BGP - Control advertisement/prefix

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Jan 29th, 2012
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Just need to know your expert opinion on this.

I have 2 provider independent prefixes advertised to our current 3 ISPs (ISP-A, ISP-B, ISP-C) via BGP. And I have another separate ISP (ISP-D) with its own IP prefix (ISP owned prefix) running default route only to ISP network (currently not member of BGP).

I plan to join ISP-D to existing BGP, advertising my 2 provider independent prefix plus the ISP owned prefix of ISP-D to this link. However I only want to advertise the ISP owned prefix to ISP-D and not to my other ISP. Meaning this prefix can only be reachable on the internet through ISP-D, inbound and outbound.

I am currently reading on Prefix List Policy Control and I just want to know if I am on the right track. Any ideas how it can be implemented? I will be testing this on lab.


My AS: AS 100

Remote As: AS 200

Provider independent Prefixes:

ISP-D prefix (ISP owned):

Appreaciate your ideas.


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ciscomanx Mon, 01/30/2012 - 00:29
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Im thinking of implementing NO-EXPORT community using prefix-list/route-map but I do not prefer applying filtering to every eBGP peer. In this case if i would go for the no-export community I will have to apply "send-community" and "route-map" to my existing peering with 3 ISPs.

Do you have any solutions for this?

ciscomanx Tue, 01/31/2012 - 09:23
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Appreciate any comments.

Have you tried the "no-export" or "no-advertise" community in real world?


Richard Burts Tue, 01/31/2012 - 11:28
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I am puzzled why you feel ok to configure all of the EBGP peers with a route map and a prefix list to set the community but do not want to configure the EBGP peer with a prefix list or a distribute list or a route map to prevent advertisement of the provider dependent prefix.

If you try to use the no export or no advertise community you will be advertising the provider dependent prefix to your immediate peers and it seems to me to be a problem if you advertise that prefix to any of the other external peers. So the best solution is some route filtering (distribute list, prefix list, or route map) on the EBGP peers.



ciscomanx Tue, 01/31/2012 - 20:29
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Hi Rick,

Thanks for your response. I currently have outbound filter-list applied on all my eBGP peerings and I worried too much filtering might affect the performance of the router. Using community might be another alternative idea but considering your comments also with Rick below prefix-list or distribute-list will help.

If say I applied both filter-list and prefix-list on the neighbor statement do you see any issue?


- Richard

Rick Morris Wed, 02/01/2012 - 05:37
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In the config all you need is to apply one.

Here is a link that has some basic info

Basically you would use one or the other but not both when performing the funciton of filtering.

router bgp 300



neighbor remote-as 100

neighbor route-map localonly out

neighbor  prefix-list  ABC in

neighbor remote-as 200

neighbor route-map ROUTE-ADV out

neighbor prefix-list  ABC in

ip prefix-list  ABC  seq 5 permit

ip prefix-list localonly seq 5 le 32

ip prefix-list ROUTE-ADV seq 5 permit le 32

ip prefix-list ROUTE-ADV seq 10 permit le 32

route-map ROUTE-ADV permit 10

match ip address ROUTE-ADV

set ip local-preference 200

route-map localonly permit 10

match ip address localonly

This is a sample config taken off the cisco link provided but also a few additions so you can see the differences and how they are applied.

Richard Burts Wed, 02/01/2012 - 12:00
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No I do not see any issues if you use both a prefix list and a filter list on a neighbor statement.



Rick Morris Tue, 01/31/2012 - 10:55
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One of the quickest and most straight forward I can think of is creating prefix-list, call it ISP-D-ADV

Create route-map, call it ISP-D-ADV

apply route-map to neighbor statement out.

clear BGP session

verify your adv by doing sh ip bgp neigh adv


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