07-15-2008 01:42 PM - edited 03-03-2019 10:44 PM
Hi All,
I need understanding the lines below. For some reason the "ge" option in the prefix list is throwing me off. So far, I understand it as BGP will NOT advertise X.X.160.0 out. Is that correct?
PREFIX LIST
ip prefix-list OUT seq 5 permit X.X.160.0/20 ge 21
ROUTE MAP
route-map OUT deny 20
match ip address prefix-list OUT
!
route-map OUT permit 100
match as-path 2
BGP
Router bpp xxxx
neighbor x.x.x.x route-map OUT out
07-15-2008 01:59 PM
Yes, that is correct. Basically GE means greater than, so GE 21 means anything that is greater than /21 will be denied such as /22, /23, etc. So this prefix-list blocks more specific routes to x.x.160.0/20
07-15-2008 02:56 PM
Thanks for the reply. Ok I think I understand.. but a few more questions to help me understand better. Does this mean 1.1.168.0 - 1.1.167.254 will be advertised? See output below. (BGP line added)
BGP
router bgp xxxx
network 1.1.160.0 mask 255.255.240.0
PREFIX INFO
whatmask 1.1.160.0/20
------------------------------------------------
TCP/IP NETWORK INFORMATION
------------------------------------------------
IP Entered = ..................: 1.1.160.0
CIDR = ........................: /20
Netmask = .....................: 255.255.240.0
Netmask (hex) = ...............: 0xfffff000
Wildcard Bits = ...............: 0.0.15.255
------------------------------------------------
Network Address = .............: 1.1.160.0
Broadcast Address = ...........: 1.1.175.255
Usable IP Addresses = .........: 4094
First Usable IP Address = .....: 1.1.160.1
Last Usable IP Address = ......: 1.1.175.254
whatmask 1.1.160.0/21
------------------------------------------------
TCP/IP NETWORK INFORMATION
------------------------------------------------
IP Entered = ..................: 1.1.160.0
CIDR = ........................: /21
Netmask = .....................: 255.255.248.0
Netmask (hex) = ...............: 0xfffff800
Wildcard Bits = ...............: 0.0.7.255
------------------------------------------------
Network Address = .............: 1.1.160.0
Broadcast Address = ...........: 1.1.167.255
Usable IP Addresses = .........: 2046
First Usable IP Address = .....: 1.1.160.1
Last Usable IP Address = ......: 1.1.167.254
07-16-2008 01:54 AM
Hello Merrylle,
prefix 1.1.160.0/21 will match the prefix-list.
May you post also the as-path access-list 2, what is permitted by this as-path filter ?
The only warning is that if prefix 1.1.160.0/21 matches as-path access-list 2 it could be advertised by the second route-map block.
All prefixes that don't match the second block will be denied (implicit deny at the end of the route-map, in some applications a last block with only permit is needed to avoid this)
Hope to help
Giuseppe
07-15-2008 02:03 PM
Hello Merrylle,
the prefix-list has the following meaning:
all IP prefixes that are contained in x.x.160.0/20 and have a prefix len Greater Equal of 21 will be permitted
Then the prefix-list is used under first route-map block with deny so those IP prefixes permitted by the prefix-list will be actually filtered
All prefixes matching the as path access-list 2 and not matching the prefix-list OUT will be advertised to the neighbor x.x.x.x
Hope to help
Giuseppe
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