From: ccie2be (ccie2be@nyc.rr.com)
Date: Thu Apr 28 2005 - 17:41:08 GMT-3
See in-line comments:
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
gladston@br.ibm.com
Sent: Thursday, April 28, 2005 4:16 PM
To: ccielab@groupstudy.com
Subject: IP Prefix-List
Sorry to repeat this topic.
Doing a scenario brought a doubt.
This prefix-list will match 30 first bits and mask /30, right?
ip prefix-list Change-Origin permit 50.180.127.8/30
4th octect -> 0 0 0 0 1 0 0 0
will check 6 firs bits (more significant to less significant)
50.180.127.8 will pass.
50.180.127.9 would pass but it is not checked (and it is not a network
address, it is a host of net 50.180.127.8)
It is checked - at least the first 30 bits are checked, not the last 2 bits.
50.180.127.12 will not pass.
Do you agree?
Agreed.
Not getting many help from doc cd:
==========================
quoted
To deny mask lengths greater than 25 bits in routes with a prefix of 192/8:
ip prefix-list abc deny 192.168.0.0/8 ge 25
==========================
So it will allow 192.5.0.0 255.255.255.128.0.
Would you agree?
Agreed.
IOS change this, as if it was not valid:
r5#c
Enter configuration commands, one per line. End with CNTL/Z.
r5(config)# ip prefix-list Change-Origin seq 15 per 192.168.0.0/8 ge 25
r5(config)#do sh run | i ip prefix-list Change-Origin
ip prefix-list Change-Origin seq 5 permit 50.180.123.0/24
ip prefix-list Change-Origin seq 10 permit 50.180.127.8/30
ip prefix-list Change-Origin seq 15 permit 192.0.0.0/8 ge 25
When you have a ge or le following the prefix, any numbers beyond the /8 in
the network address are meaningless - they're not checked.
So, in your example, the 168 in the 2nd octet isn't checked. Only the first
8 bits, so only the 192 portion of the address is meaningful. The 2nd octet
can be any number - it doesn't matter since it's not checked. So, the IOS
rewrite doesn't change anything of substance, only appearance.
HTH, Tim
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