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We have an ASA that has been doing iBGP for hosted networks. These networks include public address spaces that are routed inside our networks.

Recently we have changed providers. The new provider does not do static routing (which previously forwarded our networks to us), and requires BGP.

So, we have an existing private AS on the 5500 and all the devices behind it have private AS numbers. It all works beautifully.

I started looking into hosting multiple AS on the 5500 and found nothing really. I have configured the BGP neighbor with remote and local AS so that we can spoof the new AS number assigned by the new provider.

The issue now, is I can't find a way to manually advertise our public class C to this one specific neighbor. I do not want it in my routing tables due to its impact on the internal routes. I simply need to advertise to the provider to send anything in this subnet to me.

So, does anyone know the trick to configure BGP to only advertise x.x.x.x/24 with next hop y.y.y.y on AS zzzz to neighbor a.a.a.a?

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    "I do not want it in my routing tables due to its impact on the internal routes. I simply need to advertise to the provider to send anything in this subnet to me." The thing about BGP is that it will only advertise a route that already exists in the routing table. You cannot simply to tell it to advertise a route.
    – Ron Maupin
    Mar 24 at 23:30
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    You can control which routes are advertised to, or received from, any of your neighbors using various methods, but one of the easiest is simply using prefix-lists that are then tied to route-maps, that are then tied to neighbor entries in your BGP config. The routes you advertise have to already exist in your route table, like Ron already mentioned, though. Is that what you mean, or is it something else you’re looking to do?
    – Jesse P.
    Mar 25 at 1:45
  • So, I have specific iBGP routes to each of the "tenants" within the enterprise. Each receives a /29 of public IP space. So, the ASA has numerous routes for these subnets. I need to advertise the class C to the ISP. Update I added a static route for the class C to Null0. I then redistributed this route entry through a route-map. The last portion is that the AS PATH is failing to update. Network Next-Hop AS-Path x.x.x.x/24 y.y.y.y ?
    – Gump
    Mar 25 at 14:18
  • Found the resolution. add no-prepend and replace-as to the neighbor statement.
    – Gump
    Mar 25 at 16:12
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    "Each receives a /29 of public IP space. So, the ASA has numerous routes for these subnets. I need to advertise the class C to the ISP." Cisco has the aggregate BGP command that does all that for you, and there is the summary-only option that will only advertise the summary. Also, network address classes are dead (please let them rest in peace), killed in 1993 (two years before the commercial Internet in 1995) by RFCs 1517, 1518, and 1519, which defined CIDR (Classless Inter-Domain Routing). We have not had network address classes in this century.
    – Ron Maupin
    Mar 25 at 16:19

1 Answer 1

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The solution for me was to add the no-prepend and replace-as options on the neighbor statement with local-as.

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