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Community Member

Prefix-list Logic -- Need help

Dear All,

I have following prefix list on my internet router for which need to understand the logic.

router bgp xxxx (public AS)

nei x.x.x.x remote-as 4755

neighbor x.x.x.x prefix-list VSNL-ANNOUNCE out

nei y.y.y.y remote-as 1236

neighbor y.y.y.y prefix-list Bharti-ANNOUNCE out

network 203.99.216.0 mask 255.255.254.0
network 203.99.216.0
network 203.99.217.0

ip prefix-list Bharti-ANNOUNCE seq 5 permit 203.99.216.0/24
ip prefix-list Bharti-ANNOUNCE seq 15 permit 203.99.216.0/23
!
ip prefix-list DENY-ALL seq 5 deny 0.0.0.0/0 le 32
!
ip prefix-list  VSNL-ANNOUNCE seq 10 permit 203.99.217.0/24
ip prefix-list VSNL-ANNOUNCE seq 15 permit 203.99.216.0/23

Please help me to understand the logic of the prefix list.

Because Bharti SP is getting 203.99.216.0/24 networks only & VSNL is getting  203.99.216.0/23 networks.

2 REPLIES

Re: Prefix-list Logic -- Need help

Dear All,

I have following prefix list on my internet router for which need to understand the logic.

router bgp xxxx (public AS)

nei x.x.x.x remote-as 4755

neighbor x.x.x.x prefix-list VSNL-ANNOUNCE out

nei y.y.y.y remote-as 1236

neighbor y.y.y.y prefix-list Bharti-ANNOUNCE out

network 203.99.216.0 mask 255.255.254.0
network 203.99.216.0
network 203.99.217.0

ip prefix-list Bharti-ANNOUNCE seq 5 permit 203.99.216.0/24
ip prefix-list Bharti-ANNOUNCE seq 15 permit 203.99.216.0/23
!
ip prefix-list DENY-ALL seq 5 deny 0.0.0.0/0 le 32
!
ip prefix-list  VSNL-ANNOUNCE seq 10 permit 203.99.217.0/24
ip prefix-list VSNL-ANNOUNCE seq 15 permit 203.99.216.0/23

Please help me to understand the logic of the prefix list.

Because Bharti SP is getting 203.99.216.0/24 networks only & VSNL is getting  203.99.216.0/23 networks.

Hi Yogesh,

Prefix lists are more sophisticated forms that Cisco provides for filtering      BGP route advertisements. They filter on IP address just as distribute-lists      do, however they are easier to read, and require fewer commands to configure.For example

prefix-list xx seq 10 permit 43.10.0.0/16 ge 18

The statement above allows any route announcement in the range of 43.1.0.0 - 43.1.255.255 but that announcement must have a length greater than 18 bits in the mask. This permits you to allow announcements in the range, but not an announcement equalling the entire range (/16), or even announcements of half the range (/17). Only announcments with a length "greater than or equal to" /18 will be permitted.

With your configuration only /24 and /23 in the network will be out via prefic list for vsnl and vice versa for bharathi.

Check out the below link for more information and implementation of prefix list

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a008009456d.shtml

Hope to Help !!

Ganesh.H

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Community Member

Re: Prefix-list Logic -- Need help

Hi Ganesh,

Thanks for the reply...

As per your last statement...

With your configuration only /24 and /23 in the network will be out via prefic list for vsnl and vice versa for bharathi.

Bharti is not getting /23 their are getting /24 only.....so want to understand logic...How come one sp is getting /23 & other only /24.

appriciate if you can help me in this...

Regards

Yogesh

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