Prefix-list in BGP

[kumarpmt83]

Hi,

In our cisco 1841 there are two ISP MPLS link.

what dose mean the below configuration.

what is the use of prefix-list??how it is works??

ip prefix-list BSNL-LANROUTES seq 5 permit 172.16.0.2/32

ip prefix-list BSNL-LANROUTES seq 10 permit 10.10.30.0/24

ip prefix-list BSNL-LANROUTES seq 20 permit 10.10.30.0/23

!

ip prefix-list TATA-LANROUTES seq 10 permit 10.10.31.0/24

ip prefix-list TATA-LANROUTES seq 20 permit 10.10.30.0/23

router bgp 85001

network 10.10.30.0 mask 255.255.254.0

network 172.16.0.2 mask 255.255.255.255

neighbor 10.70.118.189 remote-as 6755

neighbor 10.70.128.198 soft-reconfiguration inbound

neighbor 10.70.128.198 route-map EBGP-TATAWAN out

neighbor 192.168.214.66 remote-as 7829

neighbor 192.168.204.76 soft-reconfiguration inbound

neighbor 192.168.204.76 route-map EBGP-BSNLWAN out

route-map EBGP-TATAWAN permit 10

match ip address prefix-list TATA-LANROUTES

route-map EBGP-BSNLWAN permit 10

match ip address prefix-list BSNL-LANROUTES

[barnesp]

Ok this is quite straight forward. But let's work backwards. You have two BGP peers with route-maps configured with the out key word. These statements filter outbound BGP prefixes based on the requisite route-map. Each route-map has one clause which is to permit and has a match statement with a prefix-list associated. The prefix-list in this case will match routes in your router's BGP table for onward advertisement to your providers. In your case only the prefixes specified will be advertised and all others will be blocked by a implicit deny at the end of each prefix-list. In this particular case you could have applied the prefix-list directly to the neighbor using - neighbor x.x.x.x prefix-list

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[kumarpmt83]

without using the prefix-list it create any problem.

[Marwan ALshawi]

as mentioned by the above poster this is for route filtering

the out direction make sure you only advertise the route in the prefix list if you remove it and there is no other ISP links or advertised route then no issue

if any then you need to be careful

the inbound direction is just to make sure this router will install in the bgp routing table only the route in the inbound prefix list if the Teleco send any other route it will be filtered out and this is something has to e build based on the network design and requriments

hope this help

please rate the helpful posts

[barnesp]

in effect the policy configured governs what traffic you will receive. In other words you will only get traffic for the prefixes you advertise out.

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