's more then sap
at a time (I know that you can for SNA SAPS - that os 0x0000 0x0d0d) - But
is this approach to filter as many saps at one time not sometimes a bit
wrong...
Let me axplain:
For instance i wan't to filter these 2 saps:
42 IEEE 802.1 Bridge Spanning Tree Protocol
4E EIA RS-511 Manufacturing Message Service
The filter in order to match those 2 saps would look like this
0x4242 mask 0x0C0C
Or in binary:
0100 0010
0000 1100
This would allow:
0100 0010 = 42
0100 0110 = 46
0100 1010 = 4A
0100 1110 = 4E
So my point is: If I had made this sap filter more restrictive I would not
have been able to match both sap 42 and 4E, but my mask allows also 46 and
4A - I know that on the Cisco SAP's page it does not say that these saps are
used for anything, but anyway...
Would it not have been better to make a sap filter for these 2 saps like
this?
0x4242 mask 0x0000
0x4E4E mask 0x0000
Any thoughts - Is my understanding of saps totally wrong?
Nic
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