From: jon (jon@storm.to)
Date: Wed Nov 16 2005 - 20:14:57 GMT-3
Ok, I'm interested in this one, so I'll work it out online...
Ignore everything except the octet you are looking at, so
0 with 254 as mask
00000000
00000001
Means that the LSB must be zero to pass the access list.
If the LSB is zero, then the number is even.
Check that with an example...
Octet is 128
10000000 Octet
00000001 Mask
00000000 Result
00000000 Compare
So true.
If that's right then maybe that's not where the problem is?
Exactly how would you use an offset list to filter routes? What EXACTLY does
an offset list do? How does RIP represent unreachable networks? I don't know
the official answer but I think I have an idea...
-----Original Message-----
From: John Matus [mailto:John.Matus@tokiom.com]
Sent: 16 November 2005 22:40
To: ccielab@groupstudy.com
Subject: prefix filtering w/ access-list.......stumped
ok,
this has always gotten me and i still don't understand why
lets say that you want to deny all routes (in rip) coming in w/ an even
second octet. when i look @ IE's solution guide, they show an offset list
with an ACL attached that is:
access-list 1 permit 0.0.0.0 255.254.255.255 ( the stipulation
is that the acl can only be 1 line), so this is supposed to say, i'm
assuming "permit odd routes"
but.....
from my understanding, if you have a 0 in the octet you are trying to
filter, then the octet is going to be even, and with a mask of 254, you are
going to be matching the 1's column, so it would seem to me that:
permit 0.0.0.0 255.254.255.255 would actually PERMIT all even routes
rather than denying them (my solution to the problem was "permit 0.1.0.0
255.254.255.255" which i believed would permit all odd routes...........but
it seems that i am mistaken somehow........
what am i not getting??
Regards,
John D. Matus
Technical Support / PAS
Fujitsu Consulting
626-568-7716
John.Matus@tokiom.com
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